Short circuit protection system for electrical discharge machining apparatus

Claim 1

In an electrical discharge machining apparatus for machining a conductive workpiece by electrical discharges passed from a tool electrode to the workpiece across a dielectric coolant filled gap, a periodically operated output switch and a power supply operatively connected to said gap for providing machining power pulses thereto, a pulse generator for providing triggering pulses, and a drive stage operatively connected between said pulse generator and said output switch for resquaring and amplifying said pulses from said, pulse generator wherein the improvement comprises a cut-off stage for interrupting pulses from said drive stage responsive to gap short circuit condition, said drive stage including a bistable flip-flop, including a pair of electronic switches, one of said electronic switches normally conductive in phase with said output switch, said cutoff stage operatively connected between said gap and the other electronic switch of said flip-flop for rendering it conductive and thus effective to hold off pulses to said output switch responsive to said condition, said cutoff stage further being diode coupled to a point between said multivibrator output and said generator output to bleed the output signal from said multivibrator during said condition.