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From: karn@envy..bellcore.com (Phil Karn)
Newsgroups: sci.electronics
Subject: Re: How to reduce noise from light dimmers?
Keywords: dimmer, RF
Message-ID: <1990Nov8.170823.20216@bellcore.bellcore.com>
Date: 8 Nov 90 17:08:23 GMT
References: <1458@eastman.UUCP> <35639@cup.portal.com> <1990Nov6.133221.7556@athena.mit.edu> <3987@osc.COM>
Sender: usenet@bellcore.bellcore.com (Poster of News)
Reply-To: karn@envy.bellcore.com (Phil Karn)
Organization: Bell Communications Research, Inc
Lines: 18

In article <3987@osc.COM> jgk@osc.COM (Joe Keane) writes:
>The problem is that they use a triac to cut off the current in the middle of a
>cycle.

No, Triacs are just AC versions of SCRs, and both always keep
conducting until the load current through them goes to zero at the end
of an AC cycle. Noise is generated in lamp dimmers because the triacs
are turned *on* in the middle of a cycle.  You may note that the noise
decreases somewhat when the dimmer is turned all the way up. That's
because the triac is being triggered at the beginning of the cycle,
when the load current is still small. Midrange is worst because the
triac is triggered halfway through the cycle, at the peak.

The best way to filter lamp dimmers is with self-contained RFI filters
as close as possible to the dimmer. Corcom, Cornell Dublier and others
make such filters, and they are very common at amateur radio hamfests.

Phil