Automatic disconnect circuit for subscriber carrier telephone system

Abstract

In a disconnect circuit including a pair of transistor switches that are connected in series between associated lines of a cable pair and a subscriber carrier charging circuit for selectively blocking central office battery line current to the latter during ringing of a physical subscriber handset and when the latter is off hook and is dialing, the capacitor controlling delayed turn-on of the switches is coupled thereto through a capacitive multiplier circuit. A first resistor, Zener diode, and second resistor are connected between the cable pair input sides of the switches. A control transistor is connected between the base electrodes of the switching transistors and one side of the diode. The capacitive multiplier here comprises a pair of transistors connected as a Darlington compound between the capacitor and diode for converting a variable input current to a constant low-level current for charging the capacitor, the multiplier transistors bypassing the remainder of this input current. This circuit operation or arrangement greatly increases the time for the capacitor to charge to a level that is sufficient to turn on the diode, control transistor, and switches without increasing the actual capacitance of the capacitor.