Thick film hybrid firing circuit
Gives phase control of most loads in the range 1 to 100 amps (depending on choice of triac, heat sink and potentiometer)

The number of complete mains cycles during fixed repeating periods (typically 1 to 10 seconds) is varied, thus altering the duty cycle or on-/off-time ratio. Trigger pulses coincide with the zero-crossing times of the mains supply thus ensuring no RFI is produced when using resistive loads.