Current detecting apparatus and its control method

A rectified voltage and a rectified current are supplied from an input terminal (Ti). Resistors (1) and (2) are serially connected between the input terminal (Ti) and an output terminal (To). A load is connected to the output terminal (To). The resistor (1) is provided between an emitter and a collector of the transistor (3) and a constant current detecting circuit (5) is also provided between them. A base of the transistor (3) is connected to a control circuit (4) and the on/off operations of the transistor (3) are controlled by the control circuit (4). The constant current detecting circuit (5) detects a current from a voltage across the resistor (1). A current detecting circuit (6) is connected to both ends of the resistor (2) and detects a current from a voltage across the resistor (2). The constant current detecting circuit (5) detects a small current. The current detecting circuit (6) detects a large current. When the small current is detected by the constant current detecting circuit (5), a signal is supplied to the control circuit (4). The control circuit (4) supplies a signal for increasing an impedance of the transistor (3) to the base of the transistor (3).