This eBook shows you how to
To do this you need "TEST GEAR." The best item of
Test Gear is a
MULTIMETER. It can
test almost 90% of all components. And that's what we will do in this
- There are two types:
DIGITAL and ANALOGUE
A Digital Multimeter has a set of digits on the display and
Analogue Multimeter has a scale with a pointer (or needle).
You really need both types to cover the number
of tests needed for designing and repair-work. We will discuss
how they work, how to use them and some of the differences between them.
DIGITAL AND ANALOGUE MULTIMETERS
BUYING A MULTIMETER
There are many different types on the market.
The cost is determined by the number of ranges and also the extra features such as diode tester,
buzzer (continuity), transistor tester, high DC current and others.
Since most multimeters are reliable and accurate, buy one with the greatest
number of ranges at the lowest cost.
This article explains the difference between a cheap analogue meter,
an expensive analogue meter and a digital meter. You will then be able
to work out which two meters you should buy.
Multimeters are sometimes called a "meter", a "VOM"
(Volts-Ohms-Milliamps or Volt Ohm Meter) or "multi-tester" or
even "a tester" - they are all the same.
USING A MULTIMETER
Analogue and digital multimeters have either a rotary selector switch or
push buttons to select the appropriate function and range. Some Digital
Multimeter (DMMs) are auto ranging; they automatically select the
correct range of voltage, resistance, or current when doing a test.
However you need to select the function.
Before making any measurement you need to know what you are
checking. If you are measuring voltage, select the AC range (10v, 50v,
250v, or 1000v) or DC range (0.5v, 2.5v, 10v, 50v, 250v, or 1000v). If
you are measuring resistance, select the Ohms range (x1, x10, x100, x1k,
x10k). If you are measuring current, select the appropriate current range DCmA
0.5mA, 50mA, 500mA. Every multimeter is different however the photo
shows a low cost meter with the basic ranges.
The most important point to remember is this:
You must select a voltage or current range that is bigger or HIGHER than the
maximum expected value, so the needle does not swing across the scale
and hit the "end stop."
If you are using a DMM (Digital Multi Meter), the meter will indicate if
the voltage or current is higher than the selected scale, by showing "OL"
- this means "Overload." If you are measuring resistance such as 1M on
the x10 range the "OL" means "Open Loop" and you will need to change the
meters show "1' on the display when the measurement is higher than
the display will indicate and some flash a set of digits to show
over-voltage or over-current. A "-1" indicates the leads should be
reversed for a "positive reading."
If it is an
AUTO RANGING meter, it will automatically produce a reading, otherwise
the selector switch must be changed to another range.
The Common (negative) lead ALWAYS
the "COM" socket. The red lead fits into the
red socket for Voltage and Resistance.
Place the red lead (red banana plug)
into "A" (for HIGH CURRENT "Amps")
or mA,uA for LOW
The black "test lead" plugs into the
socket marked "-" "Common",
or "Com," and the red
"test lead" plugs into meter socket marked "+" or
The third banana socket measures HIGH CURRENT and the positive (red lead)
plugs into this. You DO NOT move the negative "-" lead at any
The following two photos show the test leads fitted to a digital meter.
The probes and plugs have "guards" surrounding the probe tips and
also the plugs
so you can measure high voltages without getting near the
Analogue meters have an "Ohms Adjustment" to allow for the change in
voltage of the battery inside the meter (as it gets old).
"Ohms Adjust" is
called "ZERO SET"
The sensitivity of this meter is
on the DC ranges and 5k/v on the AC ranges
Before taking a resistance reading (each
time on any of the Ohms scales) you need
to "ZERO SET" the scale, by touching the two probes together and
adjust the pot until the needle reads "0" (swings FULL SCALE).
If the pointer does not reach full scale, the batteries need replacing.
Digital multimeters do not
need "zero adjustment."
cover the normal uses for a multimeter, it is interesting to note that
some Digital Multimeters (DMM) have features such as Capacitance,
Frequency and measuring the gain of a transistor as well as a number of
other features using probes such as a temperature probe. The VICHY
VC99 meter above is an example and costs about $40.00.
Most of the readings taken with a multimeter will be VOLTAGE readings.
Before taking a reading, you should select the highest range and if the
needle does not move up scale (to the right), you can select another
Always switch to the highest range before probing a circuit and keep
your fingers away from the component being tested.
If the meter is Digital, select the highest range or use the
auto-ranging feature, by selecting "V."
The meter will automatically produce a result, even if the voltage is AC
If the meter is not auto-ranging, you will have to select
voltage is from a DC source or
voltage is from an AC source. DC means Direct Current and the voltage is
coming from a battery or supply where the voltage is steady and not
changing and AC means Alternating Current where the voltage is coming
from a voltage that is rising and falling.
You can measure the voltage at different points in a circuit by
connecting the black probe to chassis. This is the 0v reference and is
commonly called "Chassis" or "Earth" or "Ground" or "0v."
The red lead is called the "measuring lead" or "measuring probe"
and it can measure voltages at any point in a circuit. Sometimes there
are "test points" on a circuit and these are wires or loops designed to hold the tip
of the red probe (or a red probe fitted with a mini clip).
You can also measure voltages ACROSS A COMPONENT. In other words,
the reading is taken in PARALLEL with the component. It may be the voltage across
a transistor, resistor, capacitor, diode or coil. In most cases this
voltage will be less than the supply voltage.
If you are measuring the voltage in a circuit that has a HIGH IMPEDANCE,
the reading will be inaccurate, up to 90% !!!, if you use a cheap
Here's a simple case.
The circuit below consists of two 1M resistors in series. The voltage at the mid point will be 5v when nothing is connected to the mid point. But if we use a
cheap analogue multimeter set to 10v, the resistance of the meter
will be about 100k, if the meter has a sensitivity of 10k/v and the reading will be incorrect.
Here how it works:
Every meter has a sensitivity. The sensitivity of the meter is the
the movement and is the amount of current required to deflect the
needle FULL SCALE.
This current is very small, normally 1/10th of a milliamp and
corresponds to a sensitivity of 10k/volt (or 1/30th mA, for a sensitivity of 30k/v).
If an analogue meter is set to 10v, the internal resistance of the meter
will be 100k for a 10k/v movement.
If this multimeter is used to test the following circuit, the reading
will be inaccurate.
The reading should be 5v as show in diagram
But the analogue multimeter has an internal resistance of 100k and it
creates a circuit shown in
The top 1M and 100k from the meter create a combined PARALLEL resistance
of 90k. This forms a series circuit with the lower 1M and the meter
will read less than 1v
If we measure the voltage across the lower 1M, the 100k meter will form
a value of resistance with the lower 1M and it will read less than 1v
If the multimeter is 30k/v, the readings will be 2v.
See how easy it is to get a totally inaccurate reading.
This introduces two new terms:
HIGH IMPEDANCE CIRCUIT and "RESISTORS in SERIES and
If the reading is taken with a Digital Meter, it will be more accurate
as a DMM does not take any current from the circuit (to activate the
meter). In other words it has a very HIGH input impedance. Most Digital
Multimeters have a fixed input resistance (impedance) of 10M - no matter
what scale is selected.
That's the reason for choosing a DMM for high impedance circuits.
It also gives a reading that is accurate to about 1%.
IN A CIRCUIT
You can take many
voltage-measurements in a circuit. You can measure "across" a component,
or between any point in a circuit and either the positive rail or earth
rail (0v rail). In the following circuit, the 5 most important
voltage-measurements are shown. Voltage "A" is across the electret
microphone. It should be between 20mV and 500mV. Voltage "B" should be
about 0.6v. Voltage "C" should be about half-rail voltage. This allows
the transistor to amplify both the positive and negative parts of the
waveform. Voltage "D" should be about 1-3v. Voltage "E" should be the
battery voltage of 12v.
MEASURING VOLTAGES IN A CIRCUIT
You will rarely need to take
current measurements, however most multimeters have DC current ranges
such as 0.5mA, 50mA, 500mA and 10Amp (via the extra banana socket)
and some meters have AC current ranges. Measuring the current of a circuit will tell you a lot of things.
If you know the normal current, a high or low current can let you know if
the circuit is overloaded or not fully operational.
Current is always measured when the circuit is working (i.e: with power applied).
It is measured IN SERIES with the
circuit or component under test.
The easiest way to measure current is to remove the fuse and take a
reading across the fuse-holder. Or remove one lead of the battery or turn the
project off, and measure across the switch.
If this is not possible, you will need to remove one end of a component
and measure with the two probes in the "opening."
Resistors are the easiest things to desolder, but you may have to cut a
track in some circuits. You have to get an "opening" so that a current
reading can be taken.
The following diagrams show how to connect the probes to take a CURRENT
Do not measure the current ACROSS a component as this will create a
The component is designed to drop a certain voltage and when you place
the probes across this component, you are effectively adding a "link" or
"jumper" and the voltage at the left-side of the component will appear
on the right-side. This voltage may be too high for the circuit being
supplied and the result will be damage.
Measuring current through a resistor
Measuring the current of a globe
Do NOT measure the
CURRENT of a battery
placing the meter directly across the terminals)
A battery will deliver a very HIGH current
and damage the meter
Do not measure the "current a battery will deliver" by placing the probes
across the terminals. It will deliver a very high current and damage the
meter instantly. There are special battery testing instruments for this
When measuring across an "opening" or "cut," place the red probe on the
wire that supplies the voltage (and current) and the black probe on the
other wire. This will produce a "POSITIVE" reading.
A positive reading is an UPSCALE READING and the pointer will move
across the scale - to the right. A "NEGATIVE READING" will make the pointer hit the
at the left of the scale and you will not get a reading. If you are
using a Digital Meter, a negative sign "-" will appear on the screen to
indicate the probes are around the wrong way. No damage will be caused.
It just indicates the probes are connected incorrectly.
If you want an accurate CURRENT MEASUREMENT, use a digital meter.
Turn a circuit off before measuring resistance.
If any voltage is present, the value of resistance will be incorrect.
In most cases you cannot measure a component while it is in-circuit.
This is because the meter is actually measuring a voltage across a
component and calling it a "resistance." The voltage comes
from the battery inside the meter. If any other voltage is
present, the meter will produce a false reading.
If you are measuring the resistance of a component while still "in
circuit," (with the power off) the reading will be lower than the true
Measuring resistance of a heater
(via the leads)
Measuring the resistance of a
piece of resistance-wire
Measuring the resistance of a
Do not measure the "Resistance of
1. Do not measure the "resistance of a battery." The resistance of a
battery (called the Internal impedance) is not measured as shown in the
diagrams above. It is measured by creating a current-flow and measuring
the voltage across the battery. Placing a multimeter set to resistance (across a battery) will destroy the meter.
Here is another well-designed resistor colour code chart:
2. Do not try to measure the resistance of any voltage or any "supply."
Resistance is measured in OHMs.
The resistance of a 1cm x 1cm bar, one metre long is 1 ohm.
If the bar is thinner, the resistance is higher. If the bar is longer,
the resistance is higher.
If the material of the bar is changed, the resistance is higher.
When carbon is mixed with other elements, its resistance increases and
this knowledge is used to make RESISTORS.
Resistors have RESISTANCE and the main purpose of a resistor is to
reduce the CURRENT FLOW.
It's a bit like standing on a hose. The flow reduces.
When current flow is reduced, the output voltage is also reduced and
that why the water does not spray up so high. Resistors are simple
devices but they produce many different effects in a circuit.
A resistor of nearly pure carbon may be 1 ohm, but when non-conducting
"impurities" are added, the same-size resistor may be 100 ohms, 1,000
ohms or 1 million ohms.
Circuits use values of less than 1 ohm to more than 22 million ohms.
Resistors are identified on a circuit with numbers
and letters to show the exact value of resistance - such as 1k
W (omega - a Greek symbol)
is used to identify the word "Ohm."
but this symbol is not available on some word-processors, so the
letter "R" is used. The letter "E" is also sometimes used and both
A one-ohm resistor is written "1R" or "1E." It can also be written
"1R0" or "1E0."
A resistor of one-tenth of an ohm is written "0R1" or "0E1."
The letter takes the place of the decimal point.
10 ohms = 10R
100 ohms = 100R
1,000 ohms = 1k (k= kilo = one thousand)
10,000 ohms = 10k
100,000 ohms = 100k
1,000,000 ohms = 1M (M = MEG = one million)
The size of a resistor has nothing to do with its resistance. The size
determines the wattage of the resistor - how much heat it can dissipate
without getting too hot.
Every resistor is identified by colour bands on the body, but when the
resistor is a surface-mount device, numbers are used and sometimes
You MUST learn the colour code for resistors and the following table
shows all the colours for the most common resistors from 1/10th of an
ohm to 22 Meg ohms for resistors with 5% and 10% tolerance.
3rd band is gold, Divide by 10
If 3rd band is silver, Divide by 100
(to get 0.22ohms etc)
The most "common" type of
resistor has 4 bands and is called the 10% resistor. It now has a
tolerance of 5% but is still called the "10% type" as the colours
increase by 20% so that a resistor can be 10% higher or 10% lower than a
particular value and all the resistors produced in a batch can be
The first 3 bands produce the resistance and the fourth band is the
"tolerance" band. Gold = 5%
(Silver =10% but no modern resistors are 10%!! - they are
5% 2% or 1%)
Download the program and save it on your desk-top for future
RESISTORS LESS THAN 10 OHMS
When the third band is gold, it indicates the value of the
"colors" must be divided by 10.
Gold = "divide by 10" to get values 1R0 to
When the third band is silver, it indicates the value of the
"colors" must be divided by 100. (Remember: more letters in the word
"silver" thus the divisor is "a larger division.")
Silver = "divide by 100" to get values R1
e.g: 0R1 = 0.1 ohm 0R22 = point 22 ohms
See 4th Column above for examples.
The letters "R, k and M" take the place of a decimal point.
e.g: 1R0 = 1 ohm 2R2 = 2 point 2 ohms 22R =
2k2 = 2,200 ohms 100k = 100,000 ohms
2M2 = 2,200,000 ohms
HOW TO REMEMBER THE COLOUR CODE:
Each colour has a "number" (or divisor) corresponding to it.
Most of the colours are in the same order as in the spectrum. You can
see the spectrum in a rainbow. It is: ROY G BIV and the colours for
resistors are in the same sequence.
brown - colour of increasing temperature
(indigo - that part of the
spectrum between blue and violet)
||No of zero's
||divide by 100
||divide by 10
||,000 or k
Here are some common ways to remember the
Bad Beer Rots Our Young Guts, But Vodka Goes Well
Bright Boys Rave Over Young Girls But Violet Gets Wed
Bad Boys Rave Over Young Girls But Violet Gets Wed with Gold and
Reading 5-band resistors:
5-band resistors are
easy to read if you remember two simple points. The first three bands
provide the digits in the answer and the 4th band supplies the number of
"STANDARD VALUES" (on
5-band resistors are also made in "Standard
Values" but will have different colours to 4-band "common"
resistors - and will be confusing if you
are just starting out. For instance, a 47k 5% resistor with 4-bands will
be: yellow-purple-orange-gold. For a 47k 1% resistor the colours will be
yellow-purple-black-red-brown. The brown colour-band represents 1%.
The first two colour-bands for a STANDARD VALUE or "common value" in 1% or 5% will be the
SAME. These two bands provide the digits in the answer.
It's the 3rd band for a 5% resistor that is expanded into two bands in a
1% resistor. But it's easy to follow.
For a standard value, the 3rd band in a 1% resistor is BLACK. This
represents a ZERO in the answer. (For 5-band resistors BLACK represents
a ZERO when in the third band. This is different to 4-band resistors
where black represents the word OHMS! If the third band is BROWN, the
answer will be 1).
So the 4th band has to represent one-less ZERO and is one colour UP THE
COLOUR CHART! In other words the 3rd and 4th bands (combined) on a 1%
resistor produces the same number of zero's as the 3rd band on a 5%
Resistors come in a range of values and
the two most common are the E12 and E24 series. The E12 series comes in
twelve values for each decade. The E24 series comes in twenty-four
values per decade.
E12 series - 10, 12, 15, 18, 22, 27, 33,
39, 47, 56, 68, 82
E24 series - 10, 11, 12, 13, 15, 16, 18,
20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91
Here is the complete list of
1% 1/4watt resistors from:
The following list covers 10 ohms (10R) to 1M.
To buy 1% resistors from Circuit Specialists, click:
Here is the list of 1%
resistors from suppliers (such as Farnell):
Surface Mount Resistors
3-digit Surface Mount resistors on a PC board
4-digit Surface Mount resistors on a PC board
The photo above shows
surface mount resistors on a circuit board. The components that are not
marked are capacitors (capacitors are NEVER marked).
All the SM resistors in the above photos conform
to a 3-digit or 4-digit code. But there are a number of codes, and the 4-digit
code caters for high tolerance resistors, so it's getting very complicated.
Here is a basic 3-digit SM resistor:
A 330k SM resistor
The first two digits represent the two
digits in the answer. The third digit represents the number of zero's
you must place after the two digits. The answer will be OHMS. For
example: 334 is written 33 0 000. This is written 330,000 ohms. The
comma can be replaced by the letter "k". The final answer is: 330k.
222 = 22 00 = 2,200 = 2k2
473 = 47 000 = 47,000 = 47k
474 = 47 0000 = 470,000 = 470k
105 = 10 00000 = 1,000,000 = 1M = one million ohms
There is one trick you have to remember. Resistances less than 100 ohms
are written: 100, 220, 470. These are 10 and NO zero's = 10 ohms = 10R
or 22 and no zero's = 22R or 47 and no zero's = 47R. Sometimes the
resistor is marked: 10, 22 and 47 to prevent a mistake.
