Method and apparatus for controlling the power supply for electrical discharge machine

Claims

What is claimed is:

1. A method for machining a workpiece using a power supply and switches for supplying pulse-shaped electric power to a machining gap between an electrode and a workpiece provided in a dielectric to machine the workpiece comprising the steps of:

switching on/off said switching devices in an optional cycle under the control of a current command value signal corresponding to an optional waveform shape of a current pulse to be supplied to said machining gap;

superimposing a current component, for compensating for a current ripple generated by said switching at the supply time of said current, on said optional waveform shape of a current pulse in order to generate a resultant current; and

supplying said resultant current to said machining gap.

2. A method for machining a workpiece as set forth in claim 1, wherein said current component is provided by selectively switching a plurality of parallel resistors.

3. A method for machining a workpiece as set forth in claim 1, wherein said current component is provided by selectively operating an analog switching device in series with at least one resistor.

4. A method for machining a workpiece as set forth in claim 1, further comprising accumulating and supplying electrical energy intermittently to said machining gap.

5. A power supply apparatus for an electrical discharge machine for supplying a predetermined pulse-shaped electric power to a machining gap between an electrode and a workpiece comprising:

a first machining circuit comprising a power supply for generating a first current and for supplying machining energy to said machining gap, and a first switching device and a first resistor, said power supply, first switching device and first resistor being connected in series;

current detection means for detecting said first current flowing in said first machining circuit;

a second machining circuit comprising at least one series circuit, said series circuit comprising at least one second switching circuit means and being connected in parallel with said first switching device and said first resistor in said first machining circuit to supply said machining gap with a second current superimposed on said first current from said first machining circuit;

means for setting a current command value signal corresponding to the waveform shape of a current pulse to be supplied to said machining gap;

first signal addition/subtraction means for operating on and outputting a difference between at least a part of said current command value signal and a part of the output from said current detection means;

first control means for outputting a signal to said first switching device in said first machining circuit according to the output of said first signal addition/subtraction means;

second signal addition/subtraction means for operating on and outputting a difference between said current command value signal and the output of said current detection means; and

second control means for outputting a switching signal to one or more of said at least one second switching circuit means in said second machining circuit according to the output of said second signal addition/subtraction means.

6. A power supply apparatus for an electrical discharge machine as set forth in claim 5, wherein said at least one second switching circuit means comprises a series connection of a second switching device and a second resistor.

7. A power supply apparatus for an electrical discharge machine as set forth in claim 5, wherein said at least one second switching circuit means comprises a semiconductor amplifier.

8. A power supply apparatus for an electrical discharge machine as set forth in claim 5, wherein said first machining circuit comprises:

an electrical energy accumulation circuit comprising said first switching device, a reactor and a first diode connected in series for intermittently supplying and accumulating electrical energy from said power supply;

a third switching device connected to supply said machining gap with an output current from said electrical energy accumulation circuit for supplying said output current to said machining gap in a pulse shape; and

a second diode connected to return to said electrical energy accumulation circuit a residual current generated in said machining gap when said third switching device is switched off.

9. A power supply apparatus for an electrical discharge machine as set forth in claim 8 wherein:

said second machining circuit comprises at least one semiconductor amplifier connected in parallel with said electrical energy accumulation circuit in said first machining circuit to supply said machining gap with said second current.

10. A power supply apparatus for an electrical discharge machine for supplying pulse-shaped electric power to a machining gap between an electrode and a workpiece provided in dielectric, comprising:

a first current source for supplying a first current to said machining gap, said current being defined by a pulse form and response speed;

current detection means for detecting said first current supplied to said machining gap by said first current source;

a second current source connected in parallel with said first current source and being operative to supply said machining gap with a second current superimposed on said first current to form a resultant current, said second current source being higher in output current response speed than said first current source;

means for setting a current command value signal corresponding to the waveform shape of the current pulse to be supplied to said machining gap;

arithmetic means for arithmetically modifying said current command value signal;

first control means for providing the output of said arithmetic means as a current command value to said first current source; and

second control means for outputting a difference between said current command value signal and an output from said current detection means as the current command of said second current source.

