Power supply control circuit for subscriber carrier telephone system

Abstract

A control circuit 16 that is responsive to voltage conditions on a cable pair transmission line for selectively disabling a battey charging circuit 12 operating off of line current comprises a resistor R5 and storage capacitor C2 connected in series across the line; a timing capacitor C1 connected between the cathode of a PUT (programmable unijunction transistor) and one wire 32 of the line; and a zener diode D1 between the PUT anode and one wire 32 for establishing a conduction threshold for the PUT. The resistor R5 and primary conduction path of a first bipolar transistor Q2, having its base connected to the PUT anode, are connected in series between the other wire 33 of the line and the charging circuit so that startup current for the charging circuit may be used for charging C2 when the charging circuit is disabled. Means is also included for driving the PUT gate voltage low and turning on the PUT in response to both high and low voltage conditions on the line. This dumps the C2 charge into C1, the latter discharing slowly for holding the charging circuit diasabled. A second bipolar transistor Q4 senses the charge voltage on C1 for decreasing the PUT gate voltage when the PUT conducts and for increasing the PUT gate voltage immediately prior to the control circuit enabling the charging circuit to draw line current in order to prevent on-off cycling of the latter on a long cable pair.