Retrotechtacular: DC to DC Conversion by Vibrator


Electricity comes in two basic forms: Alternating Current (AC) and Direct Current (DC). DC is handy to use and is easy to analyze. However, AC has some useful properties too. In particular, AC current can operate a transformer which can step it up or down easily. Power is conserved, of course (well, actually, you get less power because of losses in the transformer).

You can’t do that trick with pure DC. You can reduce a voltage, although that typically wastes power in heat (for example, a voltage divider or linear regulator). You can’t readily increase a DC voltage unless you convert it into some sort of AC first.

This was a particularly bad problem in the era of tubes–especially tubes in car radios. The car’s voltage was probably 12V but the tube’s plates might take hundreds of volts. What do you do? Some old car radios used what is called a dynamotor. This is just a motor and a generator in one box. You could spin the motor with 12V and have the generator produce a different voltage (even a DC voltage).

The Electric Dynamotor

If you think about it, a transformer is really just a generator (the secondary) with the shaft replaced by a moving magnetic field (the primary). However, with a dynamotor, you can use a DC motor to spin the input. With a transformer, you must have AC input because that’s what moves the magnetic field. Note you don’t have to have a sine wave, necessarily, just current that switches polarity fast enough to drive the primary.

So how do you get the DC in a car radio into AC? There’s a clue in another part of the car: the blinkers used to flash the lights. Today, you’d probably do the blinking with some electronics, but old cars didn’t have a lot of electronics. What they did have was a special kind of device. When you apply current to the device, it would heat up a bimetallic strip. The strip would slowly move and break the current flow. This would cool the strip down which would resume its original position and start the current flowing again.

If you wanted things to go faster, you could do the same trick with a relay. Have the relay coil take current through a normally closed contact. When you energize the coil, the contact opens and breaks the current flow. The contacts then close and the process repeats.

Testing, 1, 2, 3

RCA-tube-tester-at-Oklahoma-History-CenterOld tube testers in the drug store (see right) often had a test for vibrators. In those days, that didn’t have the meaning you’d expect today. In fact, a vibrator was a relay wired to interrupt DC current into AC. You might expect that these failed pretty often and a trip to the drug store would let you test vibrators and tubes, buy a replacement, and repair your own devices.

The name vibrator came from the characteristic buzzing noise made by these devices. Once the DC was broken up by the vibrator, a transformer could step the voltage up (or down, but it was almost always up). This produces a higher AC voltage that the circuit would then rectify and filter to get the desired higher voltage.

Heathkit_VibratorThere was actually a lot more to making a good vibrator power supply than you might think. The mechanical parts moved constantly. Also, the contacts would spark and that would eat at the contacts. It also created a lot of interference or hash. A good design would suppress sparks and hash. The resulting rectified current needed a good bit of filtering, too. The photo to the left shows a pair of Heathkit vibrators, including the internal structure that was normally hidden.

Although the vibrator seems inefficient, it beat the dynamotor. It was quieter, had fewer moving parts, was smaller, and cheaper. At the time, it seemed like progress.

Today

Vibrators are long gone except in vintage gear. Transistor power inverters became practical and edged them out. Today, you would be more likely to use a switching mode power supply to get the same effect. The principles aren’t that different, but the conversion of DC to AC is electronic and the control is more precise.

If you want to see more about vibrators–including schematics–check out the video below.

Vibrator photo By WikimediaFoundationVEVO (CC BY-SA 4.0)



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