R = ohms
k = kilo ohms = 1,000 ohms
M = Meg = 1,000,000 ohms
The 3 letters (R, k and M) are put in place of the decimal point. This
way you cannot make a mistake when reading a value of resistance.
THE COMPLETE RANGE OF SM
Click to see the complete range of SM resistor markings for 3-digit
Click to see the complete range of SM
resistor markings for 4-digit code:
0000 is a value on a surface-mount resistor. It is a
Resistances less than 10 ohms have 'R' to indicate the position of the
Here are some examples:
Three Digit Examples
Four Digit Examples
is 33 ohms - not 330
is 100 ohms -
not 1000 ohms
221 is 220
49 900 ohms, or 49k9
683 is 68 000
ohms, or 68k
is 162 000 ohms, or 162k
105 is 1 000
000 ohms, or 1M
or R56 is
is 8.2 ohms
A new coding system
has appeared on 1% types. This is known as the EIA-96 marking
method. It consists of a three-character code. The first two digits
signify the 3 significant digits of the resistor value, using the lookup
table below. The third character - a letter - signifies the multiplier.
The multiplier letters are as
X or S
Y or R
22A is a 165 ohm resistor, 68C
is a 49900 ohm (49k9) and 43E a 2740000 (2M74). This marking
scheme applies to 1% resistors only.
A similar arrangement can be used for
2% and 5% tolerance types. The multiplier letters are identical to
1% ones, but occur before the number code and the following
code is used:
arrangement, C31 is 5%, 18000 ohm (18k), and D18 is 510000
ohms (510k) 2% tolerance.
Always check with an ohm-meter (a multimeter) to make sure.
Chip resistors come in the
following styles and ratings:
Style: 0402, 0603,
0805, 1206, 1210, 2010, 2512, 3616, 4022
0402(1/16W), 0603(1/10W), 0805(1/8W), 1206(1/4W), 1210(1/3W),
2010(3/4W), 2512(1W), 3616(2W), 4022(3W)
Tolerance: 0.1%, 0.5%,
25ppm 50ppm 100ppm
EIA marking code
for surface mount (SMD) resistors
02S = 1R02
03S = 1R05
04S = 1R07
05S = 1R1
06S = 1R13
07S = 1R15
08S = 1R18
09S = 1R21
10S = 1R24
11S = 1R27
12S = 1R3
13S = 1R33
14S = 1R37
15S = 1R4
16S = 1R43
17S = 1R47
18S = 1R5
19S = 1R54
20S = 1R58
21S = 1R62
22S = 1R65
23S = 1R69
24S = 1R74
25S = 1R78
26S = 1R82
27S = 1R87
28S = 1R91
29S = 1R96
30S = 2R0
31S = 2R05
32S = 2R10
33S = 2R15
34S = 2R21
35S = 2R26
36S = 2R32
37S = 2R37
38S = 2R43
39S = 2R49
40S = 2R55
41S = 2R61
42S = 2R67
43S = 2R74
44S = 2R80
45S = 2R87
46S = 2R94
47S = 3R01
48S = 3R09
49S = 3R16
50S = 3R24
51S = 3R32
52S = 3R4
53S = 3R48
54S = 3R57
55S = 3R65
56S = 3R74
57S = 3R83
58S = 3R92
59S = 4R02
60S = 4R12
61S = 4R22
62S = 4R32
63S = 4R42
64S = 4R53
65S = 4R64
66S = 4R75
67S = 4R87
68S = 4R99
69S = 5R11
70S = 5R23
71S = 5R36
72S = 5R49
73S = 5R62
74S = 5R76
75S = 5R9
76S = 6R04
77S = 6R19
78S = 6R34
79S = 6R49
80S = 6R65
81S = 6R81
82S = 6R98
83S = 7R15
84S = 7R32
85S = 7R5
86S = 7R68
87S = 7R87
88S = 8R06
89S = 8R25
90S = 8R45
91S = 8R66
92S = 8R87
93S = 9R09
94S = 9R31
95S = 9R53
96S = 9R76
02R = 10R2
03R = 10R5
04R = 10R7
05R = 11R
06R = 11R3
07R = 11R5
08R = 11R8
09R = 12R1
10R = 12R4
11R = 12R7
12R = 13R
13R = 13R3
14R = 13R7
15R = 14R
16R = 14R3
17R = 14R7
18R = 15R
19R = 15R4
20R = 15R8
21R = 16R2
22R = 16R5
23R = 16R9
24R = 17R4
25R = 17R8
26R = 18R2
27R = 18R7
28R = 19R1
29R = 19R6
30R = 20R0
31R = 20R5
32R = 21R0
33R = 21R5
34R = 22R1
35R = 22R6
36R = 23R2
37R = 23R7
38R = 24R3
39R = 24R9
40R = 25R5
41R = 26R1
42R = 26R7
43R = 27R4
44R = 28R0
45R = 28R7
46R = 29R4
47R = 30R1
48R = 30R9
49R = 31R6
50R = 32R4
51R = 33R2
52R = 34R0
53R = 34R8
54R = 35R7
55R = 36R5
56R = 37R4
57R = 38R3
58R = 39R2
59R = 40R2
60R = 41R2
61R = 42R2
62R = 43R2
63R = 44R2
64R = 45R3
65R = 46R4
66R = 47R5
67R = 48R7
68R = 49R9
69R = 51R1
70R = 52R3
71R = 53R6
72R = 54R9
73R = 56R2
74R = 57R6
75R = 59R0
76R = 60R4
77R = 61R9
78R = 63R4
79R = 64R9
80R = 66R5
81R = 68R1
82R = 69R8
83R = 71R5
84R = 73R2
85R = 75R0
86R = 76R8
87R = 78R7
88R = 80R6
89R = 82R5
90R = 84R5
91R = 86R6
92R = 88R7
93R = 90R9
94R = 93R1
95R = 95R3
96R = 97R6
02A = 102R
03A = 105R
04A = 107R
05A = 110R
06A = 113R
07A = 115R
08A = 118R
09A = 121R
10A = 124R
11A = 127R
12A = 130R
13A = 133R
14A = 137R
15A = 140R
16A = 143R
17A = 147R
18A = 150R
19A = 154R
20A = 158R
21A = 162R
22A = 165R
23A = 169R
24A = 174R
25A = 178R
26A = 182R
27A = 187R
28A = 191R
29A = 196R
30A = 200R
31A = 205R
32A = 210R
33A = 215R
34A = 221R
35A = 226R
36A = 232R
37A = 237R
38A = 243R
39A = 249R
40A = 255R
41A = 261R
42A = 267R
43A = 274R
44A = 280R
45A = 287R
46A = 294R
47A = 301R
48A = 309R
49A = 316R
50A = 324R
51A = 332R
52A = 340R
53A = 348R
54A = 357R
55A = 365R
56A = 374R
57A = 383R
58A = 392R
59A = 402R
60A = 412R
61A = 422R
62A = 432R
63A = 442R
64A = 453R
65A = 464R
66A = 475R
67A = 487R
68A = 499R
69A = 511R
70A = 523R
71A = 536R
72A = 549R
73A = 562R
74A = 576R
75A = 590R
76A = 604R
77A = 619R
78A = 634R
79A = 649R
80A = 665R
81A = 681R
82A = 698R
83A = 715R
84A = 732R
85A = 750R
86A = 768R
87A = 787R
88A = 806R
89A = 825R
90A = 845R
91A = 866R
92A = 887R
93A = 909R
94A = 931R
95A = 953R
96A = 976R
02B = 1k02
03B = 1k05
04B = 1k07
05B = 1k1
06B = 1k13
07B = 1k15
08B = 1k18
09B = 1k21
10B = 1k24
11B = 1k27
12B = 1k3
13B = 1k33
14B = 1k37
15B = 1k4
16B = 1k43
17B = 1k47
18B = 1k5
19B = 1k54
20B = 1k58
21B = 1k62
22B = 1k65
23B = 1k69
24B = 1k74
25B = 1k78
26B = 1k82
27B = 1k87
28B = 1k91
29B = 1k96
30B = 2k0
31B = 2k05
32B = 2k10
33B = 2k15
34B = 2k21
35B = 2k26
36B = 2k32
37B = 2k37
38B = 2k43
39B = 2k49
40B = 2k55
41B = 2k61
42B = 2k67
43B = 2k74
44B = 2k80
45B = 2k87
46B = 2k94
47B = 3k01
48B = 3k09
49B = 3k16
50B = 3k24
51B = 3k32
52B = 3k4
53B = 3k48
54B = 3k57
55B = 3k65
56B = 3k74
57B = 3k83
58B = 3k92
59B = 4k02
60B = 4k12
61B = 4k22
62B = 4k32
63B = 4k42
64B = 4k53
65B = 4k64
66B = 4k75
67B = 4k87
68B = 4k99
69B = 5k11
70B = 5k23
71B = 5k36
72B = 5k49
73B = 5k62
74B = 5k76
75B = 5k9
76B = 6k04
77B = 6k19
78B = 6k34
79B = 6k49
80B = 6k65
81B = 6k81
82B = 6k98
83B = 7k15
84B = 7k32
85B = 7k5
86B = 7k68
87B = 7k87
88B = 8k06
89B = 8k25
90B = 8k45
91B = 8k66
92B = 8k87
93B = 9k09
94B = 9k31
95B = 9k53
96B = 9k76
02C = 10k2
03C = 10k5
04C = 10k7
05C = 11k
06C = 11k3
07C = 11k5
08C = 11k8
09C = 12k1
10C = 12k4
11C = 12k7
12C = 13k
13C = 13k3
14C = 13k7
15C = 14k
16C = 14k3
17C = 14k7
18C = 15k
19C = 15k4
20C = 15k8
21C = 16k2
22C = 16k5
23C = 16k9
24C = 17k4
25C = 17k8
26C = 18k2
27C = 18k7
28C = 19k1
29C = 19k6
30C = 20k0
31C = 20k5
32C = 21k0
33C = 21k5
34C = 22k1
35C = 22k6
36C = 23k2
37C = 23k7
38C = 24k3
39C = 24k9
40C = 25k5
41C = 26k1
42C = 26k7
43C = 27k4
44C = 28k0
45C = 28k7
46C = 29k4
47C = 30k1
48C = 30k9
49C = 31k6
50C = 32k4
51C = 33k2
52C = 34k0
53C = 34k8
54C = 35k7
55C = 36k5
56C = 37k4
57C = 38k3
58C = 39k2
59C = 40k2
60C = 41k2
61C = 42k2
62C = 43k2
63C = 44k2
64C = 45k3
65C = 46k4
66C = 47k5
67C = 48k7
68C = 49k9
69C = 51k1
70C = 52k3
71C = 53k6
72C = 54k9
73C = 56k2
74C = 57k6
75C = 59k0
76C = 60k4
77C = 61k9
78C = 63k4
79C = 64k9
80C = 66k5
81C = 68k1
82C = 69k8
83C = 71k5
84C = 73k2
85C = 75k0
86C = 76k8
87C = 78k7
88C = 80k6
89C = 82k5
90C = 84k5
91C = 86k6
92C = 88k7
93C = 90k9
94C = 93k1
95C = 95k3
96C = 97k6
02D = 102k
03D = 105k
04D = 107k
05D = 110k
06D = 113k
07D = 115k
08D = 118k
09D = 121k
10D = 124k
11D = 127k
12D = 130k
13D = 133k
14D = 137k
15D = 140k
16D = 143k
17D = 147k
18D = 15k
19D = 154k
20D = 158k
21D = 162k
22D = 165k
23D = 169k
24D = 174k
25D = 178k
26D = 182k
27D = 187k
28D = 191k
29D = 196k
30D = 200k
31D = 205k
32D = 210k
33D = 215k
34D = 221k
35D = 226k
36D = 232k
37D = 237k
38D = 243k
39D = 249k
40D = 255k
41D = 261k
42D = 267k
43D = 274k
44D = 280k
45D = 287k
46D = 294k
47D = 301k
48D = 309k
49D = 316k
50D = 324k
51D = 332k
52D = 340k
53D = 348k
54D = 357k
55D = 365k
56D = 374k
57D = 383k
58D = 392k
59D = 402k
60D = 412k
61D = 422k
62D = 432k
63D = 442k
64D = 453k
65D = 464k
66D = 475k
67D = 487k
68D = 499k
69D = 511k
70D = 523k
71D = 536k
72D = 549k
73D = 562k
74D = 576k
75D = 590k
76D = 604k
77D = 619k
78D = 634k
79D = 649k
80D = 665k
81D = 681k
82D = 698k
83D = 715k
84D = 732k
85D = 750k
86D = 768k
87D = 787k
88D = 806k
89D = 825k
90D = 845k
91D = 866k
92D = 887k
93D = 909k
94D = 931k
95D = 953k
96D = 976k
02E = 1M02
03E = 1M05
04E = 1M07
05E = 1M1
06E = 1M13
07E = 1M15
08E = 1M18
09E = 1M21
10E = 1M24
11E = 1M27
12E = 1M3
13E = 1M33
14E = 1M37
15E = 1M4
16E = 1M43
17E = 1M47
18E = 1M5
19E = 1M54
20E = 1M58
21E = 1M62
22E = 1M65
23E = 1M69
24E = 1M74
25E = 1M78
26E = 1M82
27E = 1M87
28E = 1M91
29E = 1M96
30E = 2M0
31E = 2M05
32E = 2M10
33E = 2M15
34E = 2M21
35E = 2M26
36E = 2M32
37E = 2M37
38E = 2M43
39E = 2M49
40E = 2M55
41E = 2M61
42E = 2M67
43E = 2M74
44E = 2M80
45E = 2M87
46E = 2M94
47E = 3M01
48E = 3M09
49E = 3M16
50E = 3M24
51E = 3M32
52E = 3M4
53E = 3M48
54E = 3M57
55E = 3M65
56E = 3M74
57E = 3M83
58E = 3M92
59E = 4M02
60E = 4M12
61E = 4M22
62E = 4M32
63E = 4M42
64E = 4M53
65E = 4M64
66E = 4M75
67E = 4M87
68E = 4M99
69E = 5M11
70E = 5M23
71E = 5M36
72E = 5M49
73E = 5M62
74E = 5M76
75E = 5M9
76E = 6M04
77E = 6M19
78E = 6M34
79E = 6M49
80E = 6M65
81E = 6M81
82E = 6M98
83E = 7M15
84E = 7M32
85E = 7M5
86E = 7M68
87E = 7M87
88E = 8M06
89E = 8M25
90E = 8M45
91E = 8M66
92E = 8M87
93E = 9M09
94E = 9M31
95E = 9M53
96E = 9M76
18F = 15M
30F = 20M
If you want an accurate RESISTANCE
measurement, remove the resistor from the circuit and use a Digital meter.
SURFACE MOUNT COMPONENTS -
Talking Electronics has packs of components for the repairman. The
following packs are available:
RESISTOR PACK consists of 1 off each standard value
10 ohms to 1M & 2M2 (60 resistors)
$14.20 including pack and post
SURFACE MOUNT CAPACITOR PACK
2 - 10p 5 - 47p 5 - 100p 5 - 470p 5 - 1n 5 - 10n 5 - 22n 5 -
5 - 1u 16v electrolytic 5 - 10u 16v electrolytic
$23.80 including pack and post
SURFACE MOUNT DIODE PACK consists
5 - 1N 4148 (marked as "A6")
$10.00 including pack and post
TRANSISTOR PACK consists of:
5 - BC 848 (marked as "1K") NPN
5 - BC858 PNP
$10.00 including pack and post
for details on how to pay by credit card or PayPal.
Any value of resistance can be created by
connecting two resistors in PARALLEL or SERIES.
CREATING ANY VALUE OF RESISTANCE
You can also create a higher wattage resistor by connecting them in SERIES
We are only going to cover two EQUAL VALUE resistors in SERIES or in
If you want to create a "Special Value," simply connect two
resistors and read the value with a Digital Meter. Keep changing the
values until you get the required value. We are not going into series or
Parallel formulae. You can easily find a value with a multimeter.
TWO EQUAL-VALUE RESISTORS IN SERIES
Two equal-value resistors IN SERIES creates a value
of DOUBLE. You simply ADD the values.
This can be done with any to two values as shown.
Three equal-value resistors in series is three times the value.
TWO EQUAL-VALUE RESISTORS IN PARALLEL
Two equal-value resistors IN
PARALLEL creates a value of HALF. Three equal-value
resistors in parallel is equal to one-third the value.
If you want a particular
value and it is not available, here is a chart.
Use 2 resistors in series or parallel as shown:
There are other ways to
combine 2 resistors in parallel or series to get a particular value. The
examples above are just one way. 4R7 = 4.7 ohms
TESTING A RESISTOR
To check the value of a
resistor, it should be removed from the circuit. The surrounding
components can affect the reading and make it lower.
Resistors VERY RARELY change value, but if it is overheated
or damaged, the resistance can increase. You can take the reading of a
resistor "in-circuit" in one direction then the other, as the surrounding components
may have diodes and this will alter the reading.
You can also test a resistor by feeling its temperature-rise. It is
getting too hot if you cannot hold your finger on it
(some "metal film" resistors are designed to tolerate quite high
There is no such thing as an "AC"
resistor. Resistors are just "resistors" and they can be in AC circuits
or DC circuits. Resistors
can be given names such as "Safety Resistor" "Ballast Resistor"
"LOAD Resistor" "Feed Resistor" "Dropper Resistor" or
"Supply Resistor." These are just normal resistors with a normal
resistance - except a "Safety Resistor."
