Wednesday, May 16, 2012

What Is Old is New Again: Soft-Switching and Synchronous Rectification in Vintage Automobile Radios

I have to admit I am a bit of a vintage electronics technologist.  One of many pass times includes bringing vintage vacuum tube automobile radios back to life. In working with modern DC sources I’ve seen innovations come about in the past decade for efficient power conversion, including soft switching and synchronous rectification. A funny thing however, for those who have been around long enough, or into vintage technologies like me, is that these issues and somewhat comparable solutions existed up to 70 years ago for automobile radios and other related electronic equipment. What is old is new again!

As we know, vacuum tubes (or valves to many) were to electronics back then as what semiconductors are to electronics today. The problem for portable and mobile equipment was that the vacuum tubes needed typically 100 or more volts DC to operate. They did have high voltage batteries for portable equipment but for automobiles the radio really needed to run off the 6 or 12 volts DC available from the electrical system. The solution: A DC/DC boost converter!

Up until the mid 1950’s most all automobile radios used vacuum tubes biased with high voltage generated from a rather primitive but clever DC/DC boost converter design. The inherent technological challenge was semiconductors did not yet exist to chop up the low-voltage, high-current DC to convert it to high-voltage, low-current DC. Of course if the semiconductors did exist this would all be a moot point! Making use of what was available the DC/DC boost converters employed what were called vibrators, which are a form of a continuously buzzing relay, to chop up the low-voltage DC for conversion. Maybe some of you are familiar with the soft humming sound heard when an original vintage automobile radio is turned on, prior to the vacuum tubes finally warming up and the audio taking over? That humming is the vibrator, the “heart” of the DC/DC boost converter in the radio.

Figure 1 below is an example circuit of vibrator-based DC/DC boost converter in a vintage automobile radio. This is just one of quite variety of different implementations created back then. Two pairs of contacts in the vibrator act in a push-pull fashion to convert the low-voltage DC into a low-voltage AC square wave. This in turn is converted to a high-voltage square wave by the transformer. Because the vibrator is an electro-mechanical device, it is limited in how fast it can switch. Switching frequencies are typically about 100 to 120 Hz. The transformers used are naturally the steel-laminated affairs similar in nature to the transformers used to convert household line voltage in home appliances. Very possibly some radio manufacturers used off- the-shelf appliance transformers in reverse to step up the voltage!  Often a small rectifier vacuum tube, such as a 6X4 (relatively modern, by vacuum tube standards) would be used to convert the high voltage AC to high voltage DC, but in this particular example I am showing here another two pairs of contacts on the secondary side switch simultaneously with the first pairs of contacts to rectify the high voltage AC. Highly efficient synchronous rectification, up to 70 years ago!

Figure 1: Representative DC/DC boost converter for a vintage automobile radio

The clever part of these DC/DC boost converters is making the vibrators last. Let’s see; 100 cycles/second, times 60 seconds/minute, times 60 minutes/hour, times ~2 hours/day, times 365 days/year; that’s 263 million cycles in one year! And while the vibrator was replaceable, it would often last for many years or more, which is quite remarkable. The trick was paying close attention to the switching as to not stress the vibrator‘s contacts. Referring to the waveforms in Figure 2, there is quite a bit of dead time between the non-overlapping switching of the contacts. This was by design. The capacitor across the secondary of the transformer in Figure 1 is carefully matched to ring with the transformer’s inductance such that the voltage is near zero across the alternate set of contacts is just as they’re closing, minimizing arcing and wear. Low-stress soft switching, again, up to 70 years ago! Ironically the cause for the vibrator failing was often due the capacitor degrading with stress and time. The capacitor was actually slightly larger than ideal value at the start to prevent overshoot and allow for aging. When resurrecting a vintage automobile radio frequently the vibrator will still work. Make certain to replace the capacitor first however or the vibrator is bound to have a very short second life.

Figure 2: Switching waveforms in a vibrator-based DC/DC boost converter

These vacuum tube automobile radios with vibrator-based DC/DC boost converters had quite a long run before being displaced, first for a very short period in the later 1950’s by hybrid radios using low voltage vacuum tubes and early germanium power transistors, and then finally overtaken by fully transistorized automobile radios in the early 1960’s.

So my hat’s off to the many design engineers of yesteryear who encountered such challenges, fully understood the principles, and just as creatively came up with solutions for them so long ago, based on what they had available. And again for those seasoned engineers who see such things come around yet once more as a new innovation, who humbly smile to themselves knowing that “what is old is new again”.

By chance are you a vintage electronics technologist?

1 comment:

  1. Oh yes.....The Olden days. I remember when I got my first transistor radio. It fit into my shirt pocket. I could take it out on the back porch, up into the attic, & out into the forest & listen to rock & roll music, & I didn't have to plug it into the wall. Remember turning on a radio & waiting for it to warm up before you got any sound? Remember watching black & white television & if Daddy turned on his electric shaver on Mommy turned on the electric egg beaters, all you got on the T.V. was buzzing & snow on ther screen? A car's ignition system driving by would do the same thing. & telephones, you could have any color you wanted as long as it was black. The receiver was heavy enough to do barbell exercises with, & the rotary dial, zik, thicka, thicka, thicka, thick! Oh, the warm, orange glow of the vacuum tubes shining on the black wall of your bedroom at night . haloween colors, while listening to the all night rock & roll radio station KLIF 1190 in Texas. Yes, the younegr kids now days have no idea what I'm talking about