Touch Lamp and RFI

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. 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 turning the lamp on or off. 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 lamp's 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.