Path: utzoo!attcan!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Looking for isolated DAC chip
Summary: Varistor limits voltage across optocoupler
Message-ID: <2940@kitty.UUCP>
Date: 17 Jan 89 22:41:18 GMT
References: <1378@ucsd.EDU> <2930@kitty.UUCP> <2758@skivs.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 34

In article <2758@skivs.UUCP>, dr@skivs.UUCP (David Robins) writes:
> >	Now, you've got your H11F1 being driven by the DAC in a controlled
> >fashion.  Take the output of the H11F1 and use it to replace the resistor
> >in a typical two-terminal TRIAC dimmer circuit using a diac as a trigger
> >device.  You may have to add a series resistor for "zero" compensation,
> >and you may want to add a parallel resistor for "keep-alive" current.
> 
> One problem with this is that the H11F1 is rated at ~30 volts.  Putting
> it in a typical triac control circuit should subject it to line
> voltages.
> 
> A solution was published in a paper I got from GE.

	You are, of course, correct.

> I'd be interested in knowing why Larry Lippman's use of the H11F1 in the 
> standard triac circuit did not over-voltage it.

	Because, in digging out the design folder from the water bath
project, I used the same circuit which you described!  I even have a
photocopy of page 285 from the 1981 GE Optoelectronics catalog which
shows the circuit.

	I had, of course, forgotten about the use of the varistor.

	However, a schematic for a second unit which we built shows
no varistor, since the voltage across the optocoupler was limited
to about 25 volts by a parallel "minimum output" variable resistor
resistor.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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