Switching circuit supply for telephone subscriber equipment - has Graetz bridge network using transistors which provides switched supply dependent upon subscriber loop polarity

摘要

A transistor supply circuit for telephone subscriber loops is designed to provide min. power demand. A Graetz bridge circuit is coupled to the telephone subscriber wires (a, b) over two Zener diodes. The bridge circuit consists of a pair of NPN transistors (T1, T2) with base resistors (R1, R2) and a pair of complementary PNP transistors (T3, T4) with base resistors (R3, R4). The bridge circuit is coupled to the telephone subscriber equipment (4). Dependent upon the polarity of the subscriber loop, either one pair of transistors (T1, T2) or the other pair (T2, T3) will conduct.