A coil can be used as a "pick-up" to detect magnetic or metal
It can be called a TRANSDUCER, Inductive transducer, magnetic transducer,
coil, pick-up, inductor, magnetic coil, pickup coil, inductive loop, inductive
sensor, metal detector, comparator, gold detector, crack detector - to detect
cracks in welds etc, head - as in
a tape recorder.
It can have a core made of iron or ferrite and be called an INDUCTOR.
It can have a moving iron core and be called a SOLENOID.
An inductor can be used to smooth the ripple in a supply, or it can be used to
generate a voltage higher than the supply and it can be wound on "poles" to
create a motor.
It can have an adjacent winding and become a transformer, isolator or step-up
device. It can be a current transformer, or even a linear motor.
And there are possibly dozens more . . .
THE INDUCTOR AS A "PICK-UP."
Now we come to an important feature of a coil when it is used as a SENSOR.
When a magnet passes a coil (this includes the action of moving towards or away from a coil), a voltage is generated in the turns in the
form of a sinewave.
We use the term "sinewave" to indicate the approximate shape of the
waveform to distinguish it from other shapes such as "square wave,"
"exponential" or "glitch." A sinewave is a continuing waveshape with a
gradual rise and fall as shown in the following example:
The same type of waveform is produced if the magnet passes the end of the coil, into and
out of the end of the coil or if the magnet passes through the centre.
If the magnet is moved rapidly, the amplitude of the waveform INCREASES
(and there will be more "up's ad down's" in the same time-interval on
the diagram above).
The amplitude is also determined by the strength of the magnet.
There are three important points to note.
The first is the voltage produced by the coil as it
passes the end of the coil.
When the magnet is directly opposite the end of the coil, the change in
magnetic flux is zero and thus the voltage produced by the coil is zero.
The second point is the change in voltage produced by the coil.
The output voltage changes from positive to negative during the very
small portion of excursion when the magnet moves from a forward to
reverse direction as seen by the end of the coil.
The third point is the coil produces zero voltage when the
magnet is not moving.
If the magnet moves at a faster rate, the voltage produced by the coil
will be greater as shown by the animation below:
If two or more coils are placed in close proximity
with one or more coils providing a magnetic field, when a magnet or metal
object is passed through the field, the waveform produced by the
sensing coil is altered. The size and shape may be very small but amplifying stages
can produce amazing information.
This is the principle of a metal detector (gold detector) or a coin
detector in a vending machine.
The resulting waveform can discriminate between a coin and a "slug," an aluminium can, a
"pull ring" or a gold nugget.
This type of detector is beyond this discussion. We will only be
discussing how to detect the frequency of the pulses and the amplitude of
the voltage from a single coil.
For an inductive pick-up to be successful, the magnet passing the end of
the coil must be powerful and pass near to the end of the coil.
The output of a coil will generally be less than 700mV and must be AC
coupled to a transistor so that the voltage produced by coil will modify
the biasing of the transistor as shown in the diagram below:
In the diagram below, the magnet is passing the coil and when it is
receding from the coil, the voltage produced by the coil turns the
transistor OFF and the voltage on the collector rises.
The coil can be connected to a two-transistor arrangement to produce a
square-wave output, suitable for circuits that require a noise-free
A very simple circuit using a coil is shown in the diagram below. It is
our METAL DETECTOR -1.
It uses a 16 turn coil (approx 6" dia) to detect metal objects such as
coins etc. The circuit oscillates at approx 140kHz and the frequency
produced is picked up by an AM radio to produce a quiet spot on the dial.
When a piece of metal is placed near the coil, the frequency of the
circuit is altered and this is picked up by the radio as a low frequency
tone. The circuit is extremely sensitive and
a shift in frequency of as little as a few hertz will be clearly heard.
A motor is an example of a rotating inductor. Most small motors are
3-pole (the minimum number of poles for self-starting) and if a motor is
used as a generator, the output will correspond to the speed of rotation.
For a 3-pole motor, three waveforms are produced for each revolution and
the following diagram shows the type of waveform produced:
If the motor increases in RPM, the waveform changes:
A detecting circuit can be used to count the pulses and determine the
By putting fan blades on the shaft of a motor, you can measure wind
velocity and the output of heating and cooling systems etc.
