| 摘要 |
PURPOSE:To reduce power consumption by providing a loop detecting circuit LDET corresponding to a subscriber line L and detecting a closed loop of a channel by the loop detecting circuit LDET to turn on a power supply only at the calling state in which a charging signal is transmitted from a telephone station. CONSTITUTION:Since a channel forms a closed loop at calling state, a loop current flows to a light emitting section of a photocoupler PF. Thus, a photodetecting section of the photocoupler PF is turned on, a power supply voltage is applied to a unit SPU1 or SPU2, and the detection of receiving of the charging signal in 50Hz, 12kHz or 16kHz transmitted from a telephone station via a subscriber line is attained by a charging signal receiving circuit section REC1 or REC2. If an excess current flows, a resistor R acts like a current limiting resistor and diodes D1, D2 are operated as a shunt circuit, allowing to protect the photocoupler PF. |