11. A power supply apparatus for an electrical discharge machine as set forth in claim 10, wherein said arithmetic means comprises means for subtracting a predetermined value from said current command value.

12. A power supply apparatus for an electrical discharge machine as set forth in claim 10, wherein said arithmetic means comprises means for multiplying a predetermined value in excess of 0 and not greater than 1 times said current command value.

13. A power supply apparatus for an electrical discharge machine for supplying pulse-waveform-shaped electric current to a machining gap between an electrode and a workpiece comprising:

a first current source for supplying a current pulse defined by an output response speed to said machining gap;

current detection means for detecting a current supplied to said machining gap by said first current source;

a second current source connected in parallel with said first current source, constituted to supply said machining gap with a current superimposed on the current from said first current source, and higher in output current response speed than said first current source;

a third current source connected in parallel with said first current source, capable of supplying a current in a direction opposite to the current supplying direction of said second current source, and higher in output current response speed than said first current source;

means for setting a current command value signal corresponding to the waveform shape of the current pulse to be supplied to said machining gap;

first control means for outputting said current command value signal to said first current source;

second control means for outputting a first polarity difference between said current command value signal and a current signal detected by said current detection means as the current command of said second current source; and

third control means for outputting a second polarity difference between said current command value signal and a current signal detected by said current detector as the current command of said third current source.

14. A method for controlling a power supply for an electrical discharge machine comprising a constant current supply section having at least a first switching device and an output current on-off section having a second switching device to supply machining power to a machining gap between an electrode and a workpiece provided in dielectric, said method comprising the steps of:

setting the output current level and output current ripple of said constant current supply section;

defining the addition result of said set output current level and output current ripple as the output current command signal of said constant current supply section and comparing said output current command signal with the output current of said constant current supply section; and

controlling the switching device of said constant current supply section according to the result of said comparing step.

15. A power supply apparatus for an electrical discharge machine comprising a constant current supply section having at least a first switching device and an output current on-off section having a second switching device to supply machining power to a machining gap between an electrode and a workpiece provided in dielectric, comprising:

detection means for detecting the output current of said constant current supply section and outputting a detection value;

output current level setting means for setting the value of the output current level of said constant current supply section;

ripple current setting means for setting the value of the output current ripple of said constant current supply section;

comparing means for comparing a set value found by adding the set value of said ripple current setting means to the set value of said output current level setting means with said detection value of said detection means; and

means for on-off controlling said first switching device in the constant current supply section according to the comparison of said comparing means.

16. The power supply apparatus for the electrical discharge machine as defined in claim 15, wherein said ripple current setting means comprises modulating means for modulating the set signal frequency of said ripple current set value according to the set value of said output current level setting means.

17. A power supply apparatus for an electrical discharge machine comprising a constant current supply section having at least a first switching device and an output current on-off section having a second switching device to supply machining power to a machining gap between an electrode and a workpiece provided in dielectric, comprising:

detection means for detecting the output current of said constant current supply section;

output current level setting means for commanding the output current level of said constant current supply section;

ripple current setting means for outputting a ripple current set value setting signal for setting the ripple in the output current of said constant current supply section;

means for outputting a synchronization signal synchronized with the ripple current set value setting signal of said ripple current setting means;

comparing means for comparing a set value, found by adding the set value of said ripple current setting means to the set value of said output current level setting means, with the detection value of said detection means;

gate means for receiving the output of said comparing means and said synchronization signal to eliminate noise generated when said first switching device is switched on/off; and

means for on-off controlling said first switching device according to the output of said gate means.

