| 摘要 |
PURPOSE:To attain large scale integration (LSI) by adding an AC voltage superimposed by a call signal to an AC voltage being retarded by a half period so as to cancel the AC voltage and obtain only a DC component thereby stabilizing the ring trip operation and making the circuit small and simplifying it. CONSTITUTION:A call signal current flows to a power supply VB from a ringer 10 through a resistor R, a subscriber line 4 and a telephone set 5, at an incoming call. In hooking off a telephone set 5, a DC current flows to a resistor R from the power supply VB through a telephone set 5. The current is converted by a current voltage conversion section 1 and is sent out as an output signal AS, and the signal is superimposed with the call signal. In giving the signal AS to a delay means 6, the signal BS having a waveform retarding by a half period is obtained. In adding the signals AS and BS to an adder section 7, the output voltage of the adder section 7 has a waveform of the DC component only while the AC components of both the output signals are cancelled. The output voltage is compared with a reference voltage VS by a comparison section 8 and when the output voltage exceeds the voltage VS by off-hook, a trip signal TP(1) is sent.
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