Identification circuit for remotely supplied telephone subscribers - has two switching transistors whose input resistance changes with line current

摘要

The circuit independent from the supply voltage polarity owing to a rectifier bridge preceding the hybrid circuit. Unit a request for a connection is detected, only a relatively low current is supplied to the subscriber line. A resistor is inserted between the subscriber line wires behind the cradle switch. When contacts of the latter are closed, it guarantees a min. current sufficient for recognition of a connection request. First switching transistors (T1, T2) are inserted between the rectifier bridge (GB) and the following hybrid circuit (Ga) instead of the above resistor. The input resistance is low at low supply current, and high at a higher supply current. Second switching diodes (G5/G6, G7/G8) controlling the first (T1, T2) are inserted in one wire (b, a) of the subscriber line. They are controlled by a voltage dropping across resistors (x, y) in the hybrid circuit.