A safety resistor is made of a flame-proof material such as
metal-oxide-film and not carbon-composition. It is designed to "burn
out" when too much current flows BUT NOT CATCH FIRE.
It is a low-value resistor and has a voltage-drop across it but this is
not intentional. The voltage-drop is to create a "heating-effect" to
burn out the resistor. In all the other types of resistor, the
voltage-drop is intentional.
A Ballast resistor is a normal resistor and can be called a Power
resistor, Dropper resistor, Supply resistor or Feed resistor. It is
designed to reduce the voltage from one source and deliver a lower
voltage. It is a form of: "in-line" resistor.
A Load Resistor is generally connected across the output of a circuit
and turns the energy it receives, into heat.
To reduce the number of resistors in a circuit, some engineers use a
set of identical resistors in a package called a Single-In-Line (SIL)
resistor network. It is made with many resistors of the same value,
all in one package. One end of each resistor is connected all the other resistors
and this is the common pin, identified as pin 1 and has a dot on the
These packages are very reliable but to make sure all the
resistors are as stated, you need to locate pin 1. All values will be
identical when referenced to this pin.
Some resistor networks have a "4S" printed on the
component. The 4S indicates the package contains
4 independent resistors that are not wired together inside. The
housing has eight leads as shown in the second image.
Current x Current
Independent resistors have an even number of pins and measuring between
each pair will produce identical values. Resistance
between any pair will indicate leakage and may be a fault.
WIRE WOUND RESISTOR
A wire wound resistor is also called a
POWER RESISTOR. This type of resistor can have a resistance as low as
0.1 ohms (one-tenth of an ohm) or as high as about 10k.
The image shows a 0.68 ohm resistor as the letter "R" represents the
DECIMAL POINT and R68 is the same a .68 and this is 0.68 ohms. The
wattage is 9 watts.
This resistor will allow xxx amps to flow. To work out the
current, use the formula:
Power = Current x Current x resistance
9 = Current x Current x .68
Divide both sides by 0.68
Find the square root of 13.2
Current = 3.6 amps
When 3.6 amps flow through the resistor, the voltage appearing across it
V = current x resistance
= 3.6 x 0.68
= 2.5v and the wattage (heat) loss will be 9 watts.
The purpose of a resistor like this is to stop or reduce "ripple."
Ripple is the noise or hum in an amplifier when the sound is turned up.
There are many reasons why you need to reduce the level of hum and this
resistor will remove ripple as large as 2.5v when 3.6 amps is flowing,
provided you have filter electrolytics on both side of the resistor to
assist in removing the ripple.
If the letter "R" is in a different position, the value of resistance
68R = 68Ω
6R8 = 6.8Ω
R68 = 0.68Ω
If you replace the R68 resistor a 6R8 resistor by mistake, the voltage
across it will rise to 25v and if 3.6 amps flows, the wattage will be:
The resistor will glow red and burn out.
A Posistor is a
resistor that connects in series
with the degaussing coil around the picture tube or
Monitor. When cold, it
has a very low resistance and a large current flows when the monitor or
TV is switched on. This current heats up the Posistor and the resistance
increases. This causes the current to decrease and any magnetism in the
shadow mask is removed. The posistor can one or two elements and it is
kept warm so the resistance remains high. Many Posistors have a second
element inside the case that connects directly to the supply to keep the
Positive Temperature Coefficient resistor high so that the current
through the degaussing coil falls to almost zero. This constant heat
eventually destroys the package.
A "BURNT" RESISTOR
- normally and
technically called a "burnt-out" resistor.
The heavy current that flows when a set is turned ON also causes the posistor
to crack and break and this results in poor purity on the screen - as
the shadow mask gradually becomes magnetic..
Posistors have different resistance values from different manufacturers
and must be replaced with an identical type.
They can be checked for very low resistance when cold but any loose
pieces inside the case will indicate a damaged component.
The resistance of a
"burnt" resistor can sometimes be determined by scraping away the outer
coating - if the resistor has a spiral of resistance-material. You may be
able to find a spot where the spiral has been damaged.
Clean the "spot" (burnt section of the spiral) very carefully and make
sure you can get a good contact with the spiral and the tip of your
probe. Measure from one lead of the resistor to the end of the damaged
spiral. Then measure from the other lead to the other end of the spiral.
Add the two values and you have an approximate value for the resistor.
You can add a small amount for the damaged section.
This process works very well for damaged wire-wound resistors. They can
be pulled apart and each section of the resistance-wire (nichrome wire)
measured and added to get the full resistance.
There is another way to determine the value of a damaged resistor.
Get a set of resistors of the same wattage as the damaged component and start with a high value.
It's handy to know if the resistor is in the range: 10ohm to 100ohms or
1k to 10k etc, but this is not essential.
Start with a very high value and turn the circuit ON. You can perform
voltage tests and if you know the expected output voltage, decrease the
resistance until this voltage is obtained.
If you do not know the expected voltage, keep reducing the value of
resistance until the circuit works as designed.
This is the best advice in a situation where you do not know the value
of a resistor.
There is a third way to determine the value and this requires measuring
the voltage drop across the resistor and the current-flow. By
multiplying the two you will get a wattage and this must be less than
the wattage of the resistor being replaced.
To check the
value of a variable resistor, it should be removed from circuit or at
least 2 legs should be removed. A Rheostat is a variable resistor using
only one end and the middle connected to a circuit.
The resistance between the two outside pins is the value marked on the
component and the centre leg will change from nearly zero to the full
resistance as the shaft is rotated.
"Pots" generally suffer from "crackle" when turned and this can be fixed
by spraying up the shaft and into the pot via the shaft with a tube fixed to a can of "spray-lubricant"
"Pre-set pots" and "trim pots" are miniature versions of a
potentiometer and they are all tested the same.
Focus pots quite
often get a spot of dirt where the wiper touches the track. Cleaning
with spray fixes the bad focus but if the pot is leaking to chassis from
inside the pot (due to the high voltage on the terminals) simply remove
it from the chassis and leave it floating (this will restore the high
voltage to the picture tube) or you can use one from an old chassis.
MAKING YOUR OWN
RESISTOR, CAPACITOR, INDUCTOR or DIODE
Quite often you
will not have the
exact value of resistance or
capacitance for a repair.
We have already covered placing resistors and capacitors in parallel and
Resistors in Parallel
Capacitors in Parallel
Here are some extras:
Two 1k 0.5watt resistors in parallel produces a 470R 1watt resistor.
Two 1k 0.5watt resistors in series produces a 2k 1watt resistor.
Two 100n 100v capacitors in series produces a 50n capacitor @200v
inductors in series - ADD THE VALUES
DIODES: Two 1Amp 400v diodes in series produces a
1Amp 800v diode
Two 1Amp 400v diodes in parallel produces a 2Amp 400v diode
ZENER DIODES: Zener diodes can be connected in series to
get a higher voltage.
Two 12v zener diodes in series produces a 24v zener.
Some multimeters have a "buzzer" that detects when the probes are touching
each other or
the resistance between the probes is very LOW. This is called a
You can use the resistance scale "x1" or "x10" to detect low values of
Set the pointer to "0" (right end of the scale) by touching the probes
together and adjusting the "zero ohms" control.
When taking a reading, you will have to decide if a low value of resistance is a short-circuit
or an "operating value."
For instance, the cold resistance of a 12v car globe is very low (about
2 ohms) and it increases (about 6 times) to 12 ohms when hot.
The "resistance of a circuit" may be very low as the electrolytics in
the circuit are uncharged. This may not indicate a true "short-circuit."
The measurement across a diode is not a resistance-value but a
"voltage-drop" and that is why the needle swings nearly full-scale.
Leads and wires and cords have a small resistance and depending on the length of the lead,
this small resistance may be affecting a circuit.
When a circuit takes 1 amp, and the resistance of the leads is 1 ohm,
the voltage drop across the leads will be 1v.
That's why a 12v battery supplying a circuit with these leads will have 11v
Turn off the equipment before making any continuity tests. The presence
of even a small voltage (from an electrolytic) can give a false reading.
You can determine the resistance of a lead very accurately by taking the
example above and applying it to your circuit.
If the battery is 12.6v and the voltage across the circuit is 10v, when
the current is 2.6 amps, the resistance of the "leads" is 12.6 -
10 = 2.6 R=V/I = 2.6/2.6 = 1ohm. By making the lead shorter or using
thicker wire, the resistance will be less and the voltage on the project
When taking readings in a circuit that has a number of diodes built-into
IC's (Integrated Circuits) and transistors, some Continuity Testers will beep and give a
The following circuit has the advantage of providing a beep when a short-circuit is
detected but does not detect the small voltage drop across a diode. This
is ideal when testing logic circuits as it is quick and you can listen
for the beep while concentrating on the probe. Using a multimeter is
You can build the circuit on Matrix
Board and add it to your Test Equipment.
You will need lots of "Test Equipment" and they can be built from
circuits in this eBook.
TESTING FUSES, LEADS AND WIRES
these components come under the heading TESTING for CONTINUITY. Turn off all
power to the equipment before testing for shorts and continuity. Use the
low resistance "Ohms Scale" or CONTINUITY range on your multimeter. All
fuses, leads and wires should have a low, very low or zero resistance. This
proves they are working.
A BLOWN FUSE
The appearance of a fuse after it has "blown" can tell you a lot about the fault in the
If the inside of the glass tube (of the fuse) is totally blackened, the
fuse has been damaged very quickly. This indicates a very high current
has passed through the fuse.
Depending on the rating of the fuse, (current rating)
you will be able to look for components that can pass a high current when
damaged - such as high power transistors, FETs, coils, electrolytics.
Before re-connecting the supply, you should test the "SUPPLY RAILS" for
resistance. This is done by measuring them on a low OHMs range in one
direction then reverse the leads to see if the resistance is low in the
A reading can be very low at the start because electrolytics need time
to charge-up and if the reading gradually increases, the power rail does
not have a short. An overload can occur when the supply voltage rises to
nearly full voltage, so you sometimes have to fit a fuse and see how
long it takes to "blow."
If the fuse is just slightly damaged, you will need to read the next
part of this eBook, to see how and why this happens:
FAST AND SLOW BLOW FUSES
There are many different sizes, shapes and ratings of a fuse. They are
all current ratings as a fuse does not have a voltage rating. Some fuses
are designed for cars as they fit into the special fuse holders. A fuse
can be designed for 50mA, 100mA, 250mA, 315mA, 500mA, 1Amp, 1.5amp,
2amp, 3amp, 3.15amp 5amp, 10amp, 15amp, 20amp, 25amp, 30amp,
35amp, 50amp and higher.
Some fuses are fast-blow and some are slow-blow.
A "normal" fuse consists of a length of thin wire. Or it may be a loop
of wire that is thin near the middle of the fuse. This is the section
that will "burn-out."
A "normal" fuse is a fast-blow fuse. For instance, a 1amp fuse will
remain intact when up to 1.25 amp flows. When a circuit is turned on, it
may take 2-3 amps for a very short period of time and a normal 1 amp
fuse will get very hot and the wire will stretch but not "burn-out." You
can see the wire move when the supply turns on.
If the current increases to 2amps, the fuse will still remain intact. It
needs about 3 amp to heat up the wire to red-hot and burn out.
If the current increases to 5 amp, the wire VOLATILISES (burns-out) and
deposits carbon-black on the inside of the glass tube.
A slow-blow fuse uses a slightly thicker piece of wire and the fuse is
made of two pieces of wire joined in the middle with a dob of
low-temperature solder. Sometimes one of the pieces of wire is a spring
and when the current rises to 2.5 amp, the heat generated in the wire
melts the solder and the two pieces of wire "spring apart."
A slow-blow fuse will allow a higher current-surge to pass through the
fuse and the wire will not heat up and sag.
Thus the fuse is not gradually being damaged and it will remain in a
perfect state for a long period of time.
A fuse does not protect electronic equipment from failing. It acts AFTER
the equipment has failed.
It will then protect a power supply from delivering a high current to a
circuit that has failed.
If a slow-blow fuse has melted the solder, it could be due to a slight
overload, slight weakening of the fuse over a period of time or the
current-rating may be too low.
You can try another fuse to see what happens.
You can replace a fast-acting fuse (normal fuse) with a slow blow if the fast-acting
fuse has been replaced a few times due to deterioration when the
equipment is turned on.
But you cannot replace a slow-blow fuse with a fast acting fuse as it
will be damaged slightly each time the equipment is turned on and
Fuses below about 100mA are very hard to make and very unreliable.
Many circuits take a high current when turned to charge the
electrolytics and a 100mA (or 50mA or 63mA fuse) will bow and stretch
and change shape, every time the equipment is turned ON.
Eventually it will break, due to it heating-up and stretching.
To produce a reliable fuse below 100mA, some manufacturers have placed a
resistor inside the fuse and connected it to a spring. One end of the
resistor is soldered to a wire with low-temperature metal and when the
resistor gets hot, the metal softens and the spring pulls the resistor
away from the wire.
Quite often you can heat up the metal and connect the wire and the fuse
This type of fuse is called a DELAY fuse and the current rating is shown
on the end-cap.
The value of the resistor determines the current rating.
There is a small voltage across this type of fuse and it means the
circuit sees a slightly lower voltage than the supply voltage.
The third photo shows the pot of solder or low-temp metal and a wire
connected to a spring. The heat generated in the wire is passed to the
solder and it softens. The spring pulls the two components apart. You
can smash the glass and set up the fuse in the two fuse-holders and
repair the fuse while you wait for a new fuse to arrive.
COILS, INDUCTORS and YOKES
Basically, an inductor NEVER prevents a fuse blowing
because an inductor prevents spikes on one lead (we will call the INPUT
lead), appearing on its other
Coils, inductors, chokes and yokes are just
coils (turns) of wire. The
wire may be wrapped around a core made of iron or ferrite.
It is labeled "L" on a circuit board.
You can test this component for continuity between the ends of the
winding and also make sure there is no continuity between the winding
and the core.
The winding can be less than one ohm, or greater than 100 ohms. A coil of wire is also called an INDUCTOR and it might look like a very
simple component, but it can operate in a very complex way.
The way it works is a discussion for another eBook. It is important to
understand the turns are insulated but a slight fracture in the
insulation can cause two turns to touch each other and this is called a
"SHORTED TURN" or you can say the inductor has "SHORTED TURNS."
When this happens, the inductor allows the circuit to draw MORE CURRENT.
This causes the fuse to "blow."
The quickest way to check an inductor is to replace it, but if you want
to measure the inductance, you can use an INDUCTANCE METER. You can then
compare the inductance with a known good component.
An inductor with a shorted turn will have a very low or zero inductance,
however you may not be able to detect the fault when it is not working
in a circuit as the fault may be created by a high voltage generated
between two of the turns.
Faulty yokes (both horizontal and vertical windings) can cause the
picture to reduce in size and/or bend or produce a single horizontal
A TV or monitor screen is the best piece of Test Equipment as it has
identified the fault. It is pointless trying to test the windings
further as you will not be able to test them under full operating
The fault may not show up when a low voltage (test voltage) is applied.
MEASURING AND TESTING INDUCTORS
Inductors are measured with an INDUCTANCE METER
but the value of some inductors is very small and some Inductance Meters
do not give an accurate reading.
The solution is to measure a larger inductor and note the reading. Now
put the two inductors in SERIES and the values ADD UP - just like
resistors in SERIES. This way you can measure very small inductors.
Question from a reader: Can I add an inductor to stop a fuse blowing?
This is the detection and prevention of current that exists for a very
short period of time.
A fuse detects an excess of current that occurs over a very long period
of time and they are entirely two different "detectors."
One cannot assist the other in any way.
TESTING SWITCHES and
Switches and relays
have contacts that open and close mechanically and you can test them for
CONTINUITY. However these components can become intermittent due to dirt
or pitting of the surface of the contacts due to arcing as the switch is
It is best to test these items when the operating voltage and current is
present as they quite often fail due to the arcing. A switch can work 49
times then fail on each 50th operation. The same with a relay. It can
fail one time in 50 due to CONTACT WEAR.
If the contacts do not touch each other with a large amount of force and
with a large amount of the metal touching, the current flowing through
the contacts will create HEAT and this will damage the metal and
sometimes reduce the pressure holding the contact together.
This causes more arcing and eventually the switch heats up and starts to
burn. Switches are the biggest causes of fire in electrical equipment
A relay also has a set of contacts that can cause
are many different types of relays and basically they can be put into
An electromagnetic relay is a switch operated by magnetic
force. This force is generated by current through a
coil. The relay opens and closes a
set of contacts.
The contacts allow a current to flow and this current can damage the
contacts. Connect 5v or 12v to the coil (or 24v) and listen for the
"click" of the points closing. Measure the resistance across the points
to see if they are closing.
You really need to put a load on the points to see if they are clean and
can carry a current.
The coil will work in either direction.
If not, the relay is possibly a CMOS relay or Solid State relay.
2. An electronic relay (Solid State Relay) does not have a
winding. It works on the principle of an opto-coupler and uses a LED and
Light Activated SCR or Opto-TRIAC to produce a low resistance on the
output. The two
pins that energise the relay (the two input pins) must be connected to 5v (or 12v) around the
correct way as the voltage is driving a LED (with series resistor). The
LED illuminates and activates a light-sensitive device.
Capacitors are one of the most difficult things to test.
because they don't give a reading on a multimeter and their value can
range from 1p to 100,000u.
