When radio moved, in the 1920s, out of the laboratory and into widespread use, some enterprising person hit on the idea of installing radio receivers in automobiles. Batteries, and lots of them, powered those early car radios; but, understandably, drivers of the time balked at the number of large batteries required for the average receiver.
A partial solution arrived with the advent of vacuum tubes with 6-volt filaments that could be lit by the car’s electrical system. This still left the problem of the high voltage, in the neighborhood of 150 – 250 volts, required by the tubes’ electrodes. If some means of converting the direct current (DC) output of the car’s battery to alternating current (AC) could be found, then that voltage could be stepped up with a transformer and rectified to produce the necessary higher voltages.
With the invention of a device then known as a vibrator, the problem was solved. The vibrator, encased in a metal can, consisted of a coil, a spring-loaded armature, and relay-style switch contacts. These contacts were used to break the energizing voltage, and to create the AC voltage necessary for the transformer/rectifier system.
In operation, voltage from the car battery energized the coil, pulling the armature toward one set of contacts. This caused the circuit energizing the coil to be broken, and the armature would snap back to its original position at the other set of the contacts. The coil was then re-energized, and the cycle repeated again (thus the “vibrating” action). At the same time, current flowed from the vibrator’s output, but alternated between positive and negative (with respect to ground), resulting in the production of an AC voltage (in the form of a square wave). This voltage was fed into a special ‘vibrator transformer’ and stepped up. The resultant high voltage could then be converted back to DC by a rectifier tube and used to provide operating voltages for the receiver’s tubes. A vibrator used in this fashion was called a non-synchronous vibrator.
Later refinements included the development of synchronous vibrators, fitted with an additional set of contacts that reversed the action of the set of contacts producing the AC voltage. Stepped-up voltage from the transformer was fed back to these additional contacts, resulting in DC voltage and eliminating the rectifier tube. Both types of systems became the standard for car radio power, and also found their way into receivers designed for areas (such as many farms of the time) without AC power lines.
Vibrator power worked well, but at a loss; most vibrator power supplies operated at around 60 percent efficiency. It was, however, the only method available until solid state receivers (with their lower voltage requirements) replaced tube-based receivers in the 1960s.
SOURCES
The American Radio Relay League Staff, The Radio Amateur's Handbook, various editions. Newington, Connecticut: ARRL Publications.