High current protection circuit for telephone interface

Abstract

A telephone test set is protected from an excessive current condition by a protection circuit that maintains sufficient loop current flow for continuous operation of the telephone equipment's functional circuitry, while by-passing and/or reducing excess current that could potentially damage the protected circuitry. A series current-limiting element is installed in the loop current path between a tip and ring interface and the circuitry of the telephone test set. A current shunting circuit is installed across the tip-ring ports of the protected circuit downstream of the current-limiting element. A current sense circuit monitors the loop current and controls the operation of the shunting circuit in accordance with the magnitude of the loop current. If a substantial overcurrent condition persists for an extended period of time, the resulting power dissipation associated with the high current flow will cause the current limiting element to change to a high impedance state. Once the overcurrent condition terminates, the series element will revert to its low impedance state.