December 1957 Popular Electronics[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. All copyrights are hereby acknowledged. See all articles from 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.
"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'!"
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 .
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 choke.
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.
Posted August 3, 2011