Dimmer, zero crossing referenced

All phase-control dimmers are referenced to the zero-crossing point of the AC waveform. A dimmer set at a high level is turned on earlier in the half-cycle after the zero-crossing point, thus allowing more power to pass to the load, while a dimmer at a low level is turned on later in the half-cycle after zero-crossing.

One of the quality measures of a professional dimmer is its zero-crossing detector circuit. A ZC detector that is immune from line noise and frequency shifts produces a quality dimmer that does not flicker in the presence of line-borne electrical noise events.