Electronic overload protection circuits

Claim 1

Apparatus for preventing excessive current flow through a semiconductor switching element having an output load circuit of a predetermined magnitude connected thereto so that the current flowing through said load circuit is controlled by said switching element and wherein said switching element has a first control electrode whereby the application of a predetermined voltage thereto will cause said switching element to change between its conducting and nonconducting states, said apparatus comprising: first transistor means with the switching path thereof being connected, via said first control electrode, across the input to said semiconductor switching element and operable for diverting said control voltage from said first control electrode thereby rendering said switching element nonconductive, said first transistor means having a second control electrode for changing the state of said first transistor means between its conductive and nonconductive states responsive to a voltage applied thereto, said first transistor means having an emitter-follower output circuit, voltage sensing means for sensing the voltage across said semiconductor element and applying same to said control electrode, said first transistor means being operable in response to a predetermined increase in the current flow through said load circuit and thereby an increase in the voltage across said semiconductor element, and second transistor means having the switching path there connected, via said first control electrode, across the input to said semiconductor switching elemenT, said second transistor means having a third control electrode connected to receive the voltage output from said emitter follower output circuit, said first and second transistors forming a two-stage transistor switching state.