Lamp dimmer circuit and operation: The Lamp dimmer circuit is, during each half-cycle, when the voltage across the capacitor (either positive or negative) exceeds the breakover voltage of the diac and “fires” the triac, current then flows through the load. The RC time constant of the series linear potentiometer and 0.47µF capacitor determines the phase delay and magnitude of the sinusoidal capacitor voltage with respect to the source voltage.
Once firing occurs, the voltage across Diac the triac collapses, the capacitor voltage goes to nearly zero, and the entire process resets at the beginning of the next half-cycle. For the circuit to work properly, a small current must flow through the load before firing occurs, but this current is miniscule with respect to full load current.
when the potentiometer is adjusted to 0 Ohms , the time constant of the RC circuit (load resistance) is 3300 • 0.1 • 10-6 = 0.330 msec, which is small compared to one-half period of 60Hz (i.e., 8.33msec). When the potentiometer is at 470kΩ, the time constant is 35.3msec, which is relatively large.100 Ohms and .01/400v are seriously connected to protect the triac.