Overcurrent protection circuit for power transistors

DETAILED DESCRIPTION I claim: 1.
A protection circuit for a power transistor of the type having its collector and emitter connected to positive and negative DC rail lines, and having a base that when supplied with positive current, causes the power transistor to conduct, and when supplied with negative current, causes the power transistor to cut off, the circuit comprising in combination: positive and negative control transistors, one having an input connected to a positive voltage source, and the other having an input connected to a negative voltage source, each control transistor having an output being connected to the base of the power transistor for providing positive current while the positive control transistor is on, and negative current while the negative control transistor is on; control means for supplying energizing signals to the control transistors to turn on the positive control transistor and off the negative control transistor, and deenergizing signals to the control transistors to turn off the positive control transistor and on the negative control transistor; latch means comprising a bistable multivibrator having an input connected to the power transistor for monitoring the voltage across the collector and emitter, and if the voltage exceeds a selected level while the power transistor is on, providing a disabling output; disabling means connected to the multivibrator and to the control means for turning off the positive control transistor and for turning on the negative control transistor when the disabling means receives a disabling output from the multivibrator; and clamp means for holding the input of the multivibrator to a low value below the threshold level while the control means is supplying a deenergizing signal to the control transistors, to prevent triggering of the multivibrator while the power transistor is off, the clamp means including a clamp line extending from the control means to the input of the multivibrator, and including blocking means in the clamp line for blocking the energizing signals from the input of the multivibrator to hold the input of the multivibrator to the low value