Lamp Acoustical Noise and the Reverse Phase Controlled Dimmer

The standard SCR dimmer, like all phase controlled dimmers, produces a distorted output waveform. When this waveform is applied to an incandescent lamp, the lamp emits an audible hum that in certain applications is intolerable. To reduce lamp acoustical noise, most lighting control manufacturers place a large inductor in series with the lamp. An alternate approach is the reverse phase controlled dimmer. Instead of inductors, it utilizes auxiliary commutated SCR with capacitor commutation circuitry to limit lamp noise. Two advantages are offered by this approach. First, eliminating the large inductor allows the dimmer to be greatly reduced in size. Secondly, elimination of the inductor makes more power available to the load. The cause of lamp acoustical noise, the design of the reverse phase controlled dimmer, and methods for measuring lamp noise are described.