Controlling LED lighting using triacs and quadracs

As incandescent lamps have been made largely obsolete, given the U.S. government’s mandate to save energy, they have frequently been replaced by LEDs due to their long life (typically 25,000 hrs.) and the ease of adapting them to many different socket and shape requirements. 

However, LED lighting control presents a few problems not encountered with incandescent lamps. For example, with much less current from the LED load, normal types of triacs may be challenged in terms of latching and holding current characteristics. Triacs make up the heart of AC light dimming controls.

Triacs used in dimmers have normally been characterised and specified for incandescent lamp loads, which have high current ratings for both steady-state conditions and initial high in-rush currents, as well as very high end-of-life surge current when a filament ruptures.

Because they are diodes, LEDs have much lower steady-state current than incandescent lamps, and their initial turn-on current can be much higher for a few microseconds of each half-cycle of AC line voltage. Therefore, a spike of current can be seen at the beginning of each AC half-cycle.

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