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TERMINOLOGY: A coil can be used as a "pick-up" to detect magnetic or metal objects. It can be called a TRANSDUCER, Inductive transducer, magnetic transducer, coil, pick-up, inductor, magnetic coil, pickup coil, inductive loop, inductive sensor, magnetic 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 signal:
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.
ROTATION
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
RPM.
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.
This
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
bridge
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
inductor.
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
inductor.
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
given load.
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.
We will look into how to chose the correct choke.
But before we do, remember this:
Ten thousand microhenries is the same as 10 millihenries.
1,000nH = 1µH
1,000µH
= 1mH
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.
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10milliHenry
2R7 Max current 3A |
10milliHenry
15R Max current 2mA |
10milliHenry 33R Max current 20mA |
The photos do not show the actual size of the components but you will see
each has a maximum current rating.
Here is a photo of comparing the size of each component:
This rating is determined by the size of the core.
In the circuit below, there are two components of the current flowing
through the inductor. There is a DC current
This can be shows on a graph thus:
Where the DC component of the current is 300mA and the AC component is
the ripple
When a current flows through an inductor it produces magnetic flux (this
current is called a "DC current") and if this current is too high it
will saturate the core and prevent any increase in current
As an experiment, we can use two different types of inductor. One
inductor will consist of a few turns of thick wire on a large core. The
other will have many turns of fine wire on a small core.
But the end-result will be different.
The large inductor will produce a large output voltage with say 50mV
ripple. The small inductor will produce a lower output voltage with the
same ripple. The 50mV ripple does not apply to the circuit above as
electrolytics have been omitted.
The reason is both inductors have a "DC resistance" due to the
resistance of the wire and this causes the "voltage-drop" (as well
as the effect of the inductor).
That's why it is important to know the DC resistance of an inductor.
Some inductors also have a "current rating."
An inductor will "heat-up" when passing a current and it is very
difficult to know how much current can be allowed without seeing its
physical size.
There is one other physical feature of an inductor that must the
remembered.
When an inductor is placed in a circuit with a "DC component," it must
have an air gap in the magnetic circuit.
The air gap is only the width of two sheets of paper but it "absorbs" or
"loses" a lot of magnetic flux and this is the flux produced by the
current flowing through the coil.
The air gap prevents the magnetic circuit being saturated by the DC
current flowing through the circuit and the inductor can now perform
the task of reducing the AC waveform (reducing the ripple).
In the following circuit, the output section has a "DC component" and a
ripple.
The "DC component" is made up of a voltage that may be 9v plus a ripple
of 30mV. In other words the output voltage will be 9.015v at its highest
and 8.985v at its lowest, and the current may be 300mA.
The inductor will be required to handle at least 300mA.
The following diagram shows an inductor with a closed magnetic circuit.
The "magnetic circuit" is shown via the arrows:
An
"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.
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
energy.
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.
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.
You need to find a simple circuit and wind your own inductor. A good place to start is the Solar Light project
20/6/07