A faulty capacitor may be "open" when measured with a multimeter,
good capacitor will also be "open."
You need a piece of test equipment called a CAPACITANCE METER to measure
the value of a capacitor.
HOW A CAPACITOR WORKS
There are two ways to describe how a capacitor works. Both are correct
and you have to combine them to get a full picture.
A capacitor has INFINITE resistance between one lead and the other.
This means no current flows through a capacitor. But it works in
Suppose you have a strong magnet on one side of a door and a piece of
metal on the other. By sliding the magnet up and down the door, the
metal rises and falls.
The metal can be connected to a pump and you can pump water by sliding
the magnet up and down.
A capacitor works in exactly the same way.
If you raise a voltage on one lead of a capacitor, the other lead will rise to the same voltage. This
needs more explaining - we are keeping the discussion simple.
It works just like the magnetic field of the magnet through a door.
The next concept is this:
equivalent to a tiny rechargeable battery.
They store energy when the supply-voltage is present and release it when
the supply drops.
These two concepts can be used in many ways and that's why capacitors
perform tasks such as filtering, time-delays, passing a
signal from one stage to another and
create many different effects in a circuit.
The basic unit of capacitance is the FARAD. (C)
This is the value used
in all equations, but it is a very large value. A one FARAD capacitor
would be the size of a car if made with plates and paper. Most
electronic circuits use capacitors with smaller values such as
1p to 1,000u.
1p is about equal to
two parallel wires 2cm long. 1p is one picofarad.
The easiest way to understand capacitor values is to start with a value
of 1u. This is one microfarad and is one-millionth of a Farad. A 1
microfarad capacitor is about 1cm long and the diagram shows a 1u
Smaller capacitors are ceramic and they look like the following. This is
a 100n ceramic:
To read the value on a capacitor you need to know a few facts.
The basic value of capacitance is the FARAD.
1 microfarad is one millionth of 1 farad.
1 microfarad is divided into smaller parts called nanofarad.
1,000 nanofarad = 1 microfarad
Nanofarad is divided into small parts called picofarad
1,000 picofarad = 1 nanofarad.
1p = 1 picofarad. 1,000p = 1n ( 1 nanofarad)
1,000n = 1u (1 microfarad)
1,000u = 1millifarad
1,000,000u = 1 FARAD.
All ceramic capacitors are marked in "p" (puff")
A ceramic with 22 is 22p = 22 picofarad
A ceramic with 47 is 47p = 47 picofarad
A ceramic with 470 is 470p = 470 picofarad
A ceramic with 471 is 470p = 470 picofarad
A ceramic with 102 is 1,000p = 1n
A ceramic with 223 is 22,000p = 22n
A ceramic with 104 is 100,000p = 100n = 0.1u
TYPES OF CAPACITOR
For testing purposes, there are two types of capacitor.
Capacitors from 1p to 100n are non-polar and can be inserted into a
circuit around either way.
Capacitors from 1u to 100,000u are electrolytics and are polarised.
They must be fitted so the positive lead goes to the supply voltage and
the negative lead goes to ground (or earth).
There are many different sizes, shapes and types of capacitor. They are
all the same. They consist of two plates with an insulating material
between. The two plates can be stacked in layers or rolled together.
The important factor is the insulating material. It must be very thin to
keep things small. This gives the capacitor its VOLTAGE RATING.
If a capacitor sees a voltage higher than its rating, the voltage will
"jump through" the insulating material or around it.
If this happens, a carbon deposit is left behind and the capacitor
becomes "leaky" or very low resistance, as carbon is conductive.
Nearly all small capacitors are ceramic capacitors as this
material is cheap and the capacitor can be made in very thin layers to
produced a high capacitance for the size of the component. This is
especially true for surface-mount capacitors.
All capacitors are marked with a value and the basic unit is: "p" for
"puff" However NO surface mount capacitors are marked and they are
very difficult to test.
||VALUE WRITTEN ON
10 or 10p
22 or 22p
47 or 47p
56 or 56p
100 on 101
220 or 221
470 or 471
560 or 561
820 or 821
POLYSTYRENE, MYLAR, METALLISED POLYESTER, ("POLY"), MICA and other types
There are many
types of capacitor and they are chosen for their reliability, stability,
temperate-range and cost.
For testing and repair work, they are all the same. Simply replace with
exactly the same type and value.
Capacitor Colour Code Table
(T) > 10pf
(T) < 10pf
ELECTROLYTIC and TANTALUM CAPACITORS
Electrolytics and Tantalums are the same for testing purposes but their
performance is slightly different in some circuits. A tantalum is
smaller for the same rating as an electrolytic and has a better ability
at delivering a current. They are available up to about 1,000u, at about
50v but their cost is much higher than an electrolytic.
Electrolytics are available in 1u, 2u2 3u3 4u7 10u, 22u, 47u, 100u,
220u, 330u, 470u, 1,000u, 2,200u, 3,300u, 4,700u, 10,000u and higher.
The "voltage" or "working voltage" can be: 3.3v, 10v,
16v, 25v, 63v, 100v, 200v and higher.
There is also another important factor that is rarely covered in text
books. It is RIPPLE FACTOR.
This is the amount of current that can enter and leave an electrolytic. This current heats up the electrolytic and that
is why some
electrolytics are much larger than others, even though the capacitance
and voltage-ratings are the same.
If you replace an electrolytic with a "miniature" version, it will heat
up and have a very short life. This is especially important in power
supplies where current (energy) is constantly entering and exiting the
electrolytic as its main purpose is to provide a smooth output from a
set of diodes that delivers "pulsing DC." (see "Power Diodes")
NON-POLAR CAPACITORS (ELECTROLYTICS)
Electrolytics are also available in non-polar values.
It sometimes has the letters "NP" on the component. Sometimes the leads
are not identified.
This is an electrolytic that does not have a positive and negative lead
but two leads and either lead can be connected to the positive or
negative of the circuit.
These electrolytics are usually connected to the output of an amplifier
(such as in a filter near the speaker) where the
signal is rising and falling.
A non-polar electrolytic can be created from two ordinary electrolytics
by connecting the negative leads together and the two positive leads
become the new leads.
For example: two 100u 63v electrolytics will produce a 47u 63v
In the circuit below, the non-polar capacitor is replaced with two
PARALLEL and SERIES CAPACITORS
Capacitors can be connected in PARALLEL and/or SERIES for a number of reasons.
1. If you do not have the exact value, two or more connected in parallel
or series can produce the value you need.
2. Capacitors connected in series will produce one with a higher voltage
3. Capacitors connected in parallel will produce a larger-value
Here are examples of two equal capacitors connected in series or
parallel and the results they produce:
VOLTAGE RATING OF CAPACITOR
Capacitors have a voltage rating, stated as WV for working
voltage, or WVDC. This specifies the maximum voltage that can be
applied across the capacitor without puncturing the dielectric. Voltage
ratings for "poly," mica and ceramic capacitors are
typically 50v to 500 VDC. Ceramic capacitors with ratings of 1kv to 5kv
are also available. Electrolytic capacitors are commonly available in
6v, 10v 16v, 25v, 50v, 100v, 150v, and 450v ratings.
THE SIZE OF A CAPACITOR - RIPPLE FACTOR
The size of a capacitor depends on a number of factors, namely the value
of the capacitor (in microfarads etc) and the voltage rating. But there
is also another factor that is most important. It is the RIPPLE FACTOR.
Ripple Factor is the amount of voltage-fluctuation the capacitor
(electrolytic) can withstand without getting too hot.
When current flows in and out of an electrolytic, it gets hot and this
will eventually dry-out the capacitor as some of the liquid inside the
capacitor escapes through the seal. It's a very slow process but over a
period of years, the capacitor looses its capacitance.
If you have two identical 1,000u 35v electrolytics and one is
smaller, it will get hotter when operating in a circuit and that's why
it is necessary to choose the largest electrolytic.
If a capacitor has a voltage rating of 63v, do not put it in a 100v circuit
as the insulation (called
the dielectric) will be punctured and the capacitor will
"short-circuit." It's ok to replace a 0.22uF 50WV capacitor with 0.22uF 250WVDC.
A capacitor can store a charge for a period of time after the equipment is
turned off. High voltage electrolytic caps can pose a safety hazard. These capacitors are in power
supplies and some have a resistor across them, called
a bleed resistor, to discharge the cap after power is switched off.
a bleed resistor is not present the cap can retain a
charge after the equipment is unplugged.
How to discharge a capacitor
Do not use a screwdriver to short between the terminals as this will
damage the capacitor internally and the screwdriver.
Use a 1k 1 watt or 3watt or 5watt resistor on jumper leads (or held with
pliers) and keep them connected
for up to 15 seconds to fully discharge the electro.
Test it with a voltmeter to make sure all the energy has been removed.
Before testing any capacitors, especially electrolytics, you should look
to see if any are damaged, overheated or leaking. Swelling at the top of
an electrolytic indicates heating (and pressure inside the case) and will
result in drying out of the electrolyte. Any hot or warm electrolytic
indicates leakage and ceramic capacitors with portions missing indicates
something has gone wrong (such as it being "blown apart").
Here is a 120u 330v electrolytic from a flash circuit in an
old-fashioned film camera.
If the flash does not "fire," the electrolytic will be charged to about
350 volts !!
Use a 1k resistor (held with pliers) to slowly discharge it. It may take
15 seconds to fully discharge.
TESTING A CAPACITOR
There are two things you
can test with a multimeter:
1. A short-circuit within the capacitor
2. Capacitor values above 1u.
You can test capacitors in-circuit for short-circuits. Use the x1 ohms
To test a capacitor for leakage, you need to remove it or at least one lead
must be removed. Use the x10k range on an analogue or digital
For values above 1u you can determine if the capacitor is charging by
using an analogue meter. The needle will initially move across the scale
to indicate the cap is charging, then go to "no deflection."
Any permanent deflection of the needle will indicate leakage.
can reverse the probes to see if the needle moves in the opposite
This indicates it has been charged. Values below 1u will not respond to charging and the needle will not
This does not work with a digital meter as the
resistance range does not output any current and the electrolytic does
Rather than spending money on a capacitance meter, it is cheaper to
replace any suspect capacitor or electrolytic.
Capacitors can produce very unusual faults and no piece of test
equipment is going to detect the problem.
In most cases, it is a simple matter to solder another capacitor across
the suspect component and view or listen to the result.
This saves all the worry of removing the component and testing it with
equipment that cannot possibly give you an accurate reading when the
full voltage and current is not present.
It is complete madness to even think of testing
critical components such as capacitors, with TEST EQUIPMENT.
You are fooling yourself. If the Test
Equipment says the component is ok, you will look somewhere else and
waste a lot of time.
FINDING THE VALUE OF A
If you want to find the value of a surface-mount capacitor or one where
the markings have been removed, you will need a CAPACITANCE METER. Here
is a simple circuit that can be added to your meter to read capacitor
values from 10p to 10u.
The full article can be found
ADD-ON CAPACITANCE METER
You can get a kit or a ready-made piece of test gear called
CAPACITOR SUBSTITUTION BOX and also RESISTOR SUBSTITUTION
I bought one of each 30 years ago and I have only used them ONCE.
They appear to be very handy but when you are testing a circuit, you
want the component next to the other parts.
It is just as easy to pick the component you need from your junk box and
connect it to the circuit via jumper leads.
CAPACITOR taken years to
select the right type of capacitor due to previous failures.
Always replace a capacitor
with the exact same type.
A capacitor may be slightly important in a circuit or it might be
A manufacturer may have
A capacitor just doesn't have a "value of capacitance."
It may also has an effect called "tightening of the rails."
In other words, a capacitor has the ability to react quickly and either
absorb or deliver energy to prevent spikes or fluctuations on the rail.
This is due to the way it is constructed. Some capacitors are simply
plates of metal film while others are wound in a coil. Some capacitors
are large while others are small.
They all react differently when the voltage fluctuates.
Not only this, but some capacitors are very stable and all these
features go into the decision for the type of capacitor to use.
You can completely destroy the operation of a circuit by selecting the
wrong type of capacitor.
No capacitor is perfect and when it gets charged or discharged, it
appears to have a small value of resistance in series with the value of
capacitance. This is known as "ESR" and stands for EQUIVALENT SERIES
RESISTANCE. This effectively makes the capacitor slightly slower to
charge and discharge.
We cannot go into the theory on selecting a capacitor as it would be
larger than this eBook so the only solution is to replace a capacitor
with an identical type.
However if you get more than one repair with identical faults, you
should ask other technicians if the original capacitor comes from a
The author has fixed TV's and fax machines where the capacitors have
been inferior and alternate types have solved the problem.
Some capacitor are suitable for high frequencies, others for low
A Decoupling Capacitor can severe one, two or three functions. You need
to think of a decoupling capacitor as a miniature battery with the
ability to deliver a brief pulse of energy when ever the line-voltage
drops and also absorb a brief pulse of energy when ever the line voltage
rises (or spikes).
Decoupling capacitor can range from 100n to 1,000u.
100n capacitors are designed to absorb spikes and also have the effect
of tightening-up the rails for high frequencies. They have no effect on
low frequencies such as audio frequencies.
These capacitors are generally ceramic and have very low internal
impedance and thus they can operate at high frequencies.
Capacitors above about 10u are used for decoupling and these are nearly
Decoupling means "tightening-up the power rails." The electrolytic acts
just like a miniature rechargeable battery, supplying a small number of
components in a circuit with a smooth and stable voltage.
The electrolytic is usually fed from a dropper resistor and this
resistor charges the electrolytic and adds to the ability of the
electrolytic to create a "separate power supply."
These two components help remove spikes as an electrolytic cannot remove
spikes if connected directly to the supply rails - it's internal
impedance is high and the spikes are not absorbed.
Decoupling capacitors are very difficult to test.
They rarely fail but if a project is suffering from unknown glitches and
spikes, it is best to simply add more 100n decoupling caps on the
underside of the board and replace all electrolytics.
Some small electrolytics will dry out due to faulty manufacture and
simply replacing every one on a board will solve the problem.
Some of the functions of a decoupling capacitor are:
Removing ripple - hum or buzz in the background of an amplifier
Removing glitches or spikes.
Separating one stage from another to reduce or remove MOTORBOATING - a
low frequency sound due to the output putting a pulse on the power rails
that is picked up by the pre-amplifier section and amplified.
Diodes can have 4 different faults.
1. Open circuit in both directions.
2. Low resistance in both directions.
4. Breakdown under load.
TESTING A DIODE ON
AN ANALOGUE METER
1. When the
diode is measured in one direction, the needle will not move at all.
The technical term for this is the diode is reverse biased. It
will not allow any current to flow. Thus the needle will not move.
Testing a diode with an Analogue Multimeter can be done on any of
the resistance ranges. [The high resistance range is best - it sometimes
has a high voltage battery for this range but this does not affect our
There are two things you must remember.
When the diode is connected around the other way, the needle will swing
to the right (move up scale) to about 80% of the scale. This position
represents the voltage drop across the junction of the diode and is NOT
a resistance value. If you change the resistance range, the needle will
move to a slightly different position due to the resistances inside the
meter. The technical term for this is the diode is forward biased.
This indicates the diode is not faulty.
The needle will swing to a slightly different position for a "normal
diode" compared to a Schottky diode. This is due to the different
junction voltage drops.
However we are only testing the diode at very low voltage and it may
break-down when fitted to a circuit due to a higher voltage being
present or due to a high current flowing.
The leads of an Analogue Multimeter have the positive of the
battery connected to the black probe and the readings of a "good diode" are
shown in the following two diagrams:
The diode is
REVERSE BIASED in the
diagram above and diodes not conduct.
The diode is
FORWARD BIASED in the
diagram above and it conducts
TESTING A DIODE ON
A DIGITAL METER
Testing a diode with a Digital Meter must be done on the "DIODE" setting
as a digital meter does not deliver a current through the probes on some
of the resistance settings and will not produce an accurate reading.
The best thing to do with a "suspect" diode is to replace it. This is because a diode
has a number of characteristics that cannot be tested with simple
equipment. Some diodes have a fast recovery for use in high frequency
circuits. They conduct very quickly and turn off very quickly so
the waveform is processed accurately and efficiently.
If the diode is replaced with an ordinary diode, it will heat up as does
not have the high-speed characteristic.
Other diodes have a low drop across them and if an ordinary is used, it
will heat up.
Most diodes fail by going: SHORT-CIRCUIT. This can be detected by
a low resistance (x1 or x10 Ohms range) in both directions.
A diode can also go OPEN CIRCUIT. To locate this fault, place an
identical diode across the diode being tested.
A leaky diode can be detected by a low reading in one direction and a
slight reading the other direction.
However this type of fault can only be detected when the circuit is
working. The output of the circuit will be low and sometimes the diode
heats up (more than normal).
A diode can go open under full load conditions and perform
Diodes come in pairs in surface-mount packages and 4 diodes can be found in
They are also available in pairs that look like a 3-leaded transistor.
The line on the end of the body of a diode indicates the cathode and you
cannot say "this is the positive lead." The correct way to
describe the leads is to say the "cathode lead." The other lead is
the anode. The cathode is defined as the electrode (or lead) through
which an electric current flows out of a device.
The following diagrams show different types of diodes:
To understand how a power diode works, we need to describe a few things.
This has NEVER been described before, so read carefully.
