Abstract:
An improved high voltage module for use in conjunction with a standard power module. The high voltage module includes a floating power supply in parallel with the standard power module and without ground connection included in the high voltage module are a plurality of stages of Darlington amplifiers. The two modules are opticoupled in the circuit with provision for fast turn-on and turn-off networks for the opticoupling network.

Description:
BACKGROUND OF THE INVENTION 
     The field to which this invention is applicable is that of electrical discharge machining, sometimes hereinafter referred to as &#34;EDM&#34; in which process material is removed from an electrically conductive workpiece by erosive electrical discharges passed across a dielectric filled gap from a tool electrode. The process is precisely controlled, usually by the use of an independent pulse generator and one or more electronic output switches in an output module. The output module switches are periodically turned on and off at a preset frequency to connect and disconnect a DC power supply from the gap thus to provide machining power pulses. In some types of machining operations, it is desirable to use a higher cutting voltage than that of a standard power module. In standard EDM operations, the power supply voltage may be of the level of 70-80 volts. When high voltage or hi pol machining is used, the voltage level is raised to as high as 180 or 200 volts particularly for finishing operations. It is important that the power supply have the capibility of being operated in either mode for both roughing and finishing operations. 
     SUMMARY OF THE PRESENT INVENTION 
     This invention provides a high voltage output module for EDM which is of the opticoupled and high voltage type. The entire high voltage module is floating and imposed on top of a standard power module. The module and its DC supply are all electrically isolated from the gap. The only gap connection is through the work leads so the module can be turned for either plus or minus output or split output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in detail in the accompanying specification and drawing in which like reference numerals are used to refer to like parts. The drawing is of the combined block and schematic type. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made to the drawing for its showing of a basic EDM power supply that includes an independent pulse generator such as multivibrator 10. The multivibrator 10 provides triggering pulses through intermediate amplifier stages 13 as required. The triggering pulses are used to trigger into machining operation either or both of a standard power module 12 or hi-pol module 14. The standard power module 12 includes output electronic switches and a DC source. At lower machining voltage levels such as, for example, 70 or 80 volts, the initial roughing stages of the EDM operation are carried on across the machining gap which includes electrode E and workpiece W. 
     Reference is now made to the high voltage module 14 which includes a DC source 16 of the order of 200 volts or more. The DC source 16 is periodically connected and disconnected from the gap by the operation of an electronic output switch transistor 18. The transistor 18 has its principal electrodes operatively connected to the positive gap terminal at workpiece W through a current limiting resistor 20. The transistor 18 has connected to it a drive stage comprising drive transistor 22 which has its collector connected to the base of the transistor 18 through a resistor 24 in Darlington configuration. A further resistor 26 is connected between the emitters of the two transistors 18, 22 as shown. 
     Also shown in the drawing are the additional stages required to provide machining power pulses to gap from the transistor 18. These include an opti-coupling network 28 that has as its input a series resistor 30 and a diode 32 connected as shown. 
     Also included in the output of the transistor 18 is a further series resistor 19 and a relay actuated contact 21 which may be closed where desired to add additional series resistance in the output of the transistor 18 thus to limit the magnitude of gap current. 
     The opti-coupling network 28 will be seen to include an opticoupler of the integrated circuit type having a light emitting diode 34 between input pins 2 and 3 and an amplifier 38 and a gate 40 coupled between pins 7, 8, 5 and 4 as shown. When the diode 34 is energized by passing a current through it, i.e. a machining power pulse from the multivibrator 10, a light is transmitted internally to a photo-diode 36. The output of the photo-diode 36 is amplified by amplifier 38 and the gate 40 is used to turn on the next following output transistor 42. The opti-coupler stage 28 operates as relatively fast acting device by reason of the inclusion of the amplifier 38 and the gate 40. 
     The output pin 6 from the opticoupler stage 28 provides a signal to the base of the transistor 42. A suitable resistor 43 is connected between pin 6 and ground. 
     The collector output from the transistor 42 is connected to ground and to a plus voltage source through a resistor 44 and a capacitor 46. The signal from the collector of the transistor 42 is then passed to the base of the first Darlington transistor 22 where the signal is inverted and applied to the base of transistor 18 for turn-on. The output Darlington transistor 18 has its emitter connected as already indicated through one or more output resistors 19, 20 so that suitable current limiting can be provided through the operation of one or more relays 21. It will be seen that the collector of the transistor 22 is connected to the terminal of the DC source 16 through a resistor 48. 
     It is necessary to develop a negative voltage signal through lead 53 as shown to provide fast turnoff. The upper portion of the drawing shows a network for developing this signal and applying it as an input to pin 5 of the opticoupling stage 28. The input to the lefthand terminal of the network is through a series resistor 54. The voltage is clamped to a 15 volt level through a zener diode 56 and associated capacitors 58, 60. The signal is then passed as an input to an astable multivibrator of the integrated circuit type 62. External resistors 64, 66 and capacitors 68 to ground are coupled as shown. The output from the astable multivibrator 62 is passed through a signal RC network 70 to the bases of a next following push-pull stage including transistors 72 and 74. Operating voltage for the push-pull stage is provided from a positive voltage source and a resistor 76 connected in series with the collector of the transistor 72. 
     A relatively small magnitude choke 78 is included in the circuit to provide necessary impedance. Capacitor 80 is connected to the next following fileter and retification stage including diodes 82, 84 and a capacitor 86. The negative voltage signal is then passed through a voltage regulator 88 and thence through the lead 53 as an input to the opticoupling stage 28. An additional capacitor 90 is connected to ground as shown. 
     It will be seen that the high voltage module 14 provides an improved high voltage machining EDM operation at high frequencies with effective turn-on and turn-off characteristics. Isolation of the DC source from the gap is complete except where it is connected through work leads. A polarity reversing switch 75 is connected in the gap circuit as shown. In standard polarity, the electrode is negative relative to the workpiece. In reverse polarity, the electrode is positive relative to the workpiece. The condition of standard polarity is illustrated in the drawing. A blocking diode 77 is included between the high voltage lead and the standard power module.