-=> Roy J. Tellason said to Greg Mayman
 -=> about "POWER SUPPLY" on 12-11-03  04:06.....

 RJT> One would think so,  only this one didn't show up with an ohmmeter.  I
 RJT> suspect that something on there is breaking down under voltage.  I need
 RJT> to be able to apply the two voltages separately,  and see what
 RJT> happens... 

Fortunately I only ever had one like that.

I finally isolated it by wiring a 100w lamp in series with the
mains input (so it wouldn't blow the fuse) and feeding the unit
from a Variac. Then I brought it up to where it was just on the
point of running away and measured lots of voltages around the
amplifier.

It turned out that one of the output transistors must have
developed a soft zener diode type action in the base-collector
junction. I've never seen any other semiconductor do it exactly
like that.

 RJT> remember where a signal went in the order FET -> optoisolator ->
 RJT> transistor -> SCR -> bridge (!) -> triac, which I thought was a bit
 RJT> much... 
 
 GM> I think I know what they were doing. OTOH it could probably have
 GM> been simpler.

 RJT> What do you think they were doing?

IMO the triac was controlling an AC line, with the gate drive
from the AC supply.

Obviously this won't suit the optocoupler which needs a unipolar
feed, so the bridge was added. And to boost the opto's output,
the SCR was added as well.

Action would be as follows:

1. FET turns on, driving the LED of the opto.
2. The opto's output connects between anode and gate of the SCR,
   turning it on when the opto is driven.
3. The SCR shorts the DC side of the bridge, giving a near short
   circuit on the AC side.
4. The AC side of the bridge is connected between anode 1 and
   gate of the Triac maybe with a series limiting resistor. When
   current flows in this path the Triac is turned on.

The current through the SCR is uni-polar due to the bridge,
although the current through the bridge to the gate of the Triac
is bipolar.

It could have been done more simply with a relay. OTOH this
circuit is fast so it might have been used in a phase controlled
dimmer -- obviously a relay can't do that.
 
From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... The light of a hundred stars does not equal the light of the moon.
___ Blue Wave/386 v2.30

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