Yes, you can use a shared neutral on single phase 120/240 or three phase Y systems. If the resistance of the neutral is too high, the switching on one phase will cause a glitch on another phase which might false the zero crossing detector. I have not had a problem with this, though. Here are a couple dimming techniques we use in the Dove Systems products at http://www.dovesystems.com . In small dimmers (like our 4 channel "Shoebox" dimmer), the PIC does the entire job. A pair of diodes on a center tap grounded power transformer provide FW rectified AC. This is run thru a current limit resistor into the INT pin on a 16c74. The INT is set to generate an interrupt on a negative edge, which causes an interrupt a little before zero cross. The capture/compare register is used to determine when to turn on the triac for an individual channel. At zero cross, we quickly look thru the four turn on times for the four channels (converted from incoming DMX and analog levels to timer clicks 'til turn on thru a 16 bit lookup table). The first one to be turned on is put in the compare register. When the compare interrupt is generated, that triac gets turned on as well as any other triac scheduled within 255 timer clicks. This prevents us from losing one due to the time it takes to execute the ISR. The next turn on time is determined and that value put in the compare register. That's pretty much it. Other code is receiving DMX and putting 4 channels starting at the address on the thumbwheel switch in RAM. Other code is reading 4 analog inputs and doing a highest takes precedence with the DMX to determine what each channel actually does. There's also code allowing each channel to operate as a nondim. The other approach we use is used on our larger dimmers (such as the DM1224). The control card for the DM1224 can actually dim 24 channels on single or three phase. Here we use a 16c74 to receive DMX and analog. This data is run through a lookup table to give the dimming curve we want. It is then sent to a single 12 bit D/A which is demuxed out to 24 channels. These 24 levels are compared to negative going linear ramps in sync with the power line phases. When the demuxed level is above the ramp, the solid state relay for that channel is turned on. This is similar to standard analog dimmer techniques. Analog dimmers, however, use additional circuitry to shape the curve of the negative ramp so lamp brightness corresponds to the control voltage. We eliminated those components, going to a linear ramp, and "predistort" the signal compared with the ramp to yield the appropriate dimming curve. Harold On Thu, 19 Apr 2001 08:17:33 -0600 "Michael C. Reid" writes: > Here is a question on phase control light dimming. We use PIC's in a > lighting control system. I have been told that dimmer manufacturers > warn > against sharing neutrals in the electrical wiring if a dimmer is > connected > on either phase. Something about phase shifting of the sine wave. > My > understanding is that the sine wave is chopped and shut off to do > the > dimming. Can dimmers be used with a shared neutral, where each side > is 180 > degrees out of phase? > > Also, does anyone out there have any cool PIC routines for dimming? > > Mike > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ 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/tagj. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.