The following schematic shows the Basic TRIAC Light Dimmer Circuit Diagram. The circuit is designed such that only a portion of the 60Hz sine wave from the VARIAC passes through the light bulb. TRIACs are electronic/ semiconductor devices equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel, that is, once they are triggered, they continue conducting current until the current drops below the hold current, typically around the zero point (null) in the mains waveform.