Short circuit protection circuit for telephone exchange - has peak current sensitive cut-out transistor permitting A.C. path for call signal

摘要

Feed lines (AC3 and AC3') connect a subscriber station (TSt-2) with the current and call signal supply circuit which includes transformer coupling (Tr2 and Tr2') for introducing the ringing alternating current supply, two series-connected transistors (Tra1 and Tra2) and a current sensor resistor (R1). A current sensor unit (A) controls a translator (Tra3) which at a predetermined max. current supplied by the central battery (G1) to render the transistors (Tra1 and Tra2) non-conductive. When a short circuit occurs in the subscriber feed line or at the current which condition is held in a bistable stage is part of the current sensor unit (A). However, on application of an external signal (u) the blockage can be removed and the line condition tested. It is possible to make the current sensor (A) generate repeated pulses (u) at selected time intervals so as to make the line operative within a reasonably short time after the short circuit is removed. The diodes (D1, D2, D3, D4) which bridge the blocking transistors allows the ringing current to pass the supply circuit at its self-limiting strength. PS.