High voltage protection circuit for telephone test set

Abstract

An overvoltage protection circuit prevents damage to the electronic circuitry of a craftsperson's test set, in the event of accidental application of a potentially destructive high voltage to one or more leads of the test set. The test set's dial pulsing circuitry employs a power-dissipating N-channel MOSFET as the operative switching component. The protection circuit is coupled to the MOSFET and includes a power-dissipation controlled current-limiting and timing circuit, either as an electronic or thermostat-based implementation. During the dialing mode, the current path through the power MOSFET is alternately turned on and off by the test set's internal dial pulse control circuitry. At all times, the protection circuit provides instantaneous overvoltage protection and constant current limiting. If the excess voltage condition last long enough, the timing circuit regulates average current flow by alternately turning the MOSFET on and off, in a manner that limits average power dissipation in the MOSFET to less than a destructive value. Modulation of the current flow path through the MOSFET is based upon the product of its (source-drain) loop current and its drain-source voltage V.sub.DS, which depends upon the magnitude of the line voltage.