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BASIC
ELECTRONICS COURSE
Page
4
INDEX
RESISTORS
Resistors are one of the most valuable components in electronics. They are
used in almost EVERY circuit. The one thing that makes an electronics person
skilled is his ability to read the coloured bands on resistors.
This is one of the hardest things to do and the best way to
master the code is to have a handful of resistors and find the value of each
using the colour-code calculator below.
It's best to work with actual devices as you get to know what to do much
faster. Resistors come in a values from less than one ohm to more than 22
million ohms.
Obviously we can't have 22 million resistors and
so the manufacturers have produced about 60 resistors to cover all the values
you will need. These resistors can be placed in groups called decade groups and
the first groups is 10 ohms to 82 ohms, the second is 100 ohms to 820, the third
is 1,000 ohms to 8,200 ohms, then 10,000 to 82,000 and 100,000 to 820,000 and
finally 1,000,000 to 22,000,000.
Each range is called a decade range and basically the numbers in each
range repeat. This means you have to learn "pairs of
numbers," plus the value for each of the ten colours.
Let's go over this in more detail.
Each range from say 10 to 82 or 100 to 820 is a "decade value"
and within each of these ranges, there are twelve resistors. The first two
digits for the twelve resistors are the same for each range, namely: "10," "12,"
"15," "18," "22," "27," "33,"
"39," "47," "56," "68," and
"82." To increase the value in the next range, zero's are added
to the two digits to
produce values such as 1000 or 15 000 or 470 000. The result is always called OHMS.
An ohm is a very small value and for electronic work we need values from one
ohm (and less) to millions of ohms. It would be impossible to produce
resistors of every value and so a set of values has been chosen that cover the whole
range. In the early days resistors were made in very rough batches and given a
tolerance value of 10%. To make sure every resistor in the batch could be banded
with a value, the 10% "incremental staircase" was produced. It
resulted in "funny" values such as 22 ohms and 68 ohms but we have to live with the decision.
It's fortunate that these "unusual values" were chosen as it
has made electronics a very specialised field. Most people look at a circuit
board filled with "coloured devices" and don't know where to
start.
By learning the resistor colour-code, you break-through this barrier.
Now we know resistors come in funny values, we have to be able to ready them
accurately. You must not make a mistake as the wrong number of zero's will make
the resistor TEN TIMES more or TEN TIMES less than its true value - or even 100 times too much!
The colour-code calculator below will get it right for you but there's one thing
you have to remember when using the calculator.
The calculator provides a decimal point in the
answer. But when drawing circuit diagrams,
it is best to eliminate decimal points as they do not reproduce very well if the
circuit diagram has to be faxed or photocopied. To get around this we place a
letter called a multiplier (R, k or M) in the position of the decimal point. This prevents any
mis-reading of values.
Three letters are used to display the values of a resistor, they are:
R = ohms, k = kilo ohms and
M = meg ohms.
This produces values such as 0R1, 4R7, 10R, 100R, 1k,
2k2, 4k7, 10k, 27k, 39k, 120k, 470k,
1M, 2M2, 4M7 22M plus all the other values within this range.
The letters R, k and M are not normally bold. This is just to
show how they are used.
For very low values of resistance (between zero ohms and about ten ohms) the letter
R acts as the decimal
point. E.g: 0R1 for "point one ohms" 2R2 for two
point two ohms, 4R7, 6R8 etc.
To determine the value of a resistor, it should be held so that the tolerance
band gold or silver (+/-5% or +/-10%) is on the right-hand side. This gives us three coloured
bands - then the gold or silver band.
Take each resistor and hold it to match the
colour-code calculator below and "dial-up" the appropriate
colours. The answer will appear in the
window. While you are doing this, remember the "pairs of digits"
so you don't get wrong answers such as 230R or 42k, these numbers don't go
together.
There is one unusual feature of the banding system. The third band can be
coloured to produce the number of zeros in the answer or it can be gold. If it
is gold, the first two digits are "DIVIDED BY 10" to produce values from
1 ohm to 8.2 ohms. If the third band is silver, the first two digits are divided
by 100. This gives resistance values from .1 ohms to .82 ohms. Putting it all
together you have values from 0R1 to 22M.
The way to remember the two divisor bands is this: Silver has more letters in
the name and thus the division is 100. Gold has less letter and the division is
10.
TOLERANCE
As we have said, no "100 ohm" resistor is exactly 100 ohms
and the tolerance band (silver or gold) indicates the value may be 10%
(silver) or 5% (gold) higher or lower than the stated value. If you require a
very accurate value, 1% resistors are available. These have 5-bands (4 bands for
the value of resistance and 1 band for the tolerance (brown=1%). A separate 1%
resistor colour-code calculator is included in this course to identify these
resistors.
The colour-code calculator presented below covers the "standard" or
"NORMAL" 4-band resistors (5% and 10%).
Now, you can try the calculator:
Here are some samples. Use the Colour-Code calculator to find the resistance
of:
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(a) red red black gold
(b) orange orange red silver
(c) yellow purple yellow gold
(d) brown green orange silver
(e) red red gold gold
(f) brown black red gold
(g) green blue red gold
(h) brown black green gold
(i) red red green gold
(j) brown black black gold
(k) red red silver gold
(l) yellow purple brown gold
(m) brown black orange gold
(n) blue grey black silver
(o) brown green red gold
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What are the colours for these? Use the
Colour-Code calculator:
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(a) 10k 5%
(b) 12k 5%
(c) 2k7 5%
(d) 10R 10%
(e) 1k 5%
(f) R22 5%
(g) 1M 10%
(h) 120k 5%
(i) 22R 5%
(j) 82R 5%
(k) 47k 5%
(l) 3k3 5%
(m) 680k 10%
(n) 1k2 5%
(o) 100R 5%
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RESISTOR COLOUR- CODE
CALCULATOR
To use the Colour-Code Calculator: Place your resistor as shown above and
select the colours to match. Read the resistance and
tolerance
from the windows.
GRAPHICAL RESISTANCE CALCULATOR in JAVASCRIPT
Version
2.0
© Danny Goodman
Analyzed and described at length in
"JavaScript Bible"
(IDG
Books ISBN 0-7645-3022-4)
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