March 1958 Popular Electronics
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Pliers of the amateur radio hobby have since the beginning put
forth a lot of effort training fledgling entrants in the realm
of electronics and communications. Up until the latter part
of the last century, there were a number of magazines - Popular
Electronics among them - that would regularly print articles
covering the basics of electronics. Other than the ARRL's
QST magazine,
it seems maybe
Nuts and Volts is the only monthly still in print that you
can go to for such information. I suppose it was inevitable
with the emergence and now domination of the Internet as a source
for most knowledge. The Among the Novice Hams column
in Popular Electronics often included short primers on subjects
like the basics of capacitors and inductors. Here is one on
inductors from March 1958. The basics still apply.
Among the Novice Hams
By Herb S. Brier, 9EGQ
In the January column, as part of our discussion of the basic
electronic theory on which the General / Conditional / Technician
license examination is based, we talked about capacitance and
capacitors. This month, we'll cover inductance and inductors,
which are also referred to as coils, chokes, and reactors. First
let's learn a bit about current and magnetism before taking
up inductance itself.
Current and Magnetism. Imagine that a source of direct current,
such as a battery, is connected across the ends of a length
of wire or other conductor. An electric current, which consists
of electrons in motion, will flow through the wire. If we bring
a magnetic compass near the wire, the compass needle will be
deflected from its normal position. The greater the current
flowing through the wire, the more the needle will be deflected.
If we reverse the battery terminals, it will be deflected in
the opposite direction.
We have shown that electrons
in motion (electric current) in a conductor generate a rotating
magnetic field around the conductor. We have also found that
the direction in which the electrons are moving determines the
direction of rotation of the magnetic lines of force. According
to the "left-hand rule," when a conductor carrying current is
grasped in the left hand with the thumb pointing in the direction
in which the current is flowing (towards the positive terminal),
the fingers point in the direction of rotation of the magnetic
field.
If we substitute a sensitive microammeter for
the battery across the ends of the conductor and rapidly move
a powerful permanent magnet across the conductor, the meter
pointer is momentarily deflected. The direction in which the
magnet is moved determines the direction in which the meter
pointer is deflected. The speed of the magnet determines how
much the pointer is deflected.
Thus, a magnetic field
moving across a conductor induces (causes to flow) a current
in the conductor. A current will also be induced in the conductor
if the magnet is held still and the conductor is swept across
its poles.
The magnitude of the effect is small in a
straight length of wire. If the wire is wound into a coil like
thread on a spool, the effect is greatly increased. Then the
magnetic lines of force around the wire act upon each turn and
on adjacent turns as well. Figure 1 illustrates this action
two-dimensionally.
If we insert a soft iron core inside the coil, even more
current flow takes place, because the magnetic lines will travel
through the iron much easier than through air. Consequently,
the iron core concentrates the magnetism around the turns of
the coil.
Fig. 1. How magnetic lines of force around
a conductor carrying current (A) are concentrated by winding
the conductor into a coil (B); total magnetic flux path around
the tightly wound coil is shown in (C).
Self Inductance.
Suppose we connect a coil containing thousands of turns of
wire wound around an iron core, a source of direct current,
a voltmeter, an ammeter, and a switch, as shown in Fig. 2. When
the switch is closed, the voltmeter immediately indicates the
full battery voltage across the coil terminals. But the ammeter
pointer moves slowly up to a position determined by the resistance
of the wire in the coil and the applied voltage.
When the switch is opened, however, the ammeter pointer
immediately drops back to its zero position, but the voltmeter
pointer flips up far beyond its previous position before it
drops back to zero. There will also probably be quite a large
spark across the opening switch contacts.
What
happened? When the switch is first closed and current starts
to flow into the coil, a strong magnetic field starts to build
up around the coil. This expanding magnetic field is moving;
therefore, it builds up an electromotive force of its own in
the coil. This induced electromotive force is exactly opposite
to the applied electromotive force. Consequently, it opposes
the flow of current into the coil-but it cannot cut off the
current completely. If it did, there would be nothing to generate
the magnetic field. So the current slowly increases to its steady
value and supports a steady magnetic field around the coil,
but the process does take time.
