PURPOSE: To limit rush current to a load in a circuit turning ON/OFF current to be supplied to the load from a DC power source by using a transistor.

CONSTITUTION: By turning ON of a switch 1, the base of a transistor Q2 is turned ON by adding voltage to it. Thus, DC current from a power source 2 flows to a resistance R3 → the emitter of a transistor Q1 → the base →a resistance R1 → the transistor Q2, the transistor Q1 is turned ON between the emitter and collector, and current is supplied to a load 3 from a power source. In the load, capacitors are connected in parallel and rush current flows for the charging of this. At this time, the voltage at the both ends of the resistance R3 rises, a transistor Q3 is turned ON by lowering the base voltage of the transistor and current flows to the emitter → collector → the resistance R1.... As a result, the base voltage of the transistor Q1 rises and the emitter →collector current of the transistor Q1 is limited.