PURPOSE:To prevent sudden change of a call current, by controlling a time average value of a call current supplied to a terminal device of a telephone exchanger so as to close to a reference current. CONSTITUTION:A call current is supplied to a subscriber line 1000 via a smoothing circuit from a DC power supply 100 and to a terminal device 300. A call current detecting circuit 200 detects a voltage across a resistor 201 connected between a switching power supply circuit 110 and the terminal device 300 and gives a DC voltage in response to the call current value to an operation circuit 400. The integration operation circuit 400 integrates an output voltage from the circuit 200 and calculates a time average of the call current value. A comparison circuit 600 compares this average value with an output voltage of a reference voltage generating circuit 500 and gives a control signal to the switching circuit 110 so as to close the averaged value to the reference voltage.
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