216
Electricity and Basic Electronics
cores. Some of these cores contain an organic
binder that has a tendency to break down with
high temperature applications. Although iron
tends to heat up in some circuits, ferrite (iron)
and steel cores are still being used in audio
frequency circuits. Selecting the right core for
the right job requires a fair amount of knowl-
edge and study.
Unit of Inductance
The unit of inductance is the henry (H).
An inductor has an inductance of one henry
when one volt is induced while the current
is changing at the rate of one ampere per
second. Most inductors fall into the range
of millihenrys (mH) to the smaller range of
microhenrys (μH).
Inductor Applications
Inductors are combined with other com-
ponents for use in many specifi c types of
circuits. For example, inductors are combined
with capacitors to form a wide variety of fi lter
circuits. Adjustable ferrite core inductors are
used in radios to tune a circuit by passing or
rejecting the specifi c frequencies.
Inductors have uses in signal processing,
switching power supplies, metal detectors, dc
to dc converters, analog circuits, and adjust-
able speed drives. Part of their job is to limit
voltage spikes, smooth out ac ripple, and
reduce surges.
There is one use for inductors that almost
everyone overlooks, even though it is very large
and is encountered almost every day. In fact, it
is so large, you have to drive your car through
it. The next time you pull up to a signal light,
you might notice something that looks like
lines cut in the road in the shape of a big box.
This is an inductor circuit. A wire is placed
into a cut in the concrete and covered with a
rubber material to seal it in place. When you
drive your car into the “box,” you are placing
a large piece of steel inside the inductor. This
creates a magnetic fi eld which triggers a timer
that changes the traffi c light signal.
Inductor Cores
A number of inductors are copper wind-
ings with air cores. Other materials are used
inside the coil to reduce their size, make them
more effi cient, and cut their overall cost. Some
of the newer inductors are built with powder
Figure 12-9. Inductors are labeled with the
capital letter L on a circuit board and come in a
wide variety of shapes, sizes, and colors, like the
blue and green ones shown here.
Inductor
Figure 12-10. This inductor, or high frequency
choke, has an adjustable ferrite core. Many of
these variable inductors are used in radios and
television sets.
Coil
Ferrite
core
Adjustment
screw
Internally threaded
coil form made from fiber