Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site lcuxc.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!gamma!pyuxww!lcuxc!wjm
From: wjm@lcuxc.UUCP (B. Mitchell)
Newsgroups: net.ham-radio
Subject: Re: light dimmers
Message-ID: <192@lcuxc.UUCP>
Date: Tue, 29-Jan-85 09:23:32 EST
Article-I.D.: lcuxc.192
Posted: Tue Jan 29 09:23:32 1985
Date-Received: Wed, 30-Jan-85 07:39:13 EST
Organization: Bell Communications Research, Inc.
Lines: 33

<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)