Hi Roy.

18-Dec-03 04:06:53, Roy J. Tellason wrote to Greg Mayman


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

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

 GM>> Fortunately I only ever had one like that.

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

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

 RJT>> remember where a signal went in the order FET -> optoisolator ->
 RJT>> transistor -> SCR -> bridge (!) -> triac, which I thought was a
 RJT>> bit 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?

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

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

 RJT> The bridge was after the SCR,  actually.  The stuff was in the
 RJT> order I gave it.

 GM>> Action would be as follows:

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

 RJT> Something like that...  It still struck me as a little bit overly
 RJT> complicated for what they were trying to do

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

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

 GM>> It could have been done more simply with a relay.

 RJT> Yes.

 GM>> OTOH this circuit is fast so it might have been used in a phase
 GM>> controlled dimmer -- obviously a relay can't do that.

 RJT> I don't recall now what the application was,  I'll have to
 RJT> look...   Found it, under "Solid State Relay"!  That's the
 RJT> caption the author had on the page,  and it also appears on the
 RJT> schematic diagram

 RJT> Looking a little closer,  the FET has the gate tied to source,
 RJT> so they're using it to limit current through the LED of the opto,
 RJT>  which is why the input can be labeled "5-25V".  There's also a
 RJT> diode in series with that,  and that leg of the circuit is marked
 RJT> "5 mA" -- I guess that depends on the specific FET you're using.
 RJT> The output side of the opto has a resistor tying the base of the
 RJT> phototransistor to ground (why?),  and an NPN transistor that
 RJT> fires the gate of the SCR that in turn fires the triac through
 RJT> the bridge

 RJT> I guess the power for the SCR, transistor, and output side of the
 RJT> opto are all supplied through the bridge,  which accounts for
 RJT> that..

 RJT> I've a couple of TRIACs here too that are in stud-mount packages,
 RJT>  though they don't have all that hefty a rating -- something like
 RJT> 6A,  I think?  Their original application was in an electronic
 RJT> organ's rotary speaker for motor control,  not much of a heatsink
 RJT> was used,  so they used a heftier part for a fairly light load,
 RJT> I guess

 RJT> That application used a couple of *very* lightweight relays to
 RJT> switch the triacs,  which carried the load.  Something like a 100
 RJT> ohm 2W resistor connted between the relay contacts and the gate
 RJT> and MT2 terminals.  I've wondered at times about using the output
 RJT> side of a MOC3010 (DIAC output) opto for a similar purpose,  and
 RJT> what resistor I might need in there

it depends how much current you want to feed to the gate. :)
my app notes for 240Vac operation with a MOC 3021 uses a network with
resistors and a capacitor

 .                 ---[390R]----[470R]--> Mt2
 .  ....|.........|......    |
 .  :  _L_    ____L___  :   _|_ 0.047uF
 .  :  /^\ //  /^\ \_/  :   ~T~ 400V
 .  :  ~T~    ~~~~T~~~  :    |
 .  :...|.........|.....:    `--> Mt1
 .      |  3021   |
 .                `-----> G

I guess for 120V operation use half the resistance and twice the
capacitance, for 60Hz instead of 50Hz reduce the capacitance by 1/6

 so about 0.082 uf (prolly 0.068 or 0.1 would be close enough given that
 capacitors are often 20% tolerance.... anyway)

 -=> Bye <=-

---