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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.ham-radio
Subject: Re: Noisy dimmers and fluorescent lights?
Message-ID: <143@redwood.UUCP>
Date: Sat, 26-Jan-85 19:37:49 EST
Article-I.D.: redwood.143
Posted: Sat Jan 26 19:37:49 1985
Date-Received: Mon, 28-Jan-85 02:17:23 EST
References: <942@vax135.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 72

*WARNING* Any suggestions made below are for discussion purposes only.
No warranty or guarantee of safety is implied. Certain of the modifications
discussed may involve hazards to persons or property, including possible fire
or shock. It is assumed that in "net.ham-radio", these hazards are understood.

+---------------
| I am planning to finish my basement.  I would like to use a combination
| of fluorescent lights and incandescent lights with wall-switch mounted
| dimmers.  However - I am afraid that they cause interference with my
| shortwave and medium wave radio reception.
| Can one buy electrically quiet versions of these devices?
| Is there some way to tame them?
| Howard Katseff | AT&T Bell Laboratories - Holmdel, NJ
+---------------

The problem with taming incandescent dimmers is *POWER*; there's a
lot of it floating around. When the dimmer is half-on, you are switching
the full load current in a microsecond or two. If you have a total of
400 watts, that's a dI/dT of several million amps per second, which
just LOVES to shock-excite any little resonances lying around in your
wiring (120 times a second).

If you look inside your dimmers, you will see that most of them have
(these days) a small hash filter, but it's usually a single L-C in an
"el" (look for the heavy wire wrapped around some ferrite). They can't
slow the fall-time across the SCR/Triac itself, since the ability of
such a small beast to handle that power depends on spending as little
time as possible in the switching transition. In fact, the L-C "filter"
is really there to speed up the switching more than it is to filter RFI.
[*DO NOT* try to add any capacitance across the SCR/Triac, unless you like
to watch fires.] The trouble is, the hash filter itself radiates locally
(due to the magnetic field?), and I have found that even when the dimmer
is relatively "clean", strong broadcast AM (KCBS in S/F) gets RFI within
several meters of the dimmer (and within a meter or so or the load wiring).

Solutions? Put the dimmer in a shielded box, and feed it with a commercial
high-power RFI filter (in the wall of the box), the sort that are sold for
computer switching power supplies. Watch out for leakage out the knob of
the dimmer; you might want to replace/extend it with plastic. Don't worry
about the load wiring. *CAUTION* Some dimmers will burn up if you put
heavy RFI filtering on them, even externally. Using the usual precautions
for working with live 110 volts, measure the steady-state temperature of
the SCR/Triac with full load at half-bright setting (using a 'scope, it
should switch on at the voltage peak -- your eyes can't estimate that well).
Then install the RFI filter, re-adjust the dimmer (the RFI filter will
probably change the switching point) let the system equilibrate, and measure
again.  Any additional temperature rise is cause for concern. As an added
safety measure, derate the dimmer's power capacity 50% or more when using
added filtering. Finally, check with the manufacturer!

Fluorescent lights aren't TOO bad by comparison, since the arc typically
re-fires early in the cycle (once the bulb is warm), the ballast is a
large inductor physically close to the lamp and in series with it, and
the power is (usually) lower. Use the commercial "shop lights", with
all-metal housings (and the ballast INSIDE the metal). In extremes, you
might have to add more RFI filtering or grounded (to the case) mesh over
the light, but I doubt it. When bulbs get old, they get "noisier" -- just
replace them.

My overall suggestion would be to avoid using dimmers at all, but light
your basement with several smaller lamps that you can turn on and off
locally to achieve the desired lighting or mood. A single master switch
near the door could retain the convenience of central control. (Most
fluorescent dimmers don't work all that well, anyway.)


Rob Warnock
Systems Architecture Consultant

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