The output from the motor can be calibrated by placing the fan outside the window of
a car traveling at a known velocity.
THE INDUCTOR AS A FILTER - also known as a CHOKE
When an inductor is placed in a circuit and reduces the
ripple, as it does in the circuit below, it is also known as a CHOKE
- it "chokes off" the ripple. This is old terminology and is called
Jargon. (Jargon are words or sentences that are only understood by those
who work in the particular
field or "area" or job.)
We have already covered this feature at the beginning of this
article but since it is so important, we will go over it again.
time we will cover some of the features of a coil and
one of its hidden "magical properties." This magical property is
the ability of a coil (inductor) to produce a BACK VOLTAGE or REVERSE VOLTAGE that
opposes an increasing or decreasing voltage.
This is how the inductor smoothes (reduces) the ripple in the voltage
emerging from the rectifier.
It all starts when a voltage increases (or decreases) in amplitude.
Any increasing voltage allows an
increasing current to flow and this current increases the magnetic flux
produced by the coil and this flux cuts all the other turns of the coil
to produce a "back voltage."
This is the situation with the inductor "L" in the circuit below:
When current flows, the
voltage-waveform after the inductor, will be smaller than
the waveform entering the inductor. The size of the waveform from the
will depend on the ripple from the transformer. The diagram above shows
a waveform before and after the inductor but this is just a
representation. It is not a true indication of the size of the waveform.
A power supply contains electrolytic called filter electrolytics and the
diagram below shows one placed before the inductor and one after the
This discussion is very
complex because we are constantly discussing voltage waveforms then
current waveforms. But this is the way it has to be.
The other thing that is hard to understand is the term "DC." When we say
"DC" we mean a steady or unchanging voltage or current. Even though the
real meaning of DC is "Direct Current" we still refer to an unchanging
voltage as "DC." Again this is jargon and that's why you need to
understand the meaning of the words.
The voltage waveform entering the inductor will actually be something like 10v DC with a ripple of say
300mV and 9.5v with a ripple of 50mV leaving the
This is shown on the diagram below:
The size of the "output" waveform (from the inductor) will
depend on the inductance of L and the value of the load. If the load is
an amplifier, the current will be changing all the time according to the
level of music or speech. If the load is a globe, we call the load a
"steady load." By this we mean the current is steady.
Even if you are supplying a steady load such as a globe, the waveform
(ripple) entering the inductor will create a "back-voltage" in the
inductor that will reduce the peaks and increase the "lows."
This is due to the input waveform have a "ripple component." If the
input voltage was "pure DC," the inductor would not be needed. This is just an example of the operation of the inductor. There
will be a voltage drop across the indicator due to the resistance of the
winding. This can be proven by connecting the inductor to a battery
(such as a car battery) and measuring the voltage drop across it for any
Now we go back to our example: The output ripple is not constant.
As the current is increased, the output ripple will increase. This is
due to a number of factors.
The transformer will not be able to supply the higher current and its
output voltage will drop. The electrolytics will provide less filtering
at the higher current (see circuit below) and the inductor will become
"magnetically saturated" and not produce the same filtering.
All these characteristics will combine to produce a varying ripple
voltage on the output.
There is another factor to consider.
You simply cannot select an inductor by inductance alone. For instance,
you cannot simply say "use a 10,000uH choke." Not all 10,000uH chokes
are the same. Here is an example of 3 different 10,000uH chokes.
But before we do, remember this:
Ten thousand microhenries is the same as 10 millihenries.
1,000nH = 1µH
1,000mH = 1Henry
The following photos show different types of chokes. All have the same
inductance (10,000uH) but they will all produce a different output
because they have different coil-resistances. The second inductor seems
to be up-side-down, but the two ends are soldered to the PC board. It is
a surface-mount item.
Max current 3A
Max current 2mA
Max current 20mA
The photos above do not show the real size of the components but you can see
each has a maximum current rating.
The current rating is the dominant factor.
That's why a 10mH choke can have so many different sizes.
THE INDUCTOR IN A POWER
An inductor can be used in a power supply to reduce the ripple.
In most power supplies this is done with electrolytics, however we will
explain how the inductor operates.
The inductor only operates on the fluctuating part of the voltage.