18. A power supply apparatus for an electrical discharge machine comprising constant current supply sections having at least first switching devices and output current on-off sections having second switching devices to supply machining power to a machining gap between an electrode and a workpiece provided in dielectric, comprising:

a first constant current supply section;

a second constant current supply section;

first detection means for detecting the output current of said first constant current supply section;

second detection means for detecting the output current of said second constant current supply section;

output current level setting means for setting the output current levels of said first and second constant current supply sections;

first ripple current setting means for setting the output current ripple of said first constant current supply section;

second ripple current setting means for setting a set value 180 degrees out of phase with the set value of said first ripple current setting means;

first comparing means for comparing a set value, found by adding the set value of said first ripple current setting means to the set value for said first and second constant current supply sections for said output current level setting means, with the detection value of said first detection means;

second comparing means for comparing a set value, found by adding the set value of said second ripple current setting means to the set value for said first and second constant current supply sections for said output current level setting means, with the detection value of said second detection means; and

means for on-off controlling said first switching devices in the first and second constant current supply sections according to the comparison results of said first and second comparing means.

19. A power supply apparatus for an electrical discharge machine comprising a constant current supply section having at least a first switching device and an output current on-off section having a second switching device to supply machining power to a machining gap between an electrode and a workpiece provided in dielectric, comprising:

detection means for detecting the output current of said constant current supply section;

output current level setting means for commanding the output current level of said constant current supply section;

comparing means for comparing the set value of said output current level setting means with the detection value of said detection means to output a signal which switches off the first switching device in the constant current supply section according to the result of said comparison;

timer means for receiving the comparison output of said comparing means and outputting a signal which switches on the first switching device in said constant current supply section when a predetermined period of time elapses after said comparing means has output a signal which switches off the first switching device in said constant current supply section; and

means for on-off controlling the first switching device according to the output of said timer means.

20. A method for controlling a power supply for an electrical discharge machine comprising a constant-current supply section with a first direct-current power supply, a first switching device, a diode and a reactor for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, comprising the steps of:

switching said first switching device on/off at desired intervals to supply said machining gap with said current corresponding to a current command value signal from said first direct-current power supply; and

adding a current for suppressing the reduction of an output current occurring at the off time of said first switching device.

21. A power supply apparatus for an electrical discharge machine comprising a constant-current supply section having a first direct-current power supply, a first switching device connected to one pole of said first direct-current power supply, a reactor connected in series with said first switching device, a first diode one end of which is connected to the other pole of said first direct-current power supply and the other end of which is connected to the connection point of said first switching device and said reactor, an output current on-off section having a second switching device for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, a series connection of a second direct-current power supply having a voltage capable of supplying said machining gap with a voltage substantially equal to or lower than an electrical discharge voltage, a third switching device and a second diode connected in parallel with said first diode of said constant-current supply section.

22. The power supply apparatus for an electrical discharge machine as defined in claim 21, further comprising:

first comparing means for comparing a current command value with a current detection value of said reactor and generating an inversion output signal;

second comparing means for comparing an overcurrent command value with said current detection value of said reactor;

timer means whose input is connected to the output of the first comparing means; and

first status storing means having a reset input connected to the output of said second comparing means and a set input connected to the output inversion signal of said first comparing means;

wherein said first switching device is controlled by the product of the output of said timer means, the output of said first status storing means and an electrical discharge signal, said third switching device is controlled by the product of the output of said first status storing means and the electrical discharge signal, said second switching device is controlled by the electrical discharge signal; and

wherein said first switching device is switched off for a period of time set by said timer means when the current detection value of said reactor exceeds the current command value, and said third switching device also is switched off when the current detection value of said reactor exceeds the overcurrent command value.

23. A method for controlling a power supply for an electrical discharge machine comprising a constant-current supply section comprising a first direct-current power supply, a first switching device, a diode and a reactor for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, comprising the steps of:

supplying said machining gap from a second direct-current power supply with a voltage higher than an electrical discharge voltage and lower than a voltage supplied by said first direct-current power supply and switching said first switching device off when an output current is at a predetermined current level; and

switching on/off a switching device different from the first switching device at desired intervals to control the current from said second direct-current power supply.

24. A power supply apparatus for an electrical discharge machine comprising a constant-current supply section having a first direct-current power supply, a first switching device connected to one pole of said first direct-current power supply, a reactor connected in series with said first switching device, and a diode one end of which is connected to the other pole of said first direct-current power supply and the other end of which is connected to the connection point of the first switching device and the reactor, and an output current on-off section having a second switching device for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, wherein a series connection of a second direct-current power supply has a voltage capable of supplying said machining gap with a voltage substantially equal to or lower than an electrical discharge voltage, a third switching device and a diode is connected in parallel with the diode of said constant-current supply section, and a series connection of a third direct-current power supply having a voltage capable of supplying the machining gap with a voltage higher than the electrical discharge voltage and lower than a voltage supplied by said first direct-current power supply, a fourth switching device and a diode is connected in parallel with the diode of said constant-current supply section.

