A potentiometer is controlling the firing point of the triac. Capacitor C4 is charged via resistors R3, R4, P1, and R5. After a certain time, dependant on the potentiometer, the charge contained in C4 is large enough for the dimmers diac D to start conducting, so that a firing pulse is applied to the gate of the triac. Consequently the triac conducts and power is transferred from the dimmer to the tungsten or LED lamp. .