If the voltage rises, it increases the magnetic flux and this is called
expanding flux. This flux cuts all the turns of the coil and produces a
voltage in the turns that is opposite to the incoming voltage.
Thus the increasing voltage does not enter the inductor.
This is how the inductor reduces the ripple on the output.
In the following circuit, the output has a reduced ripple due to the
The following diagram shows an inductor with a closed magnetic circuit.
The "magnetic circuit" is shown via the arrows:
"open" magnetic circuit is shown above via the colour-coded inductor.
The "magnetic circuit" is the core but the flux does not form a
continuous loop. This type of magnetic circuit is very inefficient and
has high losses. But it is quite acceptable if the magnetic flux is kept
to a low level where the core is not saturated.
DESIGNING AN INDUCTOR AS A FILTER (choke)
Designing an inductor for the above application is a very complex
mathematical problem. We do not know the ripple on the input to the
inductor or any of the voltage values as a small transformer as show in
the diagram has a very poor regulation factor and the output can be up
to 60% higher than the stated output so that the voltage drops to the
required level on full output current.
The best thing is to have a range of inductors and try them.
If you are winding your own inductor it is best to add too many turns to
a core and gradually remove them as needed.
Nothing beats "actual application" as there are too many variables to be
able to design something from a chart.
If you don't know where to start, look at a power supply. Remember the
frequency of this power supply is either 50Hz or 60Hz. It is not a 40kHz
design. The inductance will be much higher than for a 40kHz circuit.
The same approach is recommended for any type of inductor.
An inductor is very difficult to design.
Projects do not always consume a constant current and the ripple present
on the unfiltered side of a power supply needs to be viewed on a CRO.
Rather than using complex mathematical formulae to work out the value of
an inductor, the easiest way is to try different values.
THE INDUCTOR IN "FLYBACK"
We mentioned above, one of the "magical properties" of an inductor is
its ability to produce a "back-voltage" or "reverse voltage."
There is an even-more-magical extension to this.
If a voltage is applied to an inductor and then removed, the
back-voltage will be VERY HIGH. It can be 100 times (or more)
higher than the applied voltage.
This is the principle of the ignition system in a car. The distributor
connects the ignition-coil to the battery of the car via the points and
then the points open.
The collapsing magnetic filed in the ignition-coil is passed from the
end of the coil to the distributor rotor-cap, and this is the rotating part of the
distributor. It sends the 20,000v to the appropriate spark-plug.
So, the collapsing property of an inductor has been known for a long
time and it has also been used in electronics for many applications.
Uses such as electric fences, high voltage generators, EHT circuits in
TV's, switch-mode power supplies.
This back-voltage is also produced by relays, motors, door-latches and
where-ever a coil is energised.
In most cases this voltage is higher than the operating voltage of the
components in the circuit and it must be prevented from damaging them.
In the case of relay, the back-emf, (this is what the back-voltage is
called) is snubbed (reduced) by the diode. The back-voltage has the
opposite polarity to the supply and this voltage is placed directly
across the diode and it forms a very low resistance path to absorb the
Another way to prevent the high voltage damaging the circuit is to
provide high-voltage insulation and shielding. Apart from
spike-suppression capacitors across the chips, there is no other way to
prevent high voltages damaging a circuit.
The following circuit
uses an inductor to separate the 3.6v across a 1 watt LED from a 15v
This circuit explains the secret: "HOW AN INDUCTOR WORKS."
The resistance of the inductor is less than one ohm and it sees about
14v on the left lead and 3.6v on the right lead.
So . . . how does it work?
How does it allow 14v to be present on one side and 3.6v on the other
side. The 1R resistor between the inductor and LED only has about 0.3v
Obviously the resistance of the inductor is not creating the 14 - 3.6 =
10.4volts across the leads.
It must be something to do with the turns of wire on the ferrite core.
On the right lead of the inductor is a 100u electrolytic. This capacitor
has the effect of preventing the voltage rising or falling. It keeps the
voltage fixed and stable.
When the BC327 transistor turns ON, it sends a pulse of current to the
left lead of the inductor. This current enters the inductor and produces
magnetic flux that cuts all the turns of wire and produces a voltage in
the opposite direction.
This voltage is nearly the same value as the voltage entering the coil
and the result is very little current will flow though the inductor.