25. The power supply apparatus for an electrical discharge machine as defined in claim 24, further comprising first comparing means for comparing a current command value with a current detection value of said reactor and generating an inversion output, second comparing means for comparing an overcurrent command value with the current detection value of said reactor, timer means whose input terminal is connected to the output of said first comparing means, and first status storing means having a reset input terminal connected to the output of said second comparing means and a set input terminal connected to the inversion output of said first comparing means;

wherein said fourth switching device is controlled by the output of said timer means and a current increase signal, the output of said first status storing means and an electrical discharge signal;

said first switching device is controlled by the product of the output of said timer means, the current increase signal and the electrical discharge signal;

said third switching device is controlled by the product of the output of said first status storing means and the electrical discharge signal;

said second switching device is controlled by the electrical discharge signal; and

further, said fourth switching device is switched off for a period of time set in the timer means when the current detection value of the reactor exceeds the current command value, and said third and fourth switching devices also are switched off when the current detection value of the reactor exceeds the overcurrent command value.

26. A power supply apparatus for an electrical discharge machine comprising a constant-current supply section having a first direct-current power supply, a first switching device connected to one pole of said first direct-current power supply, a reactor connected in series with said first switching device, and a diode one end of which is connected to the other pole of said first direct-current power supply and the other end of which is connected to the connection point of the first switching device and the reactor, and an output current on-off section having a second switching device for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, said apparatus further comprising:

a series connection of a second direct-current power supply having a voltage capable of supplying said machining gap with a voltage substantially equal to or lower than an electrical discharge voltage;

a third switching device and a diode connected in parallel with the diode of said constant-current supply section; and

a series connection of a fourth direct-current power supply capable of changing a voltage, a fourth switching device and a diode connected in parallel with the diode of said constant-current supply section.

27. The power supply apparatus for an electrical discharge machine as defined in claim 26, further comprising:

first comparing means for comparing a current command value with a current detection value of said reactor;

second comparing means for comparing an overcurrent command value with the current detection value of said reactor;

timer means having an input terminal connected to the output of said first comparing means; and

first status storing means having a reset input terminal connected to the output of said second comparing means and a set input terminal connected to the inversion output of said first comparing means;

wherein said fourth switching device is controlled by the product of the output of said timer means, the output of said first status storing means and an electrical discharge signal, said first switching device is controlled by the product of a no-load voltage signal and the electrical discharge signal, said third switching device is controlled by the product of the output of said first status storing means and the electrical discharge signal, said second switching device is controlled by the electrical discharge signal, and further said fourth switching device is switched off for a period of time set by said timer means when the current detection value of said reactor exceeds said current command value, and said third and fourth switching devices also are switched off when the current detection value of the reactor exceeds the overcurrent command value.

28. A power supply apparatus for an electrical discharge machine comprising a constant-current supply section having a first direct-current power supply, a first switching device connected to one pole of said first direct-current power supply, a reactor connected in series with said first switching device, and a diode one end of which is connected to the other pole of said first direct-current power supply and the other end of which is connected to the connection point of said first switching device and the reactor, and an output current on-off section having a second switching device for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, said apparatus further comprising:

a series connection of a second direct-current power supply having a voltage capable of supplying said machining gap with a voltage substantially equal to or lower than an electrical discharge voltage;

a third switching device and a diode connected in parallel with the diode of said constant-current supply section; and

a series connection of a third direct-current power supply having a voltage capable of supplying the machining gap with a voltage higher than a voltage supplied by said first direct-current power supply, a fourth switching device and a resistor, connected in parallel with said machining gap.

