Greg Mayman wrote in a message to Roy J. Tellason:

 -=> 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. 

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

Yep.  I'll probably end up getting some MOC3010 optos and using those. 
Output side of them is a diac...

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

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

Ok.

 GM>    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...

 GM> Yup.

I guess I didn't catch that the first time around.

 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

 GM> Yeah, I know that arrangement.

What got me about that was that there were times when I worked on that
equipment and saw some signs of heating in those resistors.  You wouldn't
think they would,  much.

 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.

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

I believe it's made for that sort of application,  but I'll double-check
just to be sure.

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

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

So it'd be a very short pulse in any case.

 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... 

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

Unless it's surface mount,  at which point you change the board...

I've got a fair amount of stuff being scrapped for parts that's not going
to yield much at all,  just a few bits here and there,  and the rest is
trash,  because of surface mount.

And yet there seem to be hobbyists who are embracing that sort of thing. 
I'm not sure why that should be.  It may be that some parts are only
available that way (though I don't see the need to use those parts),  and
the compactness and light weight can't be serious considerations in the
stuff I was looking at -- other factors overrode this.

The single major factor I can see that makes surface mount an advantage in
manufacturing is that you can put stuff on both sides of a board,  and that
board layout gets a whole lot easier and more direct.  More so in
multilayer contstruction,  as in MBs.  About the only advantage I can see
for a hobbyist is not having to drill quite so many holes,  but assembly
isn't something I'd consider easier!

Personally I don't think I'm gonna bother with it until such time as
regular through-hole components just aren't available any more,  and I
don't see that as happening any time soon.  Particularly not with the pile
of stuff I've scrapped out over the years.  :-)

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