Abstract: A control for the heating section of a blow-molding machine is provided. The control permits independent selection of periods for high and low power energization of each radiant heating element of the heating section heater assembly. Power is provided by an alternating current source and switching of the power to the heating elements is accomplished at the zero crossing points of the supply voltage. To regulate the power in the heating elements against variations of supply voltage from a nominal value, a duty cycle control scheme is provided for selectively applying and withholding full cycles of the supply voltage to the heating elements. The determination of whether a cycle is to be applied or withheld is made by computing a duty cycle ratio comparing the power available from a minimum voltage expected to the power available from the actual supply voltage. Supply cycle switching is accomplished to distribute the application of cycles in two during the periods.