The 240v AC (called the "mains") consists of two wires, one is called the ACTIVE and the
other is NEUTRAL. Suppose you touch both wires. You will get a shock. The neutral is connected
to an earth wire (or rod driven into the ground or connected to a water
pipe) at the point where the electricity enters the premises
and you do not get a shock from the NEUTRAL.
But the voltage on the active is rising to +345v then goes to -345v at
the rate of 50 times per second (for a complete cycle).
345v is the peak voltage of 240v. You never get a 240v shock. (It is a
In other words, if you touch the two wires at a particular instant, you
would get a POSITIVE 345v shock and at another instant you would get a
negative 345v shock. This is shown in the diagram below.
We now transfer this concept to the output of a transformer. The
diagram shows an AC waveform on the output of the secondary.
This voltage is rising 15v higher than the bottom lead then
it is 15v LOWER
than the bottom lead. The bottom lead is called "zero volts."
You have to say one lead or wire is not "rising and falling" as you
need a "reference" or starting-point" or "zero point" for
The diode only conducts when the voltage is "above zero"
(actually when it is 0.7v above zero) and does not
conduct (at all) when the voltage goes below zero.
This is shown on the output of the Power Diode. Only
the positive peaks or the positive parts of the waveform appear on the
output and this is called "pulsing DC." This is called
"half-wave" and is not used in a power supply. We have used it to
describe how the diode works. The electrolytics charge
during the peaks and deliver energy when the diode is not delivering
current. This is how the output becomes a steady DC voltage.
Power supplies use FULL WAVE rectification and the other half of the AC
waveform is delivered to the output (and fills in the "gaps") and appears as shown in "A."
ONE FAULTY DIODE
One diode in a bridge can go open (any of the 4 diodes
will produce the same effect) and produce an output voltage that can be
slightly lower than the original voltage. The actual "voltage-drop" will depend
on the current taken by the circuit and the ability of the transformer
to produce the required voltage and current during half-wave operation.
The voltage during each half cycle (when none of the diodes is
delivering any energy to the circuit) is maintained by the electrolytic and
its size (relative to the current taken by the
circuit) will determine the size of the ripple that will result when the diode
fails. The ripple will be 100 to 1,000 times greater after the failure
of a diode, depending on the value of the filter capacitor.
To locate the faulty diode, simply get a diode and place it across each of the diodes in the bridge
(in turn) when the circuit is working.
For a bridge rectifier, the ripple-frequency will be twice the mains
frequency and its ripple will be very small if the electrolytic is
the correct value. When a diode fails, the ripple-frequency will be
equal to mains-frequency and the amplitude will increase
considerably. You may even hear background hum from audio equipment.
If you cannot find a faulty diode, the filter capacitor will be at
fault. Turn off the equipment and connect an electrolytic across the
filter capacitor via jumper leads. Turn the power ON and see if the hum
A damper diode is a diode that detects a high voltage and SQUELCHES IT
(reduces it - removes it). The signal that it squelches is a voltage that is in the
opposite direction to the "supply voltage" and is produced by the
collapsing of a magnetic field. Whenever a magnetic filed collapses, it
produces a voltage in the winding that is opposite to the supply voltage
and can be much higher. This is the principle of a flyback circuit or EHT circuit. The high voltage comes from the transformer.
The diode is placed so that the signal passes through it and less
than 0.5v appears across it.
A damper diode can be placed across the coil of a relay, incorporated
into a transistor or FET or placed across a winding of a flyback
transformer to protect the driving transistor or FET.
It can also be called a "Reverse-Voltage Protection Diode," "Spike
Suppression Diode," or "Voltage Clamp Diode."
The main characteristic of a Damper Diode is HIGH SPEED so it can detect
the spike and absorb the energy.
It does not have to be a high-voltage diode as the high voltage in the
circuit is being absorbed by the diode.
SILICON, GERMANIUM AND SCHOTTKY DIODES
When testing a diode with an analogue meter, you will get a low reading
in one direction and a high (or NO READING) in the other direction. When
reading in the LOW direction, the needle will swing nearly full scale
and the reading is not a resistance-value but a reflection of the
characteristic voltage drop across the junction of the diode. As we
mentioned before, a resistance reading is really a voltage reading and
the meter is measuring the voltage of the battery minus the voltage-drop
across the diode.
Since Silicon, Germanium and Schottky Diodes have slightly different
characteristic voltage drops across the junction, you will get a
slightly different reading on the scale. This does not represent one
diode being better than the other or capable of handling a higher
current or any other feature.
The quickest, easiest and cheapest way to find, fix and solve a problem
caused by a faulty diode is to replace it.
There is no piece of test equipment capable of testing a diode fully,
and the circuit you are working on is actually the best piece of test
equipment as it is identifying the fault UNDER LOAD.
Only very simple tests can be done with a multimeter and it is best to check a diode with an ANALOGUE MULTIMETER as it outputs a
higher current though the diode and produces a more-reliable result.
A Digital meter can produce false readings
as it does not apply enough current to activate the junction.
Fortunately almost every digital multimeter has a diode
Using this, a silicon diode should read a voltage drop between 0.5v to
0.8v in the forward direction and open in the reverse direction. For a
germanium diode, the reading will be lower, around 0.2v - 0.4v in the
forward direction. A bad diode will read zero volts in both directions.
REPLACING A DIODE
alway best to replace a diode with the same type but quite often this is
not possible. Many diodes have unusual markings or colours or "in-house"
This is only a general guide because many diodes have special features,
especially when used in high-frequency circuits.
However if you are desperate to get a piece of equipment working, here
are the steps:
Determine if the diode is a signal diode, power diode, or zener
For a signal diode, try 1N4148.
For a power diode (1 amp) try 1N4004. (for up to 400v)
For a power diode (3 amp) try 1N5404. (for up to 400v)
For a high-speed diode, try UF4004 (for up to 400v)
If you put an ordinary diode in a high-speed application, it will get
very hot very quickly.
To replace an unknown zener diode, start with a low voltage such as 6v2
and see if the circuit works.
The size of a diode and the thickness of the leads will give an idea of
the current-capability of the diode.
Keep the leads short as the PC board acts as a heat-sink.
You can also add fins to the leads to keep the diode cool.
EMITTING DIODES (LEDs)
Light Emitting Diodes (LEDs) are diodes that produce light when current
flows from anode to cathode. The LED does not emit light when it is
revered-biased. It is used as a low current indicator in many
types of consumer and industrial equipment, such as monitors, TV’s,
printers, hi-fi systems, machinery and control panels.
The light produced by a LED can be visible, such as red, green, yellow
or white. It can also be invisible and these LEDs are called Infrared
LEDs. They are used in remote controls and to see if they are working,
you need to point a digital camera at the LED and view the picture on
the camera screen.
An LED needs about 2v - 3.6v across its leads to make it emit light, but this
voltage must be exact for the type and colour of the LED. The
simplest way to deliver the exact voltage is to have a supply that is
higher than needed and include a voltage-dropping resistor. The value of
the resistor must be selected so the current is between 2mA and 25mA.
The cathode of the LED is identified by a flat on the side of the LED. The
life expectancy of a LED is about 100,000 hours. LEDs rarely fail but
they are very sensitive to heat and they must be soldered and
de-soldered quickly. They are one of the most heat-sensitive components.
Light emitting diodes cannot be tested with most multimeters because the
characteristic voltage across them is higher than the voltage of the
battery in the meter.
However a simple tester can be made by joining 3 cells together with a
220R resistor and 2 alligator clips:
Connect the clips to a LED and it will illuminate in only one direction.
The colour of the LED will determine the voltage across it. You can
measure this voltage if you want to match two or more LEDs for identical
Red LEDs are generally 1.7v to 1.9v. - depending on the quality such as
Green LEDs are 1.9v to 2.3v.
Orange LEDs are about 2.3v and
White LEDs and IR LEDs are about 3.3v to 3.6v.
The illumination produced by a LED is determined by the quality of the
crystal. It is the crystal that produces the colour and you need to
replace a LED with the same quality to achieve the same illumination.
Never connect a LED across a battery (such as 6v or 9v), as it will be
instantly damaged. You must have a resistor in series with the LED to
limit the current.
All diodes are Zener diodes. For instance a 1N4148 is a 120v zener diode
as this is its reverse breakdown voltage.
And a zener diode can be used
as an ordinary diode in a circuit with a voltage that is below the zener value.
For instance, 20v zener diodes can be used in a 12v power supply as the
voltage never reaches 20v, and the zener characteristic is never
Most diodes have a reverse breakdown voltage above 100v, while most
zeners are below 70v. A 24v zener can be created by using two 12v
zeners in series and a normal diode has a characteristic voltage of
0.7v. This can be used to increase the voltage of a zener diode by 0.7v.
See the diagram above. It uses 3 ordinary diodes to increase the output
voltage of a 3-terminal regulator by 2.1v.
To tests a zener diode you need a power supply about 10v higher than the
zener of the diode. Connect the zener across the
supply with a 1k to 4k7 resistor and measure the voltage across the
diode. If it measures less than 1v, reverse the zener.
If the reading is high or low in both directions, the zener is damaged.
Here is a zener diode tester. The circuit will test up to 56v zeners.
ZENER DIODE TESTER
TRANSFORMERLESS POWER SUPPLY
Here's a circuit
that uses zener diodes in a power supply to show how they work.
This clever design uses 4 diodes in a bridge to produce a fixed voltage
power supply capable of supplying 35mA.
If we put 2 zener diodes in a bridge with two ordinary power diodes, the
bridge will break-down at the voltage of the zener. This is what we have
done. If we use 18v zeners, the output will be 17v4.
SUPPLY USING ZENER DIODES
When the incoming voltage is
positive at the top, the left zener provides 18v limit (and the other
zener produces a drop of 0.6v). This allows the right zener to
pass current just like a normal diode. The output is 17v4. The same with the
DETERMINING THE SPECS OF A TRANSFORMER
You cannot use this type of bridge in a normal power supply as the zener
diode will "short" when the input voltage reaches the zener value. The
concept only works in the circuit above.
A Voltage Regulator takes a high input voltage and delivers a fixed
Providing the input voltage is 4v above the output voltage, the
regulator will deliver a fixed output voltage with almost no ripple.
Voltage regulators are also called "3-TERMINAL REGULATORS" or "REGULATOR
IC's" - although this name is not generally used.
In most cases, a voltage regulator gets quite hot and for this reason it
has a high failure-rate.
If a regulator is not getting hot (or warm) it has either failed or the
circuit is not operating.
A regulator can only decrease the voltage. It cannot increase the
current. This means the current being supplied to a circuit must also be
available from the circuit supplying the regulator.
All regulators have different pin-outs, so you need to find the input
pin and output pin and make sure the voltage-difference is at least 4v.
Some regulators will work with a difference as low as 1v, so you need to
read the specifications for the type you are servicing.
Some regulators are called “negative voltage regulators” and the input
voltage will be negative and the output will be negative.
You need to test a voltage regulator with the power "ON".
Make sure you
do not allow the probes to short any of the pins together as this will destroy the regulator
or the circuit being supplied.
With the power turned off or the regulator removed from the circuit, you
can test it with a multimeter set to resistance to see if it is ok. If
any resistance readings are very low or zero ohms, the regulator is
All transformers and coils are tested the same way. This includes
chokes, coils, inductors, yokes, power transformers, EHT transformers (flyback
transformers), switch mode transformers, isolation transformers, IF
transformers, baluns, and any device that has turns of wire around a
former. All these devices can go faulty.
The coating on the wire is called insulation or "enamel" and this can crack or
become overheated or damaged due to vibration or movement. When two
turns touch each other, a very interesting thing happens.
The winding becomes two separate windings.
We will take the case of a single winding such as a coil. This is shown
in the first diagram above and the winding is wound across a
former (a former is a bobbin or plastic molding or something to hold the
winding) and back again, making two layers. The bottom and top layers touch
at the point shown in the diagram and the current that originally passed
though A, B, C, D now passes though A & D.
Winding B C becomes a
separate winding as shown in the second diagram.
In other words the coil becomes a TRANSFORMER with a SHORT CIRCUIT on
the secondary winding as shown in the third diagram.
When the output wires of a transformer are
shorted together, it delivers a very high current because you have created a SHORT-CIRCUIT. This short-circuit causes the transformer
to get very hot.
That’s exactly what happens when any coil or transformer gets a “shorted
The shorted turns can be a single turn or many turns.
It is not possible to measure a fault like this with a multimeter as you don’t
know the exact resistance of a working coil or
winding and the resistance of a faulty winding
may be only 0.001 ohms less.
However when a transformer or coil is measured with an inductance meter,
an oscillating voltage (or spike) is delivered
into the core as magnetic flux,
then the magnetic flux collapses and passes the energy into the winding
to produce a waveform. The inductance meter reads this and
produces a value of inductance in Henry (or milliHenry or microHenry.)
This is done with the transformer removed from the circuit and this can
be a very difficult thing to do, as most transformers have a number of
If the coil or transformer has a shorted turn, the energy from the
magnetic flux will pass into the turns that are shorted and
produce a current. Almost no voltage will be detected from winding.
The reading from the inductance meter will be low or very low and you
have to work out if it is correct.
However there is one major problem with measuring a faulty transformer
It may only become faulty when power is applied.
The voltage between the turns may be sparking or jumping a gap and
creating a problem. A tester is not going to find this fault.
Secondly, an inductance meter may produce a
reading but you do not know if the reading is correct.
An improved tester is a RING TESTER.
The circuit for a ring tester can be found here:
It sends a pulse to the coil and counts the number of returning pulses
A faulty coil (or winding) may return one pulse but nearly all the
energy will be passed to the shorted turns and you will be able to see
this on the scale. You will only get one or two return pulses, whereas a
good winding will return more pulses.
One way to detect a faulty power transformer is to connect it to the
supply and feel the temperature-rise (when nothing is connected to the
It should NOT get hot.
Detecting shorted turns is not easy to diagnose as you really need
another identical component to compare the results.
Most transformers get very hot when a shorted turn has developed.
It may deliver a voltage but the heat generated and a
smell from the transformer will indicate a fault.
An isolation transformer is a piece of Test Equipment that
provides "Mains Voltage" but the voltage is "floating." You will still
get a shock if you touch the two output leads, but it has a special use
when testing unknown equipment.
Many electrical appliances are fully insulated and only have two leads
connected to the mains.
When you take these appliances apart, you do not know which end of say a
heating element is connected to the "live" (active) side of the
mains and which end connects to the neutral.
I am not suggesting you carry out the following tests, but they are
described to show how an isolation transformer works.
If you touch a soldering iron on the "live" (active) end of the heating
element it will create a short-circuit.
However when the appliance is connected to the mains via an isolation
transformer, you can touch an earthed soldering iron on either end of
the heater as
both leads from the isolation transformer are "floating."
As soon as you earth one lead of the output an isolation transformer, the other
lead becomes "active."
You can make your own Isolation Transformer by connecting two identical
The following diagram shows how this is done:
You can use any transformers providing the primary and secondary
voltages are the same. The current capability of the secondary winding
does not matter. However if you want a supply that has almost the same
voltage as your "Mains," you need two transformers with the same
This handy isolation transformer will provide you with "Mains Voltage"
but with a limited current.
In other words it will have a limited capability to supply "wattage." If
you are using two 15VA transformers, you will only be able to test an
appliance rated at 15 watts.
This has some advantages and some disadvantages.
If you are working on a project, and a short-circuit occurs, the damage
will be limited to 15 watts.
If you are using two transformers with different VA ratings, the lower
rating will be the capability of the combination.
If the secondaries are not equal, you will get a higher or lower "Mains
If you get two transformers from TVs or Monitors, with a rating on the compliance
plate of 45 watts, or 90 watts, you can assume the transformers are
capable of delivering this wattage and making an isolation transformer
will enable you to test similar items with the safety of being isolated
from the mains.
Colin Mitchell designs a lot of "LED lighting lamps" that are connected
directly to the mains. He always works with an isolating transformer,
just to be safe. Working on exposed "mains" devices is extremely
nerve-wracking and you have to be very careful.
The isolation transformer will prevent a BIG EXPLOSION.
Suppose you have a "mains transformer" with unknown output voltages and
unknown current capability.
You must be sure it is a mains transformer designed for operation on 50Hz
or 60Hz. Switch-Mode transformers operate at frequencies 40kHz and higher
and are not covered in this discussion.
To be on the safe-side, connect the unknown transformer to the output of
your isolating transformer.
Since the transformer will take almost no current when not loaded, the
output voltages it produces will be fairly accurate. Measure the input
AC voltage and output AC voltage.
If the transformer has loaded your isolating transformer it will be
Mains transformers are approx 15VA for 500gm, 30VA for 1kgm
50VA for 2kgm and and 100VA for 2.5kgm.
VA stands for Volts-Amps and is similar to saying watts. Watts is used
for DC circuits, while VA refers to AC circuits.
Once you have the weight of the transformer and the output voltage, you
can work out the current capability of the secondary.
For transformers up to 30vA, the output voltage on no-load is 30% higher
than the final "loaded voltage."
This is due to the poor regulation of these small devices.
If the transformer is 15VA and the output voltage is 15v AC, the current
will be 1 amp AC.
You can check the "quality" of the transformer, (the regulation) by
fully loading the output and measuring the final voltage. If the
transformer has a number of secondaries, the VA rating must be divided
between all the windings.
240v to 110v ISOLATION TRANSFORMER
Here's how to create a 110v isolating transformer:
Find a 240v:12v transformer.
Now find a transformer that has two secondary windings, such as
Connect the two transformers as shown in the circuit above. If the
output is zero, connect ONE of the 12v windings of the second
transformer around the other way.
