Dimmer For Inductive Loads

EE November 1987 DIMMER FOR INDUCTIVE LOADS The vast majority of dimmer cir- cuits is only suitable for regulating resistive (non- reactive) loads, i.e., when there is no phase difference between the mains voltage and the load current. This means that the trigger pulses can be kept rela- tively short, since the load cur- rent is in phase with the mains voltage immediately after trig- gering has taken place. Nor- mally, the load current is greater than the holding cur- rent, so that the triac or thyristor is triggered immediately, and remains on. When the load is mainly induc- tive (e.g. a transformer, or a choke for a fluorescent lamp) the load current lags the voltage, and may either not have reached, or exceeded, the holding level. The SCR then conducts briefly, but is switched off at the end of the trigger pulse. This unwanted ef- fect can be kept within limits by means of stretching of the trig- ger pulse, triggering by pulse trains, or the use of an R-C net- work. The first a...