12 to 24 volt conversion.. MORE

J. Forster jfor at QUIK.COM
Thu Oct 8 18:30:42 EDT 2009

> John
> The transformers may be more common than you think.
> They are used in power supplies of some audio power amps where equal
> opposite polarity supplies were required and some were fwct supplies.

I think you'll find that complementary-symmetry amps use higher than 24
ACV CT transformers. With such low supplies it's hard to get almost any
power into 8 Ohms.

> Many were torroid cores as well

True, but often for no really good engineering reason. However, a lot of
copper is wasted, since you are not drawing anything useful from 1/2 the

> I wasn't advocating placing a capacitor between the collectors but rather a
> buffer cap across the unused primary winding as I found on a couple
little supplies I built to obtain 120vac in a car it tamed the
waveform-made it look more respectable.

For a DC/DC converter, the closer to a perfect square wave the better.

 Those were just cross coupled power transistors
> driving Radio Shack filament transformers---and they worked first time
all the time.

I've done a couple like that, including an A,B, & C supply for a WS 58.
Runs off 6 D cells.

> As far as gauranteeing 50% duty cycle it seems to me that would be
pretty easy to do with electronics commonly available today. Probably be
more precise than an old vibrator supply and they seemed to work

Yes, but look at the OP.

> I don't understand the concern on start-ups with a switching controller.
If it starts "on" it is going to be off in 8ms or whatever period you
are working with and if it starts "off"? Perhaps you can enlighten me to
the potential problem.
> Paul K0UYA

Problem is, if startup does not go properly, one transistor can get hung
ON and blow out pretty quickly. It takes fancy storage scopes to be sure
that does not happen.

With the transformer FB configuration, you can pick the bias network so
the power transistors just barely go into conduction worst case. When the
inverter gets going, a simple diode like a 1N4007 will partly bypass the
bias network and increase the base drive to insure a very solid turn-on.

It is simple, high efficiency, and robust. I've space qualified a couple.



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