Triac Before Transformer?

[Jay From Indiana]

I'm building a power supply for a TDA8920 I got off ebay. I have a old toroidal tranny which is a PSU for an old 400W amp. It puts out two out of phase 40v and two out of phase 90v. This little amp wants max 30v so my thought is to put a motor controller or dimmer switch in front of the tranny to reduce those 40s to 25s. The PSU has a couple fat caps on there. Is that a viable plan?

[gmarsh]

It might work. Can't suggest it.

Dimmer switches don't work by lowering voltage, they do so by cutting out part of the AC cycle. Technically, if you dim below 50% duty cycle at the triac, you'll start cutting the peak voltage down a bit and the rectified output of the toroid will drop.

But the result is sharp DV/DT out of the dimmer switch and into the transformer when the triac fires, which combined with the low leakage of the toroid, could cause damage to the dimmer switch, your amp's rectifiers, output capacitors, etc.

And blowing up the dimmer switch wouldn't be good - if it fails short-circuit so you get 40V into your amp, you've got problems. Heck, if any of the above fail, it won't be good.

Even if it works, the sharp DV/DT might put an awfully bad 100/120Hz+harmonics buzz on the output of your amp.

Either use a variac, buy a different transformer... or if you're lucky and the 40V windings were the last ones wound on the toroid, you might be able to remove some windings and get the voltage down.

[Jay From Indiana]

I had considered unwrapping some of the secondary, but preferred an easy way out.

Maybe I should investigate using a pair of Radio Shack LM317T? They deliver 1.5A each, which at 25V gives 75W, plenty for this application. Maybe I could use them to drop 15V and deliver DC straight to the secondary side of the bridge. I'll have to look.

[nigelwright7557]

Just buy the right transformer, anything else will just generate a lot of heat.
Its difficult to drop volts at that current without geneating a lot of heat.

Only other option is a SMPS.