110v to 240v ISOLATION TRANSFORMER
A 110v to 240v isolation transformer can be created by connecting 3
identical transformers as shown in the diagram above. If the
output is zero, connect one of the outputs around the other way.
OPTO ISOLATORS and OPTO COUPLERS
Opto Isolators and Opto
Couplers are the same thing. A common opto-coupler is 4N35. It is
allow two circuits to exchange signals yet remain electrically isolated.
The signal is applied to the LED, which shines on a silicon NPN
photo-transistor in the IC.
The light is proportional to the signal, so the signal is transferred to the photo
transistor to turn it on a proportional amount. Opto-couplers can have
Light Activated SCR's, photodiodes, TRIAC's and other semiconductor
as an output. The 4N35 opto-coupler schematic is shown below:
An opto-Coupler using a TRIAC
TESTING AN OPTO COUPLER
Most multimeters cannot test the LED on the input of
an opto-coupler because the ohms range does not have a voltage high
enough to activate the LED with at least 2mA.
You need to set-up the test-circuit shown above with a 1k resistor on the
input and 1k5 on the output. When the 1k is connected to 12v, the output
LED will illuminate.
The opto-coupler should be removed from circuit to perform this test.
Transistors are solid-state devices and although they operate completely
differently to a diode, they appear as two back-to-back diodes when
There are basically 2 types of transistor
A transistor is sometimes referred to as BJT (Bi-polar Junction
Transistor) to distinguish it from other types of transistor such as
Field Effect transistor, Programmable Unijunction Transistor and others.
In the following diagram, two diodes are connected together and although
the construction of a transistor is more complex, we see the transistor
as two diodes when testing it.
A TRANSISTOR APPEARS AS TWO DIODES
WHEN TESTING IT
All transistors have three leads. Base (b), Collector (c), and Emitter (e).
For an NPN transistor, the arrow on the emitter points away from the
It is fortunate that the arrow on both symbols points in the direction of
the flow of current (Conventional Current) and this makes it easy to describe testing methods
using our simplified set of instructions. The symbols have been drawn
exactly as they appear on a circuit diagram.
All transistors are the same but we talk about digital and
analogue transistors. There is no difference between the two.
The difference is the circuit. And the only other slight difference
between transistors is the fact that some have inbuilt diodes and
resistors to simplify the rest of the circuit.
All transistors work the same way. The only difference is the amount of
amplification they provide, the current and voltage they can withstand
and the speed at which they work. For simple testing purposes, they are
all the same.
NPN transistors are the most common and for an NPN transistor, the
(the opposite applies for PNP)
To test a transistor, there is
you have to know:
When the base voltage is higher than the emitter, current flows though
the collector-emitter leads.
As the voltage is increased on the base, nothing happens until the
voltage reaches 0.55v. At this point a very small current flows through
the collector-emitter leads. As the voltage is increased, the
current-flow increases. At about 0.75v, the current-flow is a MAXIMUM.
(can be as high as 0.9v). That's how it works. A transistor also needs
current to flow into the base to perform this amplifying function
and this is the one feature that separates an ordinary transistor from a
If the voltage on the base is 0v, then instantly goes to 0.75v, the
transistor initially passes NO current, then FULL current. The
transistor is said to be working in its two states: OFF then ON
(sometimes called: "cut-off" and "saturation"). These are called digital
states and the transistor is said to be a DIGITAL TRANSISTOR or a
SWITCHING TRANSISTOR , working in DIGITAL MODE.
If the base is delivered 0.5v, then slowly rises to 0.75v and slowly to
0.65v, then 0.7v, then 0.56v etc, the transistor is said to be working
in ANALOGUE MODE and the transistor is an ANALOGUE TRANSISTOR.
Since a transistor is capable of amplifying a signal, it is said to be
an active device. Components such as resistors, capacitors, inductors
and diodes are not able to amplify and are therefore known as passive
In the following tests, use your finger to provide the TURN ON
voltage for the base (this is 0.55v to 0.7v) and as you press harder,
more current flows into the base and thus more current flows through the
collector-emitter terminals. As more current flows, the needle of the
multimeter moves UP-SCALE.
TESTING A TRANSISTOR ON A DIGITAL METER
Testing a transistor with a Digital Meter must be done on the
"DIODE" setting as a digital meter does not deliver a current through
the probes on some of the resistance settings and will not produce an
The "DIODE" setting must be used for diodes and transistors.
It should also be called a "TRANSISTOR" setting.
The first thing you may
want to do is test an unknown transistor for COLLECTOR, BASE AND
EMITTER. You also want to perform a test to find out if it is NPN or PNP.
That's what this test will provide.
You need a cheap multimeter called an ANALOGUE METER - a
multimeter with a scale and pointer (needle).
It will measure resistance values (normally used to test
resistors) - (you can also test other components) and Voltage
and Current. We use the resistance settings. It may have ranges
such as "x10" "x100" "x1k" "x10"
Look at the resistance scale on the meter. It will be the top
The scale starts at zero on the right and the high values are on
the left. This is opposite to all the other scales.
When the two probes are touched together, the needle swings FULL
SCALE and reads "ZERO." Adjust the pot on the side of the meter
to make the pointer read exactly zero.
How to read: "x10" "x100"
Up-scale from the zero mark is "1"
When the needle swings to this position on the "x10"
setting, the value is 10 ohms.
When the needle swings to "1" on the "x100" setting, the
value is 100 ohms.
When the needle swings to "1" on the "x1k" setting, the
value is 1,000 ohms = 1k.
When the needle swings to
"1" on the "x10k" setting, the value is 10,000 ohms =
Use this to work out all the other values on the scale.
Resistance values get very close-together (and very inaccurate)
at the high end of the scale. [This is just a point to note and
does not affect testing a transistor.]
Step 1 - FINDING THE BASE
and determining NPN or PNP
unknown transistor and test it with a multimeter set to
Try the 6 combinations and when you have the black probe on
a pin and the red probe touches the other pins and the meter
swings nearly full scale, you have an NPN transistor. The
black probe is BASE
If the red probe touches a pin and the black probe produces a
swing on the other two pins, you have a PNP transistor.
The red probe is BASE
If the needle swings FULL SCALE or if it swings for more than 2
readings, the transistor is FAULTY.
Step 2 - FINDING THE
COLLECTOR and EMITTER
Set the meter to
For an NPN transistor, place the leads on the
transistor and when you press hard on the two leads shown in the
diagram below, the needle will swing almost full scale.
For a PNP transistor, set the
meter to "x10k" place the leads on the
transistor and when you press hard on the two leads shown in the
diagram below, the needle will
swing almost full scale.
The simplest transistor
tester uses a 9v battery, 1k resistor and a LED (any colour).
Keep trying a transistor in all different combinations until you
get one of the circuits below. When you push on the two leads,
the LED will get brighter.
The transistor will be NPN or PNP and the leads will be
The leads of some transistors will need to be bent so the pins
are in the same positions as shown in the diagrams. This helps
you see how the transistor is being turned on. This works with
NPN, PNP transistors and Darlington transistors.
Heat generated by current flowing between the collector and emitter
leads of a transistor causes its temperature to rise. This heat must
be conducted away from the transistor otherwise the rise may
be high enough to damage the P-N junctions inside the
device. Power transistors produce a lot of heat, and are therefore
usually mounted on a piece of aluminium with fins, called a HEATSINK.
This draws heat away, allowing it to handle more current. Low-power signal transistors do not normally require heat sinking. Some
transistors have a metal body or fin to connect to a larger heatsink. If the
transistor is connected to a heatsink with a mica sheet (mica washer), it can
be damaged or cracked and create a short-circuit. (See
Testing Mica Washers). Or a small piece of
metal may be puncturing the mica. Sometimes white compound called
Heatsink Compound is used to conduct heat through the mica.
This is very important as mica is a very poor conductor of heat and the
compound is needed to provide maximum thermal conduction.
Transistor can fail in a number of ways.
forward and reverse voltage ratings and once these are exceeded, the
transistor will ZENER or conduct and may fail. In some cases a high
"puncture" the transistor and it will fail instantly. In fact it will
fail much faster via a voltage-spike than a current overload.
It may fail with a "short" between any leads, with a collector-emitter
short being the most common. However failures will also create shorts
between all three leads.
A shorted transistor will
allow a large current to flow, and cause other components to heat
Transistors can also develop an open circuit between base and collector,
base and emitter or collector and emitter.
The first step in identifying a faulty transistor is to check for signs of
overheating. It may appear to be burnt, melted or exploded. When the
equipment is switched off, you can touch the transistor to see if it
feels unusually hot. The amount of heat you feel should be proportional
to the size of the transistor's heat sink. If the transistor has no heat sink, yet is
very hot, you can suspect a problem.
DO NOT TOUCH A TRANSISTOR IF IT IS
PART OF A CIRCUIT THAT CARRIES 240VAC. Always switch off the
equipment before touching any components.
If you can't get an exact replacement, refer to a transistor substitution guide to identify a near
The important parameters are:
Maximum frequency of
The replacement part should have parameters equal to or higher than the
Points to remember:
Polarity of the transistor i.e. PNP or NPN.
At least the same voltage, current
and wattage rating.
or high frequency type.
Check the pinout of the
Use a desoldering pump to
remove the transistor to prevent damage to the
printed circuit board.
Fit the heat sink.
Check the mica washer and use
Tighten the nut/bolt - not too
tight or too loose.
Horizontal output transistors with an integrated diode should be
replaced with the
no such thing as a DIGITAL TRANSISTOR, however some transistors are
available with built-in resistors between base and emitter (to save
space on the board) and these transistors are often used in digital
circuits. The transistor will amplify analogue signals but when the
signal is 0v then immediately goes to a voltage above 0.7v, the
transistor is in a DIGITAL CIRCUIT and the transistor is called a
DIGITAL TRANSISTOR. It is tested like an ordinary transistor
but the low value resistor between base and emitter will produce a low
reading in both directions.
TRANSISTOR is two transistors in a single package with three leads.
They are internally connected in cascade so the
gain of the pair is very high. This allows a very small input signal to
produce a large signal at the output. They have three leads (Base,
Collector and Emitter and can be PNP or NPN) and are equivalent to the
leads of a standard individual transistor, but with a very high gain. The second advantage of a
Darlington Transistor is its high input impedance. It puts very little load
on the previous circuit.
Some Darlington transistors have a built-in diode and/or built-in
resistor and this will
produce a low reading in both directions between the base and emitter
Darlington transistors are tested the same as an ordinary transistor
and a multimeter will produce about the same deflection, even though you
will be measuring across two junctions, (and a base-emitter resistor is present).
HORIZONTAL OUTPUT TRANSISTORS,
SWITCH-MODE TRANSISTORS, FLYBACK TRANSISTORS, POWER TRANSISTORS,
VERTICAL TRANSISTORS . . . .
These are all names given to a transistor when it is used in a
particular circuit. ALL these transistors are the same for testing
We are not testing for gain, maximum voltage, speed of operation or any
special feature. We are just testing to see if the transistor is
completely faulty and SHORTED.
A transistor can have lots of other faults and the circuit using the
transistor is the best piece of TEST EQUIPMENT as it is detecting
TESTING MOSFETs and FETs
MOSFETs and JFETs
are all part of the FET family.
Metal Oxide Semiconductor Field Effect Transistor.
exactly the same as a "normal" transistor except they have different names for
the input and output leads and the voltage between the gate and the
source has to between 2v to 5v for the device to turn on fully. A FET
requires almost NO CURRENT into the Gate for it to turn on and when it
does, the voltage between drain and source is very low (only a few mV).
This allows them to pass very high currents without getting hot. There
is a point where they start to turn on and the input voltage must rise
higher than this so the FET turns on FULLY and does not get hot.
Field Effect Transistors are difficult to
test with a multimeter, but "fortunately" when a power
MOSFET blows, it
is completely damaged. All the leads will show a short circuit. 99% of bad
MOSFETs will have GS, GD and DS shorted.
The following symbols show some of the different types of MOSFETs:
MOSFET transistors cannot be tested with a
multimeter. This due to the fact that the Gate needs 2v - 5v to turn on
the device and this voltage is not present on the probes of either meter
set to any of the ohms ranges.
You need to build the following Test Circuit:
Touching the Gate will increase the
voltage on the Gate and the MOSFET will turn ON and illuminate the LED.
Removing your finger will turn the LED off.
Large devices such as the TO-220 types shown above do not like static
electricity on the gate and you have to be careful not to "spike" the
gate with any static. Generally this type of device is not "super
sensitive" and you can use your finger or a large value resistor.
When replacing one of these devices, there are 2 things to match-up.
Voltage and Current.
In most cases, the "turn-ON" resistance (the resistance between Source
and Drain) will be the same (something like 22 milli ohms) and the speed
of operation will be ok.
Check the voltage needed to turn the gate ON and make sure you can
supply the required voltage.
SILICON CONTROLLED RECTIFIERs (SCR)
Controlled Rectifier (SCR)
is a semiconductor device that is a member of a family of control
devices known as Thyristors. It is a 3-leaded device and when a
small current enters the Gate, the thyristor turns on. AND STAYS
ON. It only conducts current
between Anode and Cathode in one direction and it is mainly only used in
DC circuits. When it is used with AC, it will
only conduct for a maximum of half the cycle.
To understand how an SCR "latches"
when the gate is provided with a small current, we can replace it with
two transistors as shown in diagram B above.
When the ON button is pressed, the BC547 transistor turns on. This turns
ON the BC557 and it takes over from the action of the switch.
To turn the circuit off, the OFF button removes the voltage from the base
of the BC547.
Testing an SCR
Using the SCR Tester
An SCR can be tested with some multimeters but a minimum
current Anode-to-Cathode is needed to keep the device turned on. Some
multimeters do not provide this amount of current and the SCR Tester
circuit above is the best way to test these devices.
Shorted SCRs can usually be detected with an ohmmeter check (SCRs
usually fail shorted rather than open).
Measure the anode-to-cathode resistance in both the forward and reverse
direction; a good SCR should measure near infinity in both directions.
Small and medium-size SCRs can also be gated ON with an ohmmeter (on a
digital meter use the Diode Check Function). Forward bias the SCR with
the ohmmeter by connecting the
black ( - ) lead to the anode and the
red ( + ) lead to the cathode
(because the + of the battery is connected to the negative lead, in most
analogue multimeters). Momentarily touch the gate lead to the anode
while the probes are still touching both leads;
this will provide a small positive turn-on
voltage to the gate and the cathode-to-anode resistance reading will
drop to a low value. Even after removing the gate voltage, the SCR will
stay conducting. Disconnecting the meter leads from the anode or cathode
will cause the SCR to revert to its non-conducting state.
When making the above test, the meter impedance acts as the SCR
load. On larger SCRs, it may not latch ON because the test current
is not above the SCR holding current.
Connect an SCR
and press Switch2. The lamp should not illuminate. If it illuminates,
the SCR is around the wrong way or it is faulty.
Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will
turn ON and remain ON.
Release Sw 2 and press it again. The Lamp or motor will be OFF.
A triac is a
bidirectional, three-terminal dual, back-to-back thyristor (SCR)
switch. This device will conduct current in both directions when a small
current is constantly applied to the Gate.
If the gate is given a small, brief,
current during any instant of a cycle, it will remain triggered during
the completion of the cycle until the current though the Main Terminals
drops to zero.
This means it will conduct both the positive and negative half-cycles of
an AC waveform. If it is tuned on (with a brief pulse) half-way up the
positive waveform, it will remain on until the wave rises and finally reaches zero. If it is
then turned on (with a brief pulse) part-way on the negative wave, the
result will be pulses of energy and the end result will be about 50% of
the full-energy delivered at a rate of 100 times per
second for a 50HZ supply.
TRIACs are particularly suited for AC power control applications such
as motor speed control, light dimmers, temperature control and many
Using the TRIAC Tester
Connect a TRIAC
and press Switch2. The lamp should not illuminate. If it illuminates,
the TRIAC is faulty.
Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will
turn ON and remain ON. If the lamp does not turn on, reverse the TRIAC
as the current into the gate must produce a slight voltage between
Gate and Main Terminal 1.
Release Sw 2 and press it again. The Lamp or motor will be OFF.
MICA WASHERS AND INSULATORS
sheets (washers) between a transistor and heatsink are most often made from mica
but some are plastic and these get damaged over a
period of time, turn dark and become cracked.
The plastic eventually becomes carbonized and conducts current and can
affect the operation of the appliance. You can see the difference
between a mica sheet (washer) and plastic by looking where it extends
from under the transistor. Replace all plastic insulators as they
Some TV's and monitors with a CRT (picture tube), have spark
gaps either on the socket at the end of the tube or on the chassis.
These can consist of two wires inside a plastic holder or a glass tube
or special resistive device.
The purpose of a spark gap is to take any flash-over (from inside the
tube), to earth. This prevents damage to the rest of the circuit.
However if the tube constantly flashes over, a carbon track builds up
between the wires and effectively reduces the screen voltage. This can
cause brightness and/or focus problems. Removing the spark-gap will
restore the voltage.
These are not available as a spare component and it's best to get one
from a discarded chassis.
Co-Ax cables can produce very
high losses and it seems impossible that a few metres of cable will
reduce the signal. The author has had a 3 metre cable reduce the
signal to "snow" so be aware that this can occur. Faults can also come
from a splitter and/or balun as well as dirty plugs and sockets. This
can result in very loud bangs in the sound on digital reception.
