A quick look at the circuit... The 1K only has full voltage across it for a very short time. Once the triac fires, the voltage across the triac drops to about 1.5 volts. A 500mw resistor works fine here (unless the triac blows open, then the resistor will blow). I don't see why the 10K to ground driving the opto is required. The PIC output goes all the way to ground, so the pull down is not needed. The bridge and opto zero cross detector could be replaced with an AC opto. In dimmers where we use this (typically our three phase dimmers), we use an H11AA1. We also put a resistor across the LED here to adjust the width of the zero cross pulse. In other circuits (such as our DM406), we have a separate full wave rectifier on the secondary of the power transformer that we use for zero cross detect. For receiving DMX, we use the MAX 488, which has a higher ESD rating than chips in the 75 series. Also, when used as a DMX transmitter, it is slew rate limited, which reduces reflections in poorly terminated lines. Finally, the DMX standard calls for a 5 pin XLR connector, not a BNC. Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com On Fri, 14 Dec 2001 20:24:43 -0500 Eben Olson writes: > Hi all. I'm trying to build some PIC-controlled light dimmers for my > school theater. After looking at some schematics from the internet, > I pieced together the attached basic diagram. But if anyone has > experience with this kind of thing, I have a few questions. I'm very > confused about how to calculate the wattages required for the > resistors. Using V=IR, and P=VI, it seems like the 1k resistor > between AC supply and the MOC triac trigger would have to take about > 15 watts. Is this correct? > TIA, > Eben Olson ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu