Surge voltage shunt protection circuit for phone line junction - has two diodes for one polarity protection and two transistor threshold circuits for opposite polarity protection

摘要

A subscriber telephone junction (1) is suppli ed through an exchange battery (9) in series with an auxiliary supply source (10). The junction output lines (4,5) are protected against surge voltages by two diodes (16,17) connected between the respective lines and one battery terminal and two circuits (20,21) connected between the respective lines and the opposite battery terminal to pass current in the inverse direction to the diodes. Each circuit comprises a resistor (24) diode (25) resistor (26) series connected from the line to battery, both resistances being high value. The base- emitter junction of one transistor (27) is connected across the first resistor and drives the base of a second opposite polarity transistor (29) whose emitter connected to the terminal and collector to the first transistor base. An excessive voltage rapidly turns on both transistors to present a very low resistance to the surge voltage.