-=> Roy J. Tellason said to Greg Mayman
 -=> about "POWER SUPPLY" on 12-18-03  04:06.....

 RJT> Yeah,  but you're supposed to be able to trigger a triac with either
 RJT> polarity of voltage,  for either polarity of MT2 voltage.  There are
 RJT> differences in sensitivity in the four "quadrants" of operation, 
 RJT> nontrivial ones,  but with sufficient drive I don't see this as being a
 RJT> problem. 

Very true, but it is easiest to derive the trigger voltage from
the MT2 voltage. In that case the trigger is naturally the same
polarity as MT2 ;-)

 RJT> of the opto has a resistor tying the base of the phototransistor to
 RJT> ground (why?),

To de-sensitize it. Sometimes you don't want the thing to go off
until you reach a certain current threshold.

   and an NPN transistor that fires the gate of the SCR
 RJT> that in turn fires the triac through the bridge. 
 RJT> I guess the power for the SCR, transistor, and output side of the opto
 RJT> are all supplied through the bridge,  which accounts for that...

Yup.

 RJT> That application used a couple of *very* lightweight relays to switch
 RJT> the triacs,  which carried the load.  Something like a 100 ohm 2W
 RJT> resistor connted between the relay contacts and the gate and MT2

Yeah, I know that arrangement.

 RJT> terminals.  I've wondered at times about using the output side of a
 RJT> MOC3010 (DIAC output) opto for a similar purpose,  and what resistor I
 RJT> might need in there.

Check that the opto can withstand full peak voltage in the "off"
condition, = 1.414 * the RMS voltage.

The resistor will depend on what current the diac can carry,
remembering that it could turn on when the voltage is at the peak.

OTOH once the Triac is triggered there is zero volts available,
so the current is also zero, until the next half cycle.

 RJT> I also did a similar mod,  in Leslie tone cabinets.  Those things used
 RJT> to burn up relays,  which were used to switch motors directly.  If I
 RJT> could get the okay on it before the contacts were too burned,  I'd
 RJT> stick a couple of triacs in there and let the relay switch _them_
 RJT> instead of the motors directly,  saving them from further
 RJT> deterioration.  Too bad I don't get the opportunity for little tweaks
 RJT> like that these days... 

Yeah, too much of the stuff is large scale integration that you
can't do a darn thing with except change the chip :-(((((

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Of all our possessions, wisdom alone is immortal. - Socrates
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