This current is exactly the correct amount to fully illuminate the LED.
The inductor can only prevent too much current flowing FOR A VERY SHORT
PERIOD OF TIME.
That's why the oscillator is a very HIGH FREQUENCY.
That's the secret behind the operation of the inductor.
It creates a voltage across it that limits the current flowing through the
Of course the inductor must be selected to have the correct number of
turns and the correct wire-size and correct ferrite-size.
This discussion has covered only a fraction of the mysteries of the
inductor. There are special ways to wind an inductor for high voltage
circuits. There are special ways to reduce the inductance and there are
special cores to increase the inductance.
There are ways to tap an inductor to get all sorts of results such as
high current, high impedance, low impedance and impedance-matching. You
can add a second, third and fourth winding to get isolation,
impedance-matching, high voltage or high current. Inductors can be made
long, short, fat or any shape to fit onto a circuit.
That's why there are so many shapes, sizes and pin-outs.
To understand "the Inductor" you need to make a simple circuit that you
can adjust and change the physical properties of the inductor.
The following circuit is a perfect example for experimenting. It uses a
single 1.5v cell to illuminate a white LED. We know a white LED requires
at least 3.2v for it to illuminate, so the circuit is "magically"
increasing the 1.5v to a higher voltage.
The coil (the two coils) is actually a transformer and it is acting in
It has no core (just air) and you can separate the turns and see what
effect it has on the operation.
BIKE FLASHER - Amazing!
This bike flasher uses a single transistor
to flash two white LEDs from a single cell. And it has no
core for the transformer
- just AIR!
All Joule Thief circuits
you have seen, use a ferrite rod or toroid (doughnut) core and the turns
are wound on the ferrite material. But this circuit proves the
collapsing magnetic flux produces an increased voltage, even when the
core is AIR. The fact is this: When a magnetic filed collapses quickly,
it produces a higher voltage in the opposite direction and in this case
the magnetic field surrounding the coil is sufficient to produce the
energy we need.
Wind 30 turns on 10mm (1/2" dia) pen or screwdriver and then another 30
turns on top. Build the first circuit and connect the wires. You can use
1 or two LEDs. If the circuit does not work, swap the wires going to the
Now add the 10u electrolytic and 100k resistor (remove the 1k5). The
circuit will now flash. You must use 2 LEDs for the flashing circuit.
The original 30 turns + 30 turns coil is
shown on the right. The circuit took 20mA to illuminate two LEDs.
BIKE FLASHER - AMAZING!
THE IMPROVED BIKE FLASHER CIRCUIT
The secret to getting the maximum energy from the coil (to flash the
LEDs) is the maximum amount of air in the centre of the coil. Air cannot
transfer a high magnetic flux so we provide a large area (volume) of low
flux to provide the energy. The larger (20mm) coil reduced the current
from 20mA to 11mA for the same brightness. This could be improved
further but the coil gets too big. The two 30-turn windings must be kept
together because the flux from the main winding must cut the feedback
winding to turn ON the transistor HARD.
When the transistor starts to turn on via the 100k, it creates magnetic
flux in the main winding that cuts the feedback winding and a positive
voltage comes out the end connected to the base and a negative voltage
comes out the end connected to the 100k and 10u. This turns the
transistor ON more and it continues to turn ON until fully turned ON. At
this point the magnetic flux is not expanding and the voltage does not
appear in the feedback winding.
During this time the 10u has charged and the voltage on the negative
lead has dropped to a lower voltage than before. This effectively turns
off the transistor and the current in the main winding ceases abruptly.
The magnetic flux collapses and produces a voltage in the opposite
direction that is higher than the supply and this is why the two LEDs
illuminate. This also puts a voltage through the feedback winding that
keeps the transistor OFF. When the magnetic flux has collapsed, the
voltage on the negative lead of the 10u is so low that the transistor
does not turn on. The 100k discharges the 10u and the voltage on the
base rises to start the next cycle.
You can see the 100k and 1k5 resistors and all the other parts in a
"birds nest" to allow easy experimenting.
Note: Changing the turns to 40t for the main winding and 30t for
the feedback (keeping the turns tightly wound together by winding wire
around them) reduced the current to 8-9mA.