When the switch
is opened, the incoming current drops instantly to zero and
kicks the props out from under the magnetic field, which is
thus forced to collapse instantaneously. While it collapses,
the energy it contains is instantly converted back into an electromotive
force in the coil, which builds up in voltage until it is sufficient
to arc across the open switch contacts.
These
effects are due to the inductance of the coil, which is measured
in henrys. By definition, a change of one ampere per second
in the amount of current flouring through an inductance of one
henry generates an electromotive force of one volt in it. The
technical name for a coil containing inductance is an inductor.
In radio work, the terms millihenry (0.001 henry), abbreviated
mh., and microhenry (0.000001 henry), abbreviated μh., are also
used.
Applying A.C. Figure 3 shows what happens to the
current and voltage in an inductor if an a.c. generator is substituted
for a d.c. generator.
For simplicity, let us assume
that the a.c. generator voltage is maximum (point A) when we
close the switch. Immediately, this voltage tries to force current
through the inductor. But zip! The resulting magnetic field
immediately generates a counter voltage in the inductor, which
sharply limits the amount of current that can flow into it.
However, as time passes, the generator gradually forces more
current into the inductor, even though the generator voltage
is decreasing at the same time, until 1/4 cycle or 90° later
(point B), the current reaches its maximum value, just as the
generator voltage has decreased to zero.
Immediately, the a.c. generator voltage starts increasing
in the opposite (negative) direction and tries to force a current
through the inductor in that direction. But, as soon as the
current tries to reverse direction, the magnetic field generated
by the current flowing in the original direction starts to collapse,
and its energy is converted back into an electromotive force
that tends to keep the current flowing in the original direction.
Fig. 2. Theoretical circuit used to illustrate
the meaning of inductance as discussed in the text.
Fig. 3. Current and voltage relationships
in an inductive circuit when alternating current is applied.
At first, the electromotive force from the collapsing field
is strong; so the current is high. As the cycle continues, however,
this energy is used up, while the generator voltage is increasing.
Thus, at the end of 1/2 cycle or 180° (point C), the current
has decreased to zero, just as the generator voltage reaches
its maximum negative value.
At this point, current starts flowing into the inductor
in the opposite direction, and the action of the current and
voltage is like that of the previous half cycle. At the end
of a complete cycle (point E), the current and voltage relations
are exactly as they were when the switch was closed. These series
of actions continue as long as alternating current is fed into
the inductor.
Inductive Reactance. Obviously, inductance
opposes the flow of alternating current through it. This opposition
is called inductive reactance and is measured in ohms. The formula
for calculating it is:
X
_{L} = 2 π FL;
where π (pi) 3.14, F is the frequency in cycles per second,
and L is the inductance in henrys. The formula is also correct
if the frequency is expressed in kilocycles and the inductance
in millihenrys, or the frequency in megacycles and the inductance
in microhenrys.
Don Jensen, KN6VXM, worked the 48 states
and Europe with a home-brew 6146 transmitter running 50 watts.
Now he uses a new Johnson Ranger transmitter.
An example
will show that there is nothing mysterious about the formula.
Question: What is the inductive reactance of a 10-henry choke
(inductor) at a frequency of 60 cps? Answer: X
_{L} =
2 X 3.14 X 60 X 10 = 3768 ohms. At 600 cycles, its reactance
is 37,680 ohms. Inductive reactance is directly proportional
to frequency and inductance.
This is just the opposite of capacitive reactance, where
the reactance is inversely proportional to frequency and capacitance.
Another difference between inductive and capacitive reactance
is that, in a purely capacitive circuit, the current leads the
voltage by 90°, while in a purely inductive circuit the current
lags the voltage by 90°.
Posted August 13, 2012