Abstract: Ballast control circuit for regulating power supply to high intensity discharge lamp by compensating for changes in line voltage and lamp voltage. Current flowing in power supply to lamp is controlled by a circuit including a triac inductively coupled to the supply line, a triggering circuit for firing the triac at predetermined time intervals for phase control of the alternating current supply to the lamp, a non-linear amplifier circuit for controlling the phase interval of firing of the triac, a synchronizing circuit for synchronizing the triac firing with the lamp voltage, and a voltage reference circuit for controlling the operation of the non-linear amplifier circuit in response to changes in the lamp voltage. A power supply circuit is connected to the alternating current supply for supplying direct current with both regulated and unregulated voltage to the above-described control circuit.