PURPOSE: To make any special circuit protecting element unnecessary, by a method wherein a discharging element is permitted to be employed for the protections of a power source circuit and a load commonly except the detection of overvoltage.

CONSTITUTION: An AC input is rectified by a rectifying circuit constituted of diodes D1∼D4, then, is smoothed by capacitors C1, C2 and is supplied to loads L1, L2 through a DC/DC converter. when a switch SW is opened, the rectifying circuit becomes a full wave rectifying circuit but it becomes a voltage multiplying rectifier when the switch is closed. The series connecting body of a resistor R and a discharge tube NL is connected between the terminals 3, 4 of the rectifying circuit. when the switch SW is closed by error or when the circuit is abnormal, the discharge tube NL is conducted and a current is conducted through the resistor R, therefore, a fuse F is blown to protect the title power source circuit and the loads.