Electric discharge machining apparatus with current control means

Claim 1

A SPARK DISCHARGE MACHINING APPARATUS COMPRISING A PAIR OF ELECTRODES SEPARATED BY A SPARK GAP, ONE OF SAID ELECTRODES CONSISTING OF A TOOL AND THE OTHER OF A WORKPIECE, A CAPACITOR CONNECTED ACROSS SAID ELECTRODES, AN MEANS TO CHARGE SAID CAPACITOR TO THE IGNITION POTENTIAL OF THE GAP BETWEEN SAID ELECTRODES, INCLUDING A SOURCE OF DIRECT-CURRENT POTENTIAL HAVING POSITIVE AND NEGATIVE END TERMINALS AND A MID TAP ADAPTED TO PROVIDE A POTENTIAL INTERMEDIATE THAT EXISTING BETWEEN SAID END TERMINALS, A RESISTOR AND A DIODE CONNECTED IN SERIES BETWEEN ONE OF SAID END TERMINALS AND SAID MID TAP, SAID RESISTOR AND DIODE BEING CONNECTED TOGETHER BY A JUNCTION AND SAID DIODE BEING POLED TO PERMIT CURRENT TO FLOW FROM SAID SOURCE THROUGH SAID RESISTOR, AND A TRANSISTOR HAVING EMITTER AND COLLECTOR ELECTRODES CONNECTED IN SERIES WITH SAID CAPACITOR BETWEEN SAID JUNCTION AND THE OTHER OF SAID END TERMINALS OF SAID SOURCE, WHEREBY THE POTENTIAL ON THE TRANSISTOR IS LIMITED TO THE POTENTIAL BETWEEN SAID MID TAP AND SAID OTHER END TERMINAL, AND THE CURRENT FLOW THROUGH THE TRANSISTOR IS LIMITED BY THE IMPEDANCE OF SAID RESISTOR.