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Touch Lamp


RF Touch lamps are RF-operated devices that often cause, or are susceptible to, EMI problems. They have a free running oscillator that is very broad and rich in harmonic energy. This oscillator is hooked up to a touch plate that changes the frequency of the oscillator when a hand is placed near the plate. Unfortunately, this plate also acts as an antenna, radiating some of the energy of the oscillator, or picking up nearby radio signals. When the former happens, it can interfere with other services. When the latter happens, the circuitry inside the lamp reacts the same way that it would when the plate is touched -- the lamp changes states from "off" to "on".   Although cases of moderate interference can sometimes be cured by using a "brute-force" type AC-line filter and/or a common-mode choke, most cases will require internal modification to the lamp. For a number of different reasons (you may be blamed if anything EVER goes wrong with the lamp or house wiring) you do not want to perform this modification on equipment that is not your own. Remember -- house AC power is dangerous. These modifications must only be performed by qualified service personnel!


Web Links

The Lutron FAQ Page provides information about light dimmers, including RFI.

Hints & Kinks

  • Just spent two hours fixing interference to a touch lamp--it was fine at 75 watts, but 80 to 100 watts on 80M would occasionally turn on the lamp.  I found that the wire between and RF choke and the input to the IC chip needs to be as short as possible.  Ideally, one would use a small RF choke instead of the large PI wound one I found in my junk box.  --Zack Lau, W1VT

Note:  Ed Hare, W1RF suggests trying a ferrite core if it can be used.

RFI and Touch-Controlled Lamps

I have found a simple cure for those touch-controlled lamps that turn themselves on and off during nearby radio transmissions. In my case, 40-meter operation gave the most trouble, with 75-meter operation a close second. Higher frequencies presented no problem. (I use a ground-mounted vertical antenna for 80, 40 and 15 meters, and the lamp is approximately 150 feet from the antenna. An AC-line filter at the lamp did not eliminate the problem.)

A 1k ohm resistor (in series with the signal input lead to the encapsulated circuit that operates the lamp) cured the problem for me. I suppose the required resistor value would vary with the RF-field intensity and frequency.

-- John M. Adams, W7OTC, Sun City, CA

More on RFI to Touch-Controlled Lamps

I had the same problems as W7OTC with a touch-controlled lamp switched on and off by my transmissions (100 W to a roof-mounted vertical, with two radials per band). The problem occurred during operation on the 80- through 15-m bands, but 10-m operation had no effect. A 1-k ohm resistor was not a complete cure in my case.

A 3.3k ohm resistor in series with the signal input on the lamp helped on all bands except 80m (an additional 1.8k ohm prevented the lamp from functioning). When the resistor was replaced with an RF choke (100 uH, 139 mA), the problem abated on all bands except for 80 m. On 80 m, the interfering signal was considerably attenuated by the choke, but the lamp still switched. The choke alone may be enough to clear up the problem in some cases.

The final answer turned out to be both the RF choke and a 1.8k ohm resistor in series with the signal-input lead to the touch-control circuit.

-- Colin Hall, G4JPZ/W6, Marina Del Rey, CA

Touch-Lamp Transceiver

When my wife told me she had bought a three-way lamp that switched on and off at the touch of any of its metallic parts, I did not realize she had purchased a transceiver. I found that my transmitted signal would cause the lamp to operate exactly as if I had touched its metal parts. Later I discovered a raspy, S8 signal at 1875 kHz -- it was coming from the lamp, which was located three rooms away on a different AC circuit.

The lamp signal is present from 40 meters down. At frequencies from 20 meters up, my operation is undisturbed.

A box inside the lamp contains a circuit board through which AC line voltage is routed and which has a wire connected to the metal base of the lamp. When the lamp is plugged in, the lamp signal is present at all times, regardless of whether the lamp is on or off. In my attempts to eliminate the interference, I tried a commercial AC filter, coiling the lamp cord on some ferrite material and other such approaches without success.

To make sure the lamp my wife had was not defective, I borrowed a similar lamp from a neighbor to try it. I found it to perform in exactly the same manner except that the frequency of oscillation was somewhat different. There is no manufacturer or distributor name on the lamp or packing container. The lamp was made in Taiwan.

I am writing so that others who may be experiencing similar difficulties may have some idea of the probable source of interference. After I described what I discovered to a ham friend, he realized that such a unit had been causing interference to his station for more than a month.

-- Cal Enix, W8EN, 209 S Kalamazoo St, White Pigeon, MI 49099

If these cures don't work, it may be possible to shield the electronic switch module, but this must be done safely! You may also want to contact the manufacturer and send a report of your problem to:

Light-dimmer Interference Reduction

Radio Amateurs who've have been cursed with RFI from solid-state light dimmers will be interested to know that at least one domestic manufacturer -Lutron - produces light dimmers that incorporate RFI suppression techniques. The Lutron NOVA series uses toroidal chokes that provide a significant level of RFI suppression.

I bought a Lutron model N-600, which will handle up to 600 watts of incandescent lighting. Temporarily installed in my radio shack, a generic light dimmer produced an S9+ reading at 230 kHz (an arbitrary noisy frequency). The N-600 produced a reading of S3, a difference of about 40 dB. Admittedly, this is not zero, but installing the N-600 some distance away provided a reduction in RFI that is very gratifying. Indeed, I new hear new noise sources, heretofore undetectable through the dimmer din.

You're not likely to find these dimmers at your local discount store, and they are not inexpensive. Check for the availability of these dimmers at a lighting fixture store and expect to pay about $25 apiece for them.

-- Richard G. Brunner, AA1P, 10 Brookside Dr., Foxboro, MA 02035


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