Output circuit having electronic overload protection activated by

Abstract

An output circuit has multi-stage protection circuitry for protecting against different fault current conditions. The protection circuitry operates by detecting voltage changes across the output transistor of the output circuit. The output circuit comprises first, second and third switching devices. The first switching device is connected to the output device and power conditions the output signal. The second and third switching devices override the output signal and turn off the first switching device when a magnitude of current through the first switching device exceeds predetermined magnitudes for predetermined amounts of time. A protection circuit is also disclosed which has a sensing device that senses a current flow condition by sensing a voltage drop produced across two terminals of the first power conditioning transistor, the two terminals being the terminals through which the current flows. Advantageously, the output circuit utilizes highly flexible two-stage protection circuitry. The protection circuitry separately identifies surge current and overcurrent conditions. In addition to the flexibility provided by the use of two-stage protection circuitry, flexibility is also provided by the fact that component values may be easily varied to vary the magnitudes/durations of current within each stage which activate the protection circuitry. Moreover, the protection circuitry has a very fast response time. As a result, an output transistor which is protected by the protection circuitry of the present invention suffers much less stress in the event of a short circuit condition.