----- Original Message ----- From: Wagner Lipnharski To: Sent: 16 July 1999 16:11 Subject: Re: Lamp Dimming, DAC's and other Jiggery-Pokery > A TRIAC device triggers by a specific voltage at its gate. Once the > triac is gated it short circuit itself as a switch. It stays conducting > while exist current through. The AC line switch phases 120 times per > second, so if the triac receives a pulse at its gate, it would enter in > conductance status and still conducting until one of those "120 phase > changes per second" happens. If you keep the gate voltage constant, the > triac will still conducting (in real it restart conduction 120 times per > second). > > The DAC output is a variable voltage. Suppose you rectify (without > capacitor filtering) the extremes of a center tap coil of a 6Vac > transformer, to have only 120 positive semi-senoids. Then compare this > 120 positive half senoids signal to the DAC voltage output, it will be a > time period when the AC voltage would be higher and then lower. If you > increase the DAC output the "AC higher time" would be shorter, if you > reduce the DAC output voltage, the "AC higher time" would be longer. > The output of this comparer circuit (LM339 for example) could be > directly tied to the triac gate, so the time the triac would be > conducting would be longer or shorter, controlled by the DAC voltage > output, controlling the lamps power. > > This is the general idea. Not exactly the final circuit. > Wagner. Thanks, but what I was actually looking for (apologies if I worded the first post wrong) is just the variable resistor bit. I already have the power switching gear, and I don't really want to start working with high voltage/current for a first project. The PIC is only there to replace one of the control sliders. From talking to someone who works with stage lights, at switch on they draw 100+ amps, which tends to fry the triacs of homemade systems...no idea how the commercial ones work, but to me it makes sense to use the stuff I already have rather then start from scratch. What would be helpful is any information / websites that deal with PC serial port (RS232?) interfacing to pic. Just flicking through an electronics catalogue now, there's a digital volume control made by Dallas which might be usable...any ideas/thoughts? Are there any other commercial digital-analogue variable resistors available?