STOPPING COMMON SOURCES OF INTERFERENCE TO RADIO EQUIPMENT TOUCH LAMPS 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. 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 signal generated by the lamps circuitry signal is present at all times, regardless of whether the lamp is on or off. 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. The easiest route here is simply to get rid of these pesky things, and return them to the store, if possible. A simple cure for those touch-controlled lamps that turn themselves on and off during nearby radio transmissions on 40 and 80 meter operation. A 1k ohm resistor in series with the signal input lead (from the lamp base) to the encapsulated circuit that operates the lamp may cured the problem for me. If this isnt sufficient, add an RF choke (100 uH, 139 mA) in series with the resistor. The choke alone may be enough to clear up the problem in some cases. If these cures dont work, it may be possible to shield the electronic switch module, but this must be done safely! LIGHT DIMMERS Radio Amateurs who 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, such as their model N-600, which will handle up to 600 watts of incandescent lighting. Another brand light dimmer produces an S9+ reading at 230 kHz (an arbitrary noisy frequency). The N-600, by comparison, produced a reading of S3, a difference of about 40 dB. Admittedly, this is not zero, but installing the Lutron model should provide a reduction in RFI that is very gratifying. NIGHT LIGHTS Some night lights with sensor eyes can be a source of HF hash. The offending light type is a CdS photocell, an SCR (or TRIAC) and a small incandescent lamp. When the light level drops, the thing switches on and off at a rapid rate and hashes up everything in the area! The lamp light will visually feed back to the photocell. In this design, the little porthole for the photocell is removable. Take it out and cement a 3/8 inch ID x 1 inch long kraft paper tube in its place to shield the photocell from the lamp light. This should prevent the feedback oscillation. You can also look for better designs such as an electroluminescent disk which plugs in, runs all the time, draws about 12ma and shows nothing on a spectrum analyzer. They are supposed to have a minimum 10 year life and cost about $4.
Posted on: Wed, 04 Jun 2014 02:19:16 +0000
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