On Fri, 17 Apr 1998 15:32:40 -0400 "Montaigne, Mike" writes: > I wanted to build a PIC controlled zero crossover lighting >dimmer, for our church and briefly discussed this a few months ago. I >was using a 555 timer triggered on the zero crossover to get my delay. >The problem was I wanted 0-10 volt remote control so I could locate my >controls in the balcony and the power unit under the stage. (The >other >problem is, we have no money). A standard analog method of generating phase control is to generate a negative going ramp that is reset to +10 volts at each AC line zerocrossing. You can then use analog comparators to compare the control voltage with the ramp. If the control voltage is greater than or equal to the ramp voltage, turn on the triac. This technique is used in lots of theatrical dimmers. They use a separate ramp for each power line phase and have a comparator for each dimmer output. > > The standard way of dimming lights is to turn on a triac some >time after the zero crossover of the ac waveform. The problem with >this >is, the triac turning on causes rf noise (which you then have to >filter >with an inductor and capacitor). By the way, the TEA1007 chip does a >real nice job for this. See the Vellem K8003 DC controlled dimmer >kit, >about $25 (change triac to 25 amp & add larger current inductor). Yes, the turn-on can cause singing of the lamp filaments. The rise-time can be delayed either by adding a series inductor or by using a slow turn on device. Some manufacturers are using a FET with a slow turn on, but this is substantially less efficient than using a triac (which has a lower voltage drop than a pair of FETs and has either high voltage across it with no current or low voltage across it with high current, both resulting in relatively low dissipation). > > I then thought, using a PIC I could hit the light with (only >one) 1/2 sine wave, say once per second, and increase the number of >1/2 >sine waves every second until the light was full on. I could then >send >(serial 422 from a pc) a light number and a count of 0-256 to tell the >PIC how dim or bright. > > I tried this with a zero crossover triac and a signal >generator >that I could vary the pulse period and the frequency and found that >even >when I had 1/2 cycle every say 10 seconds, you could still see the >flash >of the filament on a 100 watt bulb. I know that a larger bulb, say >1000 >watt would not respond as quickly, but I am trying to dim 1500 watts >of >xmas tree bulbs. Your're right. The flicker is going to be horrendous. I think you're going to have to stick with phase control. > > (Other than a variac) is there any (cheap) way of zero >crossover >dimming for this project - am I missing something very basic? I don't >mind using the TEA1007 phase shift solution, but I thought industry >lighting control for theater, used zero crossover, and I was trying to >understand how it worked. The lighting industry (at least where I work http://www.dovesystems.com) uses phase controlled triacs (or solid state relays) with a series choke to limit rise time. Again, some use FETs or IGBTs, still switching at 60 (or 50) Hz, but with a slower rise time, making the choke unnecessary, but suffering low efficiency. I've looked at the possibility of high frequency switching of the AC to give a "solid state variac", but costs are high (four FETs and associated drivers, snubbers, and output filters per channel) and efficeincy is lower than the same circuit with a triac (due to required diode drops, FET or IGBT saturation voltage, etc.). Besides the analog comparator technique discussed above, we also receive DMX512 and generate phase control all in a single 16c74a (see the Shoebox dimmer at dovesystems). Harold _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]