hpk@vax135.UUCP (Howard Katseff) (01/25/85)
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
rpw3@redwood.UUCP (Rob Warnock) (01/27/85)
*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 UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404
dmmartindale@watcgl.UUCP (Dave Martindale) (01/28/85)
Another (expensive) alternative to triac dimmers: use an autotransformer. They're physically too large to install in an outlet box, and require ventilation air in the higher current ratings, but they do work. I have one controlling the reading lamp over my bed, and the computer graphics lab here uses incandescent lamps on autotransformers as dimmers. In both these cases, the reason was avoiding audio noise, not RFI. Incandescent lamps driven by the waveform put out by a triac dimmer "buzz" audibly; with the autotransformer the only sound is the (very quiet) hum of the transformer itself. As a side benefit, the lamp can be taken smoothly right down to zero light output, something that just doesn't work with the triac dimmers. When picking an appropriate size of autotransformer dimmer, remember that incandescent lamps are not constant-impedance devices - use the transformer ratings intended for constant-current loads.
Stephen Carter <SCARTER@RUTGERS.ARPA> (01/30/85)
>I am planning to finish my basement. I would like to use a combination >of fluorescent lights and incandescent lights .... When I worked for an electrician installing lighting systems awhile back, almost every book and most experts agreed that mixing the two types in one room produced ill effects. I guess you could play games with choosing complimentary bulb types, but if you ever have been in a room where both types are used, you would see what I mean....they just don't work well with each other.. You might want to consider track lighting instead of the tubes. It's a bit more expensive (ok, ALOT more expensive), but I think it would give your room a better atmosphere. SCarter -------