🚀 go-pugleaf

In article <2930@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <1378@ucsd.EDU>, brian@ucsd.EDU (Brian Kantor) writes:
>> I've got 7 or 8 bits of TTL on a latch output that specify a lamp 
>> brightness level, and I want to control a TRIAC or other simple 
>> light-dimmer circuit with it.  The idea is to have a microprocessor
>> control a lighting panel.
....
>	However, you asked for "simple", so here it is...  The heart
>of this approach is the GE H11F1 optocoupler having a bilateral analog
>FET output.  In simple terms, changing the LED current on this device
>changes a two-terminal output resistance.
....
>	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.  William Sahm was
at GE Semiconductor about 5 years ago, and he sent me:
Chen and Sahm; A Bilateral Analog FET Optocoupler
	It was in a 1978 IEEE journal, I don't have the reference.

In an example, they showed a
220 volt remote lamp dimmer circuit which used a 22K resistor in series with a
27volt MOV across the triac, the MOV at the gate end of the triac.
The voltage now clipped to ~27 volts is applied to the H11F1 in series
with a 0.033 mfd cap, the cap at the "bottom" end. The phase control
voltage at the junction of the H11F1 and the cap is coupled to the
triac thru a 2N4992, a low voltage bilateral trigger; if I recall, it was
about a 7 volt breakover.  Most other diac triggers are about 25-35 volts.

I used this circuit with the same 22K resistor in a 120 volt
application.  I used a resistor in series with the LED in the H11F1 so
the current thru it was controlled by a voltage.  

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

-- 
David Robins, M.D.  (ophthalmologist / electronics engineer)
The Smith-Kettlewell Institute of Visual Science,  ***  net:  uunet!skivs!dr
2232 Webster St, San Francisco CA 94115            ***  415/561-1705 (voice) 
The opinions expressed herein do not reflect the opinion of the Institute!