29. The power supply apparatus for an electrical discharge machine as defined in claim 28, further comprising:

first comparing means for comparing a current command value with a current detection value of said reactor;

second comparing means for comparing an overcurrent command value with the current detection value of said reactor;

timer means whose input terminal is connected to the output of said first comparing means; and

first status storing means having a reset input terminal connected to the output of said second comparing means and a set input terminal connected to the inversion signal of the output of said first comparing means;

wherein said first switching device is controlled by the product of the output of said timer means, the output of said first status storing means and an electrical discharge signal, said third switching device is controlled by the product of the output of said first status storing means and the electrical discharge signal, said second switching device is controlled by the electrical discharge signal, and further said first switching device is switched off for a period of time set by said timer means when the current detection value of said reactor exceeds the current command value, said third switching device also being switched off when the current detection value of said reactor exceeds the overcurrent command value, and said fourth switching device is switched on by the product of a high-voltage pulse signal and the electrical discharge signal.

30. A power supply apparatus for an electrical discharge machine comprising a constant-current supply section having a first direct-current power supply, a first switching device connected to one pole of said first direct-current power supply, a reactor connected in series with said first switching device, and a diode one end of which is connected to the other pole of said first direct-current power supply and the other end of which is connected to the connection point of said first switching device and said reactor, and an output current on-off section having a second switching device for supplying a machining power to a machining gap between an electrode and a workpiece disposed in a dielectric fluid, wherein:

said first direct-current power supply comprises a plurality of direct-current power supplies having predetermined voltages and connected in series with each other, one of said plurality of direct-current power supplies being a second direct-current power supply having a voltage capable of supplying said machining gap with a voltage substantially equal to or lower than an electrical discharge voltage;

one end of a series connection of a third switching device and a diode is connected to the connection point of said second direct-current power supply and a third direct-current power supply, and the other end thereof is connected to the connection point of said first switching device and said reactor.

31. The power supply apparatus for an electrical discharge machine as defined in claim 30, wherein one of said plurality of direct-current power supplies is employed as a fourth direct-current power supply having a voltage capable of supplying said machining gap with a voltage higher than the electrical discharge voltage and lower than a voltage supplied by the first direct-current power supply in conjunction with said second direct-current power supply, one end of a series connection of a third switching device and a diode is connected to the connection point of said third direct-current power supply and fifth direct-current power supply other than said second direct-current power supply, and the other end thereof is connected to the connection point of said first switching device and said reactor.

32. The power supply apparatus for an electrical discharge machine as defined in claim 30, wherein a sixth direct-current power supply is connected to said first direct-current power supply, one end of a series connection of a fifth switching device and a resistor is connected to one end of said sixth direct-current power supply, and the other end, thereof is connected to one of the electrode and the workpiece.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for controlling a power supply of an electrical discharge machine which supplies machining power to a machining gap between an electrode and a workpiece provided in dielectric.

2. Description of the Background Art

An electrical discharge machine supplies a constant current pulse to a machining gap to melt a workpiece and remove molten material therefrom and to machine the workpiece by the discharge of energy. Generally, the following four conventional power supply circuit arrangements are used to supply the constant current pulse.

One known circuit arrangement for a first power supply apparatus is shown in FIG. 54. This arrangement is, for example, disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 as a "Pulse Generator Used with the Electrical Erosion Machine Tool."

In FIG. 54, the numeral 1 indicates an electrode, 2 denotes a workpiece, 3 designates a control circuit for a switching device 4, 4 represents a switching device, 5 indicates a power supply for supplying a machining current, 6 designates a diode for causing a residual current to flow, 7 represents a current detection resistor, 8a and 8b denote stray inductances of wiring, 9 indicates a comparator, 10 represents an envelope signal generator, and 18 designates a servo device for exercising the servo control of the electrode 1.