TESTING EARTH LEAKAGE DETECTORS or
Residual Current Devices or
Ground Fault Circuit Interrupters or GFCI
An Earth Leakage
Detector or Sensor is a circuit designed to continuously monitor the
imbalance in the current in a pair
of load carrying conductors.
These two conductors are normally the Active and Neutral. Should the
imbalance current reach 30mA the sensor will "trip" and remove the
voltage (and current) from the line being monitored.
Some detectors will trip at 15mA.
You cannot alter the sensitivity of the device however there are a
number of faults in these devices that can be fixed.
In some devices the contact pressure for the 10Amp or 15 Amp contacts is very weak
and they arc and produce an open circuit. The result is this: When
you press the rest button, power is not restored to the output.
Clean the contacts with a small file and bend the metal strips to the contacts so they make a
very strong contact.
The other fault is the trip mechanism.
The magnetism from the coil does not allow the pin to move and "trip"
the contacts. It may be due to a small metal filing or the pin not
moving freely enough.
All good Earth Leakage Detectors have a TEST BUTTON. This connects a
resistor between the active line and earth so that 15mA or 30mA flows.
The detector should trip immediately. Make sure the trigger mechanism
trips when the test button is pressed.
None of the electronics in the detector can be replaced however you can test the
mechanical operation and the pressure on the contacts when the unit is removed from the power.
Do not work on the device when it is connected to the mains.
TESTING CELLS AND BATTERIES
There is an enormous number of batteries and cells on the market
and a number of "battery testers." Instead of buying a battery tester
that may give you a false reading, here is a method of testing cells
that is guaranteed to work.
There are two types of cell: a rechargeable cell and a non
The easiest way to test a rechargeable cell is to put a group of
in an appliance and use them until the appliance "runs down" or fails to
work. If you consider the cells did not last very long, remove them and
check the voltage of each cell. The cell or cells with the lowest
voltage will be faulty. You can replace them with new cells or good
cells you have in reserve.
There is no other simple way to test a rechargeable cell.
You cannot test the "current of a cell" by using an ammeter. A
rechargeable cell can deliver 10 amps or more, even when nearly
discharged and you cannot determine a good cell for a faulty cell.
Dry cells are classified as "non-rechargeable" cells.
DRY CELLS and MANGANESE CELLS are the same thing. These produce
1.5v per cell (manganese means the Manganese Dioxide depolariser inside
the cell. All "dry cells" use manganese dioxide).
ALKALINE CELLS produce between 2 - 10 times more energy than a "dry
cell" and produce 1.5v per cell.
Alkaline cells can fail for no reason at any stage in their life and are
not recommended for emergency situations.
The output voltage of some Alkaline cells can fall to 0.7v or 0.9v for
not apparent reason.
There are lots of other cells including "button cells," hearing-aid
cells, air cells, and they produce from 1.2v to 3v per cell.
Lithium cells are also called "button cells" and they produce 3v per cell.
Lithium cells are non-rechargeable (they are generally called
"button cells") but some Lithium cells can be recharged. These are
Lithium-ion cells and generally have a voltage of 3.6v. Some Lithium-ion
cells look exactly like 3v Lithium cells, so you have to read the data
on the cell before charging.
You cannot test the voltage of a cell and come to any conclusion as to
the age of the cell or how much energy remains. The voltage of a cell is
characteristic to the chemicals used and the actual voltage does not
tell you its condition.
Some "dry cells" deliver 1.5v up to the end of their life whereas others
drop to about 1.1v very quickly.
Once you know the name of the cell that drops to 1.1v, avoid them as the
operation of the equipment "drops off" very quickly.
However if you have a number of different cells and need to know which
ones to keep, here's the solution:
1. Check the voltage and use those with a voltage above 1.1v
2. Next, select 500mA or 10A range on a meter and place the probes on a
cell. For a AAA or AA cell, the current should be over 500mA and the
needle will swing full scale very quickly.
Keep the testing short as you are short-circuiting the cell but it is
the only way to determine the internal impedance of the cell and this
has a lot to do with its stage-of-charge.
This will give you a cell with a good terminal voltage and a good
This also applies to button cells, but the maximum current they will
deliver will be less.
If you want to get the last of the energy out of a group of cells they
can be used in the following circuits:
and PIEZO BUZZERS
There are two types of piezo
devices that produce a sound.
They are called PIEZO DIAPHRAGMS and PIEZO BUZZERS.
A piezo diaphragm consists of two metal plates with a ceramic
material between. The ceramic expands and contracts when an alternating
voltage is placed on the two plates and this causes the main plate to
"dish" and "bow."
This creates a high-pitched sound. There are no other components inside
the case and it requires an AC voltage of the appropriate frequency to
produce a sound.
A piezo buzzer has a transistor and coil enclosed and when
supplied with a DC voltage, the buzzer produces a sound.
Both devices can look exactly the same and the only way to tell them
apart is by connecting a 9v battery. One device may have "+' and "-" on
the case to indicate it is a piezo buzzer, but supplying 9v will make
the buzzer produce a sound while the piezo diaphragm will only produce a
How a PIEZO BUZZER WORKS
A piezo diaphragm will produce a click
when connected to 9v DC.
A piezo buzzer will produce a tone when
connected to a DC voltage.
A Piezo Buzzer contains a transistor, coil, and piezo diaphragm and
produces sound when a voltage is applied. The buzzer in the circuit
above is a PIEZO BUZZER.
The circuit starts by the base receiving a
small current from the 220k resistor. This produces a small magnetic
flux in the inductor and after a very short period of time the current
does not increase. This causes the magnetic flux to collapse and produce
a voltage in the opposite direction that is higher than the applied
3 wires are soldered to pieces of metal on the top and bottom sides of a
ceramic substrate that expands sideways when it sees a voltage. The
voltage on the top surface is passed to the small electrode and this
positive voltage is passed to the base to turn the transistor ON again.
This time it is turned ON more and eventually the transistor is fully
turned ON and the current through the inductor is not an INCREASING
CURRENT by a STATIONARY CURRENT and once again the magnetic flux
collapses and produces a very high voltage in the opposite direction.
This voltage is passed to the piezo diaphragm and causes the electrode
to "Dish" and produce the characteristic sound. At the same time a small
amount is "picked-off" and sent to the transistor to create the next
TESTING A SPEAKER
A speaker (also called a loud
speaker) has coil of wire wrapped around a magnet but it does
not touch the magnet as it is wound on a thin cardboard former so that
the coil will be pulled closer to the magnet when a current flows in the
When the current flows in the other direction, the coil moves away from
The coil is called voice coil and it is connected to a sheet of
thin card called a CONE and as the cone vibrates, the speaker reproduces
music or noise.
Use a multimeter on a low ohms scale to read the value of resistance of
It can be as low as 2 ohms or as high as 100 ohms.
Most speakers have an 8R voice coil and the actual resistance may be
slightly lower than this.
Some speakers have a resistance of 16R, 32R or 50R and even 75 ohms.
You would think putting a 16R speaker in place of 8R would reduce the
sound output, but this is not always the case.
You can even use 50R or 75R and get the same performance.
This may sound amazing, but here is the reason.
The cone is deflected a certain amount due to the current flowing and
the number of turns.
These two values are multiplied together to produce a value called
If we have an 8R speaker with 80 turns and 100mA, the result is 0.1 x 80
If we use a 16R speaker, the average current flow will be 50mA and the
number of turns will be about 160. The multiplication of
0.05 x 160 = 8.
The author then tried a 50R speaker and the sound output was equal to 8R
and the same with 75R speaker.
This might not apply in all situations, but the 75R speaker was slightly
larger and the ticking sound form the Metal Detector kit was
louder than using an 8R mini speaker.
To see if the cone of a speaker is undamaged, push it slightly and it
will move towards the magnet. If it does not move, it is bent or
damaged. If the cone is scratchy when pushed, it is rubbing against the
A cone should be able to be pushed and pulled from its rest-state. If
not, it will produce a distorted sound.
TESTING A CIRCUIT
Whenever you test a circuit, the TEST EQUIPMENT puts "a load"
or "a change" on it.
It does not matter if the test equipment is a multimeter, Logic Probe,
CRO, Tone Injector or simply a LED and resistor.
There are two things you need to know.
1. The IMPEDANCE of the circuit at the location you are testing, and
2. The amount of load you are adding to the circuit via the test
There is also one other hidden factor. The test equipment may be
injecting "hum" due to its leads or the effect of your body at absorbing
hum from the surroundings or the test equipment may be connected to the
These will affect the reading on the test equipment and also any output
of the circuit.
Sometimes the test equipment will prevent the circuit from working and
sometimes it will just change the operating conditions slightly. You
have to be aware of this.
The last section of this eBook covers High and Low
Impedance and understanding impedance is something you need to know.
The point to note here is the fact that the equipment (and the reading) can be upset by
hum and resistance/capacitance effects of test equipment. This is
particularly critical in high impedance and high frequency circuits.
TESTING INTEGRATED CIRCUITS (IC's)
can be tested with a LOGIC PROBE. A Logic Probe will tell you
if a line is HIGH, LOW or PULSING.
Most logic circuits operate on 5v and a Logic Probe is connected to the
5v supply so the readings are accurate for the voltages being tested.
A Logic Probe can also be connected to a 12v CMOS circuit.
You can make your own Logic Probe and learn how to use it from the
LOGIC PROBE with
This is a very
simple transistor circuit to provide HIGH-LOW-PULSE indication for
digital circuits. It
can be built for less than $5.00 on a piece of matrix board or on a
small strip of copper clad board if you are using surface mount
components. The probe will detect a HIGH at 3v and thus the project can
be used for 3v, 5v and CMOS circuits.
using CD4001 and CD4011
Here is a simple Logic Probe using a
single chip. The circuits have been designed for the CD4001 CMOS
quad NOR gate and CD4011 CMOS NAND gate. The output has an
active buzzer that produces a beep when the pulse LED illuminates (the
buzzer is not a piezo-diaphragm but an active buzzer containing
SUPER PROBE MkII has 20 different features including a Logic Probe,
capacitance tester, Inductance tester, and more.
SUPER PROBE MkII Circuit
SUPER PROBE MkII
To test an IC,
you need a circuit diagram with waveforms. These diagrams will show the
signals and are very handy if a CRO (cathode ray Oscilloscope ) is used to diagnose the
problem. The CRO will reproduce the waveform and prove the circuit is
A Logic Probe will just show activity and if an output is not producing
a "pulse" or "activity," you should check the power to the IC and
test the input line.
It is beyond the scope of this eBook to explain how to diagnose waveforms, however
it is important to know if signals are entering and exiting an IC and a
Logic Probe is designed for this.
This circuit is rich in harmonics and is ideal for
testing amplifier circuits. To find a fault in an amplifier, connect the
earth clip to the 0v rail and move through each stage, starting at the
speaker. An increase in volume should be heard at each preceding stage.
This Injector will also go through the IF stages of radios and FM sound
sections in TV's.
TESTING AUDIO AMPLIFIERS and AUDIO IC's
(also see above)
The Super Probe MII described above has a "noise" function and a tone
function that allows you to inject a signal into an audio stage,
amplifier (made from discrete components) or an audio chip, and detect the output on a speaker.
Audio stages are very difficult to work-with if you don't have a TONE
GENERATOR or SIGNAL INJECTOR.
The signals are very small and not detected by a multimeter.
You can start anywhere in an amplifier and when a tone is heard, you
can keep probing until the signal is not present or louder. From this
you can work out which way the signal is travelling.
A Signal Injector is very handy for finding shorts and broken wires in switches, plugs, sockets
and especially leads to headphones.
You can determine the gain of a stage (amplification) by probing before
and after a chip or transistor and listen for the
relative increase in volume from the speaker.
You can also use your finger to produce "hum" or "buzz" if a
Injector is not available.
Nearly all audio problems are plugs, sockets and cracks in the PC board,
but finding them takes a lot of time and skill.
TESTING IC's - also called "CHIPS"
An Integrated Circuit is also called a "chip." It might have 8 pins or
as many as 40.
Some chips are ANALOGUE. This means the input signal is rising and
falling slowly and the output produces a larger version of the input.
Other chips are classified as DIGITAL and the input starts at 0v and rises
to rail voltage very quickly. The output does exactly the same - it
rises and falls very quickly.
You might think the chip performs no function, because the input and
output voltage has the same value, but you will find the chip may have
more than one output and the others only go high after a number of
clock-pulses on the input, or the chip may be outputting when a combination of inputs is recognised or
the output may go HIGH after a number of clock pulses.
Analogue chips are AUDIO chips or AMPLIFIER chips.
To test these chips you will need three pieces of test equipment:
1. A multimeter - this can be digital or analogue.
3. A Mini Bench Amplifier.
The Mini Bench Amplifier is available as a kit.
MINI BENCH AMPLIFIER
MINI BENCH AMPLIFIER
Start by locating the power pin with a multimeter.
If the chip is receiving a voltage, you can use the Mini Bench Amplifier
to detect an output.
Connect the Ground Lead of the Mini Bench Amplifier to 0v and touch the Probe tip on each of the
You will hear faint audio on the Input pin and very loud audio on the
If no input is detected, you can use a
Signal Injector to produce a tone.
Connect the clip of the Signal Injector to 0v and the probe to the
input pin of the amplifier chip. At the same time, connect the Mini
Bench Amplifier to the output pin and you will hear a very loud tone.
These pieces of test equipment can also be used to diagnose an amplifier
circuit constructed with individual components.
Amplifier circuits using discrete components are very hard to trouble-shoot and these pieces of
test equipment make it very easy.
It is always best to have data on the chip you are testing, but if this
is not available, you will need three pieces of equipment:
1. A multimeter - this can be digital or analogue.
2. A Logic Probe,
3. A logic Pulser.
Firstly test the chip to see if power is being delivered. This might be
anything from 3v3 to 15v.
Place the negative lead of the multimeter on the earth rail of the
project - this might be the chassis, or the track around the edge of the
board or some point that is obviously 0v.
Try all the pins of the chip and if you get a reading, the chip will
Identify pin 1 of the chip by looking for the "cut-out" at the end of
the chip and you may find a small dimple below the cut-out (or notch).
This is pin 1 and the "power pin" can be directly above or any of the
Next you need to now if a signal is entering the chip.
For this you will need a LOGIC PROBE.
A Logic Probe is connected to the same voltage as the chip, so it
will detect a HIGH and illuminate a red LED.
Connect the Logic Probe and touch the tip of the probe on each pin.
You will not know if a signal is an input or output, however if you get
two or more active pins, you can assume one is input and the other is
output. If none of the pins are active, you can assume the signal is not
reaching this IC.
If only one pin is active, you can assume the chip is called a CLOCK (or
Clock Generator). This type of chip produces pulses. If more than two
pins are active, you can assume the chip is performing its function and
unless you can monitor all the pins at the same time, you don't know
what is happening.
This is about all you can do without any data on the chip.
If you have data on the chip, you can identify the input(s) and output(s).
A Logic Probe on each of these pins will identify activity.
A Logic Probe has 3 LEDs. Red LED indicates a HIGH, Green indicates a
LOW and Orange indicates a PULSE (activity).
Some Logic Probes include a piezo and you can hear what is happening, so
you don't take your eyes off the probe-tip.
It is important not to let the probe tip slip between the pins and
create a short-circuit.
If you have a board or a single chip and want to create activity (clock
pulses), you can use a Logic Pulser. This piece of test equipment will
produce a stream of pulses that can be injected into the clock-line
(clock input) of a chip.
You can then use a Logic Probe at the same time on the outputs to
observe the operation of the chip.
You can also use the Mini Bench Amplifier to detect "noise" or activity
on the inputs and outputs of digital chips.
This only applies if the frequency is in the audio range such as
scanning a keyboard or switches or a display.
This is how to approach servicing/testing in a general way. There are
thousands of digital chips and if you want to test a
specific chip for its exact performance, you will need to set-up a
There are two types of remote control - Infrared and RF. Infrared is
used for short-range, line-of-sight for TV's DVD's etc.
A few faults can be fixed, but anything complex needs a new remote control.
Check the batteries and battery-contacts. See if the IR LED is illuminating by
focusing it into a digital camera and looking on the screen for
The only other things are a sticky button, a worn-out button or a crack
in the PC board. Water damage is generally too much work to repair.
RF remote controls for cars, garage doors etc
need a second working unit to check the power output.
Here is a simple circuit that can be connected to an analog multimeter
to detect the signal strength at a very close range:
To hear the tone from a transmitter, the
Mini Bug Detector circuit can be used:
VOLTAGES ON (in) A CIRCUIT
Any further investigation requires a circuit diagram so you can work out
what is actually being sent from the transmitter.
Most of the time it is a faulty switch, battery or contacts. Make sure
the setting is correct on the "dip switches" and use a working unit to
compare all your testing.
There are basically two
different types of circuit.
1. ANALOGUE CIRCUIT
An analogue circuit can also be called an AUDIO CIRCUIT and the voltages
at different points in a circuit can be measured with a multimeter but
the changes (the waveforms) will be quite small or changing at a rapid
rate and cannot be detected by a multimeter.
You need a CRO to "see" the signals or a Signal Injector to
inject a waveform into the circuit and hear the result on the circuit's
2. DIGITAL CIRCUIT
A digital circuit can also be called a "Computer Circuit" or "Logic
Circuit" and some of the voltages can be measured with a multimeter
(such as supply voltages) but the "signal lines" will be be changing
from HIGH to LOW to HIGH very quickly and these signals are detected
with a Logic Probe.
Here are some circuits with details of how to test the voltages.
Most circuits do not show voltages at various different points and we
will explain what to expect on each "stage."
