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.
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. To do this, a bilateral DIAC is used to control the current flow to the TRIAC.
In this circuit design, the time constant can be seen as the 3.3k ohm resistor plus the value of the potentiometer times the 0.1uF capacitance. Thus, to control the intensity of the light, one simply controls the value of the resistance on the potentiometer, which determines the rate at which the capacitor reaches the break over voltage.