PURPOSE: To pretect an internal circuit from being damaged when excessive current flows by connecting a voltage detecting circuit to detect that a constant current circuit consisting of transistors and resistances operates.

CONSTITUTION: At normal operation, a transistor TR9 maintains off status during the period a detecting signal (b) is "L", and current flows from an AC zero point into a load 3. When the load 3 is shorted, the voltage applied to both the ends of a resistance 15 is quickly increased and the base current of a TR6 starts to flow through a TR7. Since impedance between the collector and emitter of the TR6 is increased at that time, the collector and emitter of the TR6 is increased at that time, the voltage of a point (a) rises, current flows through SSS12, the TR9 is turned on, and TRs 8, 6 are turned off. When excessive current is going to flow into the TR6, the TR6 is turned off at the time of constant current, protecting the TR6 from being damaged.