PURPOSE: To obtain a normal function of a transformer with no saturation of an iron core, by turning on and off a TRIAC based on an on-off command and simultaneously with the output of a circuit that detects an AC zero-cross point.

CONSTITUTION: When a command signal C supplied to the D terminal of an FF14 is changed from on to off, the Q output of the FF14 changes from H to L level. As a result, a transistor TR15 is turned off to eliminate the output (e) of a gate circuit 19. Thus a TRIAC6 is turned off at the zero-cross point where the AC (a) shifts to an adverse phase from a forward phase. When a command signal (c) is turned on again by a request of the secondary side load, the Q output of FF14 is turned to H level synchronously with the rise of a pulse (b). Accordingly, the TR15 is turned on again, and the TRIAC6 is turned off at the zero-cross point where the forward phase shifts to the adverse phase by the output (e) of the circuit 19 and like the off mode. Thus the residual magnetic field stored in the off-state is negative by the current which flows in the on-state. As a result, the iron core is never saturated.