| 摘要 |
PURPOSE:To obtain an inexpensive wire disconnection system by flowing a minute current being a degree not detected by a conventional telephone exchange without interrupting the current at the end of call and providing a means monitoring this minute current. CONSTITUTION:A resistor Rs=40kOMEGA is connected in parallel with a subscriber (telephone set or the like) TEL in Figure and a current as small as 1mA is flowed thereto at all times. A contact C is a contact of a telephone exchange and closed normally. A relay L is a call detecting relay of the telephone exchange, restored normally and operated at the closing of a subscriber loop. Resistors RF1, RF2 and capacitors CF1, CF2 of a noise absorbing circuit FIL constitute a low pass filter, which absorbs a noise voltage such as AC induction induced in a subscriber line and gives only the change in a DC potential to an open wire detector DET through a diode DF. If a subscriber wire is opened, the DC potential of said subscriber line is dropped to a power supply voltage -48V. When any subscriber line potential in a subscriber group is lower than a reference potential, a current flows to a resistor RD1 through the diode DF and its terminal potential is made lower than the reference potential. Since the resistance of the L relay is 1kOMEGA, the change in the subscriber line potential depending on the presence of the Rs=40kOMEGA is nearly 1V. The result of detection is given to a controller CONT. |
