December 1957 Popular Electronics
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This installment of the After Class series in
the December 1957 edition of Popular Electronics deals with inductors. It is a beginner level introduction
to how reactive components behave in circuits. I am tempted to say that back in the era, people were
less familiar with the relatively new concept of electronics, but in thinking about it, your typical
2011 reader is probably even less likely to know anything at all about electronics or the way basic
components work. I would bet that maybe 1% could even tell you the difference between AC and DC current
- or the difference between current and voltage for that matter.
After Class: Special Information on Radio, TV, Radar and Nucleonics
"I know what an inductor is - I think but I'm pretty hazy as to its use in an electronic circuit.
And while we're talking about inductors, exactly what's the difference between a solenoid and a toroid,
which I understand are two forms of inductors'!"
Picture a complicated highway interchange with no directional signposts, traffic lights, or highway
police, and you have a pretty good idea of what an electrical circuit would be like without "oppositional"
components that guide, cajole, and coax electrons to follow prescribed paths. Resistors, capacitors,
and inductors - occurring singly or in various combinations comprise the fundamental oppositional
elements that direct electronic traffic so skillfully even in complex circuits.
are undiscriminating components; they oppose d.c., a.c., high frequency, and low frequency alike. A
given current of any type passing through a specific resistor causes the same voltage drop. Capacitors
are different; they oppose changes of voltage in any circuit of which they form a part but do not react
significantly to rapid current variations.
Inductors vs. Capacitors. An inductor
may be viewed as the exact opposite of a capacitor. Made up of turns of wire in the form of a coil,
an inductor opposes current rather than voltage changes, especially when these variations occur at a
high frequency. This characteristic makes the inductor a useful complement to the capacitor in a power
supply filter circuit.
A full-wave rectifier circuit changes alternating current to pulsating direct current (Fig. 1). This
"hill and valley" formation of pulsating d.c. is usually not suited for operating d.c. electronic equipment
until the "valleys" are filled in .
Fig. 1. Rectifier changes a.c. to pulsating d.c.
Consider the filter circuit shown in Fig . 2. As the pulsation appears across the capacitor C, the
latter charges to the peak voltage (points labeled A) of the input wave. When a valley appears (points
labeled B), the capacitor attempts to discharge through the inductor L into the load, which may be any
device that uses direct current.
Fig. 2. Filter smoothes pulsating direct current.
Since such a discharge would constitute a sudden surge of current, the opposition that inductor L
offers to such an abrupt change forces C to retain most of its charge until the next peak appears to
bring it back to full charge again. In this way, L assists C in maintaining the circuit voltage constant,
an action necessary to change pulsating current into pure d.c. In this circuit, L is termed a choke.
Opposition Factors. The opposition an inductor or choke presents to varying currents
depends upon two factors: (1) how fast the current varies (frequency), and (2) the physical construction
of the coil - the number of turns, turn spacing, coil diameter, and kind of core. A filter choke of
the type just described has hundreds of turns of fine wire, an iron core, and is said to have a high
inductance. Just as resistance is measured in ohms, e.m.f. in volts, and current in amperes, inductance
is measured in henrys. The symbol "L" signifies inductance. Smoothing chokes usually run from 10 to
Any time a circuit condition calls for choking off a rapidly varying current, an
inductor of the correct size used in conjunction with a properly chosen capacitor can do the job. For
example, suppose an electron tube is generating a current of high frequency, say 1,000,000 cycles per
second (1 mc.). This tube is fed d.c. voltage and current from a power supply, but the high-frequency
current is not to be permitted to flow backward through the supply.
Connecting an inductance and capacitor as shown in the partial schematic (Fig. 3) fulfills this requirement
by permitting the d.c. to flow through the inductor but opposing the high-frequency current back-flow
through the inductor. The capacitor provides an easy path for the high-frequency current to return to
the tube - a necessary arrangement to assure a complete circuit for this energy. An inductor like this
has relatively few turns wound on a ceramic or plastic core; its inductance might range from 2 to 10
millihenrys (0.002 to 0.01 henry). Because the frequency is so high, not much inductance is needed to
place a barrier in front of the incoming varying current.
Fig. 3. "L" blocks high frequency, "C" passes it.
In summary, an iron core coil of many turns has a high inductance and is especially useful as a choke
in low-frequency circuits; high-frequency hookups call for low-inductance coils of few turns on nonmagnetic
Toroids and Solenoids. Over the years, toroids have been used desultorily by many of the larger overseas
communications companies for the special filter applications in which they perform particularly well.
The last decade has seen a steady growth in the number of circuits and devices in which toroid ally
wound coils have come to be preferred over solenoid types.
The word toroid describes a method of coil winding.
Fig. 4. Turns are placed around a doughnut-shaped core which forms an absolutely closed magnetic
circuit as illustrated. A solenoid, on the other hand, is generally defined as a coil fabricated on
a straight core, sometimes iron, and other times air or non-magnetic solids like plastics or fiber -
see Fig. 5. Certain types of transformers have two separate solenoid coils on a common core.
Fig. 4. The toroid has a doughnut-shaped core.
Fig. 5. Solenoid is wound around straight core.
What is the secret of the toroid's sudden rise to prominence? Known for many years to be vastly superior
to solenoids in certain important respects, toroids were avoided by designers and manufacturers for
two reasons. Suitable core materials were not available, especially in applications involving the higher
audio frequencies and the radio frequencies. And automatic machines for winding turns around a doughnut
core were virtually unobtainable at any price.
OPEN MAGNETIC CIRCUIT-STRONG EXTERNAL FIELD
Neither of these problems exists
today. High-speed automatic toroid winders form these inductors just as quickly as solenoids; and modern
core materials like Permalloy dust, the ferrites and mu-metal make them practical for use at the higher
The "open" magnetic circuit of the solenoid forces its lines of force to travel
through the surrounding open space to link with the core at both ends, whereas the "closed" path of
the toroid core confines the field to the molecules of the core so that there is little or no external
field. Two very significant advantages arise from this behavior: first, it makes possible higher inductances
with fewer turns of wire, and second-probably much more important it does away with the need for careful
shielding. Where there is no external field surrounding inductors, there can be no electromagnetic coupling
to cause unwanted regeneration and oscillation.
In general, toroids may be stacked one on top
of the other in the same case without interaction for the reason given above. Their most important use
at present is in wave-filters (resonant and non-resonant traps and peaking circuits). They may be made
very stable and unresponsive to temperature changes and vibration, and may be designed with very high
Q's even at frequencies up to 60 kc. or 70 kc.
Look for toroid transformers to replace solenoid
types in many of the forthcoming high-fidelity systems still on the design boards.
Posted August 3, 2011