This is an A.C. power control system in which push-pull load current controlliung switches (24,25) are each switched once in each duty cycles which consists of a number of supply cycles (23), the switching being at any phase angle in one of the cycles. There is a half cycle delay circuit (7) arranged so that switching of one switch will be at the same instant in a positive half cycle as the switching of the other switch in the next or previous negative half cycle. Capacitors (17) slow down the switching action of the switches.