wjm@lcuxc.UUCP (B. Mitchell) (01/29/85)
<gulp> Agreed, RFI from el-cheapo solid state light dimmers can be a serious problem. I had one in my living room, but took it out when it started spitting QRM into my hi-fi system. The problem is that the SCR or triac does the dimming by "chopping" the AC sine wave part way through the cycle. Since incandescent lamps have a fairly long time lag - chopping a 60Hz wave reduces the RMS voltage level causing the bulb to dim without *too* much flicker. But this chopped wave has a rather high harmonic content and that's what's causing the RFI. You can try the standard RFI suppression tricks. Make sure the light dimmer is in a metal box (even if your local electrical code allows nonmetallic outlet boxes) and that the box is solidly grounded (again, the National Electrical Code requires metallic outlet boxes to be grounded). Also try ferrite beads on all the dimmer leads. However, I'd suggest getting away from the solid state dimmer altogether and using one of the following alternatives: 1) Wire the lights on several switches - this is probably the least expensive approach, although it only gives you a few levels of light - you can get three levels of light from two switches by putting 1/3 of the lamps on one and 2/3 on the other. 2) Use an autotransformer for dimming the lights. Admittedly, autotransformers aren't cheap, and they are too large to fit in a standard box, but their output is a sine wave and they don't produce RFI. Also, autotransformers can be used to dim fluorescent lamps as well as incandescent ones. (You do need special dimmer ballasts in the fluorescent fixtures and the temperature should stay above 55-60 degrees F when the lamps are dimmed). Keep in mind the National Electrical Code requirements that autotransformers be grounded and that the grounded circuit conductor (the neutral or white wire) be common to both input and output circuits). Also, an autotransformer is NOT an isolating device, so consider the secondary circuit to be part of the AC line. 73's Bill Mitchell, WB2IAU, (ihnp4!lcuxc!wjm)
jans@mako.UUCP (Jan Steinman) (02/01/85)
In article <192@lcuxc.UUCP> wjm@lcuxc.UUCP (B. Mitchell) writes: >The problem is that the SCR or triac does the dimming by "chopping" the AC >sine wave part way through the cycle. I once designed a dimmer based upon zero-switching and digital cycle-counting. To dim the light to 40%, only allow 40 out of 100 half-cycles to pass. This method is easier on the triac and lamp, but takes more hardware and won't really fit in an outlet box. A side advantage is that it is more easily interfaced to a computer than the phase-control technique. A more serious problem is that there is excessive flicker at very low duty cycles -- at 2% light, a cycle is passed about once a second! Practical experience showed levels as low as 10% (6 Hz flicker rate) were tolerable, (due to filiment, not visual, persistance) with the typical useage range of 30% - 70% working quite well. Best of all, absolutely no RFI! -- :::::: Jan Steinman Box 1000, MS 61-161 (w)503/685-2843 :::::: :::::: tektronix!tekecs!jans Wilsonville, OR 97070 (h)503/657-7703 ::::::