Over voltage protection for telephone terminal transistors - blocks two electronic switches that replace dialling pulse contacts for duration of over voltage

摘要

The overvoltage protection, for a telephone terminal transistor, has two electronic switches replacing the mechanical dialling switch pulsing contacts. The first electronic switch is connected to the voice circuit, and the second electronic switch is in parallel with the series combination of first switch and voice circuit. When an overvoltage occurs, the two electronic switches are blocked for as long as that overvoltage is present. A voltage stabiliser (ST) and a voltage divider (R27, R28) are connected between the positive and negative poles of the rectifier bridge (GL). A threshold voltage diode (ZS) consisting of two diodes connected inversely in series, are across the input terminal lines to the bridge.