A stage is a set of components with an input and output. A "stage" can
also be called a "Building Block."
Sometimes it has a capacitor on the input and one on the output.
This means the stage is completely isolated as far as DC is concerned.
The stage has a supply (a DC supply) and it is producing its own
voltages on various points on the "stage." It can only process (amplify)
Sometimes the stage can be given a name, such as small-signal amplifier,
push-pull amplifier or output.
If the stage has a link or resistor connected to a previous stage, the
previous stage will have a "DC effect" on the stage. In other words it
will be biasing or controlling the voltages on the stage. The stage may
be called a "timer" or "delay" or "DC amplifier."
It is important to break every circuit into sections. This makes testing
easy. If you have a capacitor at the input and output, you know all the
problems lie within the two capacitors.
In a digital circuit (no capacitors) you need to work on each IC
(integrated Circuit) and test the input for activity and all the
Once you have determined if the circuit is Analogue or Digital, or a
combination of both, you have to look at the rail voltage and work out
the size or amplitude of the voltage or waveform.
This is done before making a test, so your predictions are confirmed.
You will need a multimeter (either Digital or Analogue) a
Logic Probe and a Signal Injector (Tone Generator).
An analogue meter has the advantage that it will detect slight
fluctuations of voltage at a test-point and its readings are faster than
a digital meter. A digital meter will produce an accurate
voltage-reading - so you should have both available.
HIGH IMPEDANCE AND LOW IMPEDANCE
Every point in a circuit has a characteristic called "IMPEDANCE." This
has never been discussed before in any text book. That's why it
will be new to you.
In other words, every point will be "sensitive to outside noise."
An audio amplifier is a good example. If you put your finger on the
active input, it will produce hum or buzz in the speaker. This is
because it is a HIGH IMPEDANCE line or high impedance section of the
The same applies to every part in a circuit and when you place Test
Equipment on a line for testing purposes, the equipment will "upset" the
line. It may be very slight but it can also alter the voltage on the
We have already mentioned (above) how a cheap multimeter can produce a
false reading when measuring across a 1M resistor.
That's why you need high impedance test Equipment so you do not
"load" the point you are testing and create an inaccurate reading.
The word Impedance really means resistance, but when you have
surrounding components such as diodes, capacitors, transistors, coils,
Integrated Circuits, supply-voltages and resistors, the combined effect is very
difficult to work out as a "resistance" and that's why we call it
The term "High and Low Impedance" is a relative term and does not
have any absolute values but we can mention a few points to help you
In general, the base of a transistor, FET input of an IC are classified
as HIGH IMPEDANCE.
The output of these devices are LOW IMPEDANCE.
Power rails are LOW IMPEDANCE.
An oscillator circuit and timing circuit are HIGH IMPEDANCE.
A LOAD is low impedance.
And it gets tricky: An input can be designed to accept a low-impedance
device (called a transducer or pick-up) and when the device is
connected, the circuit becomes LOW impedance, but the input circuitry is
actually high impedance.
The impedance of a diode or LED is HIGH before the device sees a voltage
higher than the junction voltage and then it becomes LOW Impedance.
Impedance is one of the most complex topics however it
all comes down to testing a circuit without loading it.
That's why test equipment should have an input impedance
higher than 1M.
The first circuit we will investigate is the Mini Bug Detector,
shown above and below. Points on the circuit have been labelled A, B, C etc:
Point A - The first transistor is "self-biased" and will have 0.6v
on the base. The antenna is connected to a 20 turn coil and you might
think the coil will "short" the signals to earth.
But the coil and 470p capacitor form a circuit that oscillates at a high
frequency when the antenna wire picks up stray signals. The coil and
capacitor actually amplify the signals (see Talking Electronics website:
Spy Circuits to see how a TANK CIRCUIT works) and these signals
enter the base of the first transistor.
This is classified as a HIGH Impedance section because the signals are
small and delicate and any loading via test equipment will kill them.
The first transistor amplifies the signals about 70 times and they
appear at Point B.
The signal passes though a 22n to Point
C and the transistor amplifies the signal about 70 times to point
D. Point C is classified as high impedance as any voltage
measurement at this point will upset the biasing of the stage as a few
millivolts change in base-voltage will alter the voltage on the
collector considerably. Point D is classified as low impedance as
any voltage-testing will not alter the voltage appreciably.
The output of the second stage passes through a capacitor to the join of
two diodes. These two diodes are not turned on because the voltage at
Point E can never rise above 0.7v as this is the voltage produced by
the base-emitter of the third transistor.
The purpose of the two diodes is to remove background noise. Background
noise is low amplitude waveforms and even though the transistor is
turned on via the 220k, low amplitude signals will not be received. The
third transistor works like this: It cannot be turned ON any more
because any waveform from the 22n will be "clipped" by the bottom diode
and it will never rise above 0.6v.
So, the only signal to affect the transistor is a negative signal - to
turn it OFF.
Firstly we have to understand the voltage on the 22n. When the second
transistor is sitting at mid-rail voltage, the 22n gets charged via the
2k2 and lower diode. When the transistor gets tuned ON, the collector
voltage falls and the left side of the 22n drops. The right side of the
22n also drops and when it drops 0.6v, the top diode starts to conduct
and when the voltage on the 22n drops more than 0.6v the third
transistor starts to turn OFF. This effect is amplified by the
transistor at least 100 times and appears at Point F. All the
voltages around the two diodes are classified as HIGH Impedance as any
piece of test equipment will upset the voltage and change the output.
There are some losses in amplitude of the signal as it passes through
the 22n coupling capacitors but the end result is a very high strength
signal at point G. The 4th transistor drives a 10mH
choke and the mini piezo is effectively a 20n capacitor that detects the
"ringing" of the inductor to produce a very loud output.
The 22n capacitor on the collector eliminates some of the background
noise. The choke and piezo form an oscillatory circuit that can
produce voltages above 15v, even though the supply is 3v.
The 47n capacitor at Point J is to keep the supply rails "tight" (to
create a LOW Impedance) to allow weak cells to operate the circuit.
The "Power-ON" LED tells you to turn the device off when not being used
and Point L is the power supply - a low impedance line due to the
Testing the Mini Bug Detector
To test the Mini Bug Detector, you will need a Signal
Place the Injector on Point G and you will hear a tone. Then go
to E, C and A. The tone will increase in volume. If
it does not increase, you have pin-pointed the faulty stage.
The next circuit is a combination of digital and analogue signals.
It is a Logic Probe:
The voltage on a circuit (to be tested) is detected by the probe at
Point A of the circuit above and the "tip" is classified as
"reasonably high impedance" as it has a 220k resistor between the tip
and 0v rail. The 1M reduces the impedance by about 20% but the inputs of
the two inverters have no effect on the "tip" impedance as they are
extremely high input-impedance devices.
The 1M trim pot is designed to put put a voltage on point B that
is slightly higher than mid-rail so the green LED is turned off.
Point A will see a voltage below mid-rail and point C will
be HIGH. Point C and F are low-impedance outputs.
When the tip of the probe is connected to a LOW voltage, Point B sees a
LOW and Point F goes LOW to illuminate the green LED. At the same time
it removes the "jamming voltage" produced by the diode between pin 4 of
the 4049 and pin 3 of the 74C14 and the oscillator between points H
and J produces a low-tone via the 100k resistor and 22n to indicate
When the probe tip sees a HIGH, a lot more things happen.
Point C goes LOW and turns on the red LED. At the same time
the 100p is in an uncharged state and the right lead goes LOW. This
takes the left lead LOW as the left lead connects to a HIGH Impedance
line and pin 9 goes LOW. This makes point E HIGH
and since the 1u is in an uncharged state, pin 11 goes HIGH. This makes
point G LOW and the diode between pins 9 and 12 keeps pin 9 LOW
and takes over from the pulse from the 100p. The yellow LED is
illuminated. The 1u starts to charge via the 470k and when it is approx
half-charged, pin 11 sees a HIGH and point G goes low. This
creates the length of pulse for the yellow LED.
At the same time, Point L goes LOW because the "jamming diode"
from pin 2 of the 4049 goes low and allows the inverter between point L
and N to produce a tone for the piezo.
In addition, Point I goes HIGH and quickly charges a 1u
electrolytic. This removes the effect of the jamming diode on pin 5 of
the 74C14 and a low frequency oscillator made up of 68k and 1u between
pins 5&6 turns on and off an oscillator between points O and R
to get a beep. The mini piezo is driven n bridge mode via the two gates
between points QT and PS.
Point U is a 1u electrolytic to reduce the impedance of the power
rail and Point V is a protection diode to prevent damage if the
probe is connected to the supply around the wrong way.
Testing the Logic Probe
You can test the Logic Probe with the simple Logic
Probe with Pulse project described above. It will let you know if
each point in the circuit is HIGH or LOW. You will also find out the
difficulty in testing the points that are HIGH Impedance, as the Probe
will upset the voltage levels and the reading may be inaccurate.
More circuits will be added here in the future.
Here are three 30-minute videos on
TESTING A MOTOR
Strictly speaking, a motor is not an electronic component, but since a
website gave a useless description on testing motors, I have decided to
supply the correct information.
The only REAL way to test a motor is to have two identical motors and
check the torque by connecting them to a low voltage and trying to stop
the shaft with your fingers. This will give you two results.
Firstly it will let you know the torque of the motor.
This is the twisting effect of the shaft. There is no way to determine
the torque by knowing the voltage or current.
The unknown factor is the strength of the field magnets (permanent
magnets) and this
determines the torque.
Secondly, feeling the shaft will let you know if the torque is even for
a complete revolution.
By having two identical motors, you can see if one has a lower torque.
Almost nothing can go wrong with a motor except for the
brushes. If the brushes wear out, additional resistance will be produced
at the interface between the brush and commutator and this can be
detected by allowing the shaft to rotate slowly and feeling the
resistance as it revolves. A 3-pole motor will have three places where
the strength is greatest and each should have the same feeling. A 5-pole
motor will have five places of strength.
If the strength is weak or not uniform, the motor is faulty.
You cannot test a motor with a multimeter as the resistance of the
armature winding is very low and if the motor is allowed to spin, the
back voltage produced by the spinning, increases the reading on the
meter and is false.
Micro motors have a coreless armature. This means the 3 windings for the
armature are wound on a machine then bent slightly into shape and glued.
A circular magnet with 3 poles is in the centre and the armature rotates
This type of motor is reasonably efficient because the armature is the
greatest distance from the point of rotation, and the motor reaches full
RPM very quickly.
I have not heard of the armature-winding flying apart but if you hear
any scraping noise, it may be the winding.
3-pole, 5-pole and micro motors can be found in printers, eject
mechanisms of CD players, toys, RC helicopters, cars etc and rarely
fail. This discussion has been presented to clarify the testing
procedure and expose the absurd claims of another site.
TESTING COMPONENTS "IN-CIRCUIT"
You can test components while they are IN CIRCUIT, but the
surrounding components will have an effect on the results.
You can get all sorts of "In-Circuit" testers. They are expensive and
offer little more accuracy than a multimeter.
In-Circuit testing with a multimeter can give you the same results as a
All you have to do is turn the project ON and use a multimeter (set to
voltage) to determine the voltage at various points. It is best to have
a circuit of the equipment so you can what to expect at each point.
Only major departures from the expected can be located in this way.
Obviously the first thing to look for is burnt-out components. Then feel
components such as transistors for overheating.
The look for electrolytics that may be dry. Sometimes these have changed
colour or are slightly swollen.
If they are near hot components, they will be dry.
For the cost of a few dollars I change ALL THE ELECTROLYTICS in some
pieces of equipment, as a dry electrolytic is very difficult to detect.
Testing a transistor "in-circuit" is firstly done with the supply ON.
That's because it is quicker.
Measure the voltage between ground and collector.
In most cases you should get a voltage of about half-rail. If it is
zero, or close to rail voltage, you may have a problem.
Turn off the supply and use the multimeter on low-ohms to measure all
six resistances between the leads.
A low resistance in both directions on two leads will indicate a fault.
Resistors almost NEVER go "HIGH." For instance, a 22k will never go to
50k. However a low-value resistor will "burn-out" and you will read the
value of the surrounding components.
Don't forget, some low-value resistors are designed to burn-out (called
fusible resistors) and anytime you find a damaged low-value resistor,
you will need to look for the associated semiconductor.
You can replace the resistor quickly and turn the circuit ON to see it
burn out again.
Alternatively you can trace though the circuit and find the shorted
It's always nice to "see the fault" then "fix the fault."
Sometimes a transistor will only break-down when a voltage is present,
or it may be influenced by other components.
When the piece of equipment is turned OFF, you can test for resistance
values. The main thing you are looking for is "dry joints" and
continuity. Dry joints occur around the termination of transformers and
any components that get hot. Rather than wasting time checking for dry
joints, it is better to simply go over the connections with a hot iron
and fresh solder.
You may need to check the continuity of a track (trace) and it may go
from one side of the PC board to the other.
Use a multimeter set to low-ohms and make sure the needle reads
It is very dangerous to do any testing on a project using a multimeter
set to "amps" or "milliamps."
You cannot test "current flowing through a component" by placing the
probes across a component. You will simply over-load the rest of the
circuit and create a problem.
To find out if current is flowing though a circuit or a low-value
resistor, turn the project ON and measure the voltage either across the
component or the voltage on one end then the other.
A voltage-drop indicates current is flowing.
That's about it for testing "in-circuit." Use the rest of this eBook to
help you with diagnosis.
Don't think an IN-CIRCUIT COMPONENT TESTER is going to find a fault any
faster than a multimeter. They all use a multimeter principle.
every component can fail and produce an effect called a SHORT CIRCUIT.
This basically means the component takes more current than normal and it
may fail completely or simply take more current and the operation of the
circuit may be reduced only a small amount.
The resistance of the component may reduce a very small amount but this
may have a very large effect on the operation of the circuit.
For instance, two turns in the horizontal or vertical winding of a yoke on the
picture tube or monitor may arc and weld together and reduce the size of
the picture on the screen, but measuring the winding will not detect the
difference in resistance.
The same with the windings on a motor and a short between two winding in
If the "short" is between two near-by turns, the change in resistance
will be very small. If the "short" is between to different layers, the
resistance will be reduced and it may be detected.
When a "short" occurs, the winding turns into a transformer. To be
exact, an AUTO-TRANSFORMER.
In the following diagram you can see a normal winding in fig A:
Fig B shows two turns touching each other and if the wire is enamelled,
the coating has been damaged so the copper wire from the two turns is
touching. This is called a SHORTED TURN.
In fig C you can see two turns touching.
In fig D the shorted-turn has been moved to the other side of the symbol
to show the effect it has on the operation of the winding.
The shorted-turn is exactly like the secondary of a transformer with a
"jumper" across the output.
This will produce a very high current in the secondary.
high current flows through the shorted turn and this changes the
operation of the rest of the winding.
1. In most cases a SHORT CIRCUIT can be detected by feeling the
additional heat generated by the component.
2. Next, turn off the supply and measure the resistance of the
component. If it is lower than expected, the component will be faulty.
3. Next, measure the voltage across the component. If it is lower than
normal, the component will be faulty.
4. Next, measure the current taken by the component. If it is higher
than normal, the component will be faulty.
5. If the component is an inductor, such as a motor, coil or
transformer, you can use an inductance meter. Compare a good winding
with a faulty winding. Sometimes the fault will disappear because an arc
develops across the fault when the component is operating.
INTERNAL AND EXTERNAL SHORTS
An internal short refers to
two windings shorting
together and the winding has a very high resistance between the winding
and the frame on which it is wound. An external short refers to a winding shorting to the frame of
the component - such as one of the armature windings shorting to the
metal core, around which the wire is wound.
This may not be important unless another winding shorts to the metal
frame and creates "inter winding" problems (inner winding
problems is within the same winding).
The opposite to a short circuit is an OPEN CIRCUIT.
This is generally a broken lead or contact or a wire that has
"burnt-out" or been "eaten-away" by acid attack or galvanic action by
water and voltage (current).
1. No current will flow when an OPEN CIRCUIT exists.
2. The voltage on each end of the OPEN CIRCUIT will not be the same.
3. Measure the current across the OPEN CIRCUIT and determine if excess
current is flowing.
4. Join the two ends of the OPEN CIRCUIT and see if the circuit operates
This is not the full story to learning about servicing. It is just the
We have only covered the simplest tests and shown how 90% of faults can
be found by checking voltages, waveforms and looking for obvious things
such as burnt out components, cracks in PC boards.
The author has fixed over 35,000 TV's, radios, stereos, VCRs and all
those things that were on the market 30 years ago.
Things have not changed. It's just that some repairs cost nearly as much as
buying a new product and half the customers opt for dumping a faulty
item and buying the latest "flat screen" version. That's why
you have to get things through the workshop as fast and as cheaply as
possible, to make a living.
If you want any more devices
added to this list, email
To help with understanding how a
transistor circuit works, we have produced an eBook:
The Transistor Amplifier. It covers a whole range of circuits
using a transistor.
Not copyright by Colin Mitchell
You can use any of this material. Please pass this eBook to
your friends and let them know that everything on the web is FREE. I
have looked at all the "Pay Sites" and found the information they "sell"
is available on the web at NO COST.
Nearly all text books are also downloadable for free on "Download.com"
etc and when you see a used copy of a $74.00 textbook on Amazon for
$12.00 you realise many users have already discarded their copy. A
good textbook never gets thrown out or sold for $12.00!!!
See the enormous amount of information on
Talking Electronics website.