| 摘要 |
PURPOSE:To reduce the heat generation of a commercial power source mixed contact detection circuit at the time of commercial power source mixed contact to a subscriber's line relating to the commercial power source mixed contact detection circuit provided in the subscriber's circuit of a switchboard. CONSTITUTION:The commercial power source mixed contact detection circuit 10 inside the subscriber's circuit 1 is provided with an attenuation circuit 11 for attenuating an AC voltage inputted from the subscriber's line 2 at the time of the commercial power source mixed contact, an AC detection circuit 12 for inputting and rectifying the AC voltage outputted from the attenuation circuit 11, comparing it with a threshold voltage and outputting an AC detection pulse and an AC detection control circuit 13 for comparing the AC detection pulse with a reference pulse set beforehand and when the pulse width of the AC detection pulse exceeds the pulse width of the reference pulse, increasing the attenuation amount of the attenuation circuit so that the pulse width of the AC detection pulse is within the width of the reference pulse and also controlling the threshold voltage of the AC detection circuit to be lowered in a direction for compensating a detection voltage decline due to the increase of the attenuation amount. |
