After Class: Using Inductors
December 1957 Popular Electronics
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.
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April
1985. As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby acknowledged.
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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.
Ordinary resistors 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.
Fig. 1. Rectifier changes a.c. to pulsating d.c.
Fig. 2. Filter smoothes pulsating direct current.
Fig. 3. "L" blocks high frequency, "C" passes it.
Fig. 4. The toroid has a doughnut-shaped core.
Fig. 5. Solenoid is wound around straight core.
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 .
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.
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
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 30 henrys.
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.
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 cores.
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.
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.
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 frequencies.
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.