12 to 24 volt conversion.. MORE

J. Forster jfor at QUIK.COM
Thu Oct 8 16:04:04 EDT 2009

That would obviously work, if you can find a unit with two 24 high current

For the primary, you have to have smothing like 24 VAC CT (You could use a
 winding w/o a CT, but that would require a full bridge driver).

For the secondary a 24 VAC CT or a 12 VAC w/o CT would work. The latter
takes 4 diodes, rather than two.

As  suggested, a small filament transformer could be used for feedback.

I don't like the power cross-coupled astable MV much. I prefer to let the
onset of core saturation do the commutation.

Also, you need to be very sparing in the use of capacitors between the
inverter collectors as they can increase the switching times
significantly, raising the instantaneous dissipation in the transistors.

I don't like external oscillators much because you have to have almost
exactly 50% duty cycle to avoid saturation issues. If you guarantee that
with a flip-flop, you have to be careful about startups that so you don't
destroy transistors. It gets complicated.



> How about a surplus transformer with two filament windings? The
otherwise empty 115v winding could have a buffer cap across it to tame
the transformer
> ringing. Plus low power 115vac would be available for ???
> A base winding, although the preferred way to go, is not eessential.
Just cross connect the bases to the opposite collector multivibrator
fashion through a couple small resistors. Or, if you really want the
base windings,
> they can often be squeezed in on top of the other windings. For out of
the box thinking, use a power fet to switch the transformer. Something
like a 555 timer chip could possibly drive the fet. Don't forget the
back diode across the switching fet.
> Paul K0UYA
> ----- Original Message -----
> From: "J. Forster" <jfor at QUIK.COM>
> Sent: Thursday, October 08, 2009 11:54 AM
> Subject: 12 to 24 volt conversion.. MORE
>> The problem of winding a transformer might be solved using a kludge of
three filament transformers.
>> First off, you'd need a pair of transformers capable of handling the
needed output power, with the 115 VAC windings connected back-to-back.
Thansformers roughly 115 VAC input and 24 VAC CT @ 10 A would be about
right. One 24 VAC winding would be used to the collectors of the
>> transistors,m with the CT to +12 VDC. The other 24 VAC winding would be
used w/ a FW half bridge. This isolated 12 - 15 VDC would be stacked on
top of the battery.
>> The trick is to use a smaller filament transformer, something like 115 VAC
>> to 6 VAC @ 1A as a feedback winding to ythe transistor bases. All three
115 VAC windings would be connected in parallel.
>> BTW, it'd likely be more efficient to build a stacking PS like this,
rather than inverting the full power for the load.
>> FWIW,
>> -John

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