Path: utzoo!attcan!uunet!skivs!dr From: dr@skivs.UUCP (David Robins) Newsgroups: sci.electronics Subject: Re: Looking for isolated DAC chip Message-ID: <2758@skivs.UUCP> Date: 16 Jan 89 18:37:24 GMT References: <1378@ucsd.EDU> <2930@kitty.UUCP> Reply-To: dr@skivs.UUCP (David Robins) Organization: Smith-Kettlewell Eye Research Institute, San Francisco, CA Lines: 48 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!