The operation of this circuit will now be described. Before a discharge is started, the switching device 4 is conducting and a machining voltage is applied to the machining gap between the electrode 1 and the workpiece 2 by the power supply 5. Upon the start of the discharge, a pulse command 16 corresponding to a machining current waveform to be supplied to the machining gap is output from a control apparatus (not shown in FIG. 54) to the envelope signal generator 10. The pulse command 16 is output by the envelope signal generator 10 as envelope signals 13, 14. FIG. 55 shows the shapes of the envelope signals 13, 14. In the comparator 9, the current flowing in the machining gap is detected by the current detection resistor 7 to obtain a present machining current value 15, thereby comparing the envelope signals 13, 14 with the present machining current value 15 and outputting a control signal 12 to the control circuit 3. The control circuit 3 switches on/off the switching element 4 under the control of the control signal 12 to control the machining current within a predetermined value. Namely, when the present machining current value 15 exceeds the envelope signal 13, the switching device 4 is turned off. Conversely, when the present machining current value 15 falls below the envelope signal 14, the switching device 4 is turned on. The machining current is controlled in the above method.

In this method, the rising speed of the machining current waveform is determined by the current detection resistor 7 and the magnitude of the inductances 8a, 8b of a machining current supply feeder, i.e., the resistor and inductances are used as loads to carry out switching control.

A second conventional circuit arrangement for a power supply apparatus is shown in FIG. 58, which is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. SHO57-33949 as a "Pulse Generation Circuit Controlled for Formation by Intermittent Electrical Discharges". This power supply apparatus has been improved in rising and falling speeds of the machining current, as compared to the first power supply apparatus, in order to ensure faster operation. In FIG. 58, an auxiliary power supply 28, a first switching device 4, a current detector 24, a reactor 22 and a diode 23 constitute a first auxiliary circuit. A power supply 5, the auxiliary power supply 28, the first switching device 4, the current detector 24, the reactor 22, an electrode 1, a workpiece 2 and a second switching device 20 constitute a main circuit.

The operation of this circuit will now be described. In the first auxiliary circuit, the switching device 4 is driven by a control circuit 27 under the control of the detection signal of the current detector 24. The control circuit 27 carries out the switching control of the switching device 4 so as to render the current flowing in the current detector 24 constant. In this case, the reactor 22 inserted in the circuit allows the current flowing in the first auxiliary circuit to be kept constant.

This second power supply apparatus is fitted with a second switching device 20 exclusively employed to switch the discharge pulse on/off. When the discharge pulse is off, a current within a predetermined range flows in the first auxiliary circuit on a steady-state basis, and as soon as the discharge is started, the machining current is supplied from the first auxiliary circuit. This enables the current to rise extremely fast. The current during the discharge flows in the main circuit which consists of the power supply 5, the auxiliary power supply 28, the first switching device 4, the current detector 24, the reactor 22, the electrode 1, the workpiece 2 and the second switching device 20. When the discharge has ended, the current which had been flowing in the reactor 22 of the main circuit flows to the second diode 23 in the first auxiliary circuit, thereby intercepting the current of the machining gap rapidly.

A first diode 25 is provided to raise power supply efficiency by forming a second auxiliary circuit and causing the current flowing in the reactor 22 to return to the power supply 5 when the first switching device 4 and the second switching device 20 are both switched off. The second auxiliary circuit is constituted by the first diode 25, the current detector 24, the reactor 22, the second diode 23 and the main power supply 5. FIG. 59 shows a machining current waveform generated by the second power supply apparatus.

Also, there is a third conventional circuit arrangement for a power supply apparatus shown in FIG. 60, which is disclosed, for example, in Japanese Laid-Open Patent Publication No. HEI2-34732 as a "Control Method for the Electrical Discharge Machining Power Supply." In FIG. 60, 30a to 30e indicate drive devices which cause switching devices 32a to 32e to conduct and which constitute a logic circuit 35. 33a to 33e represent limiting resistors which control a machining current and which have different values individually. Between an electrode 1 and a workpiece 2 is a detector 36 for detecting a discharge start. This detector 36 transmits a discharge detection signal 37 to the logic circuit 35. The logic circuit 35 selects the switching devices 32a to 32e to be driven under the control of the output signal of an oscillator 34 and the discharge detection signal 37.

The operation of this circuit will now be described. In the circuit, a power supply 5 is provided for supplying a current and a parallel connection of circuits, each comprising series connections of the switching devices 32a to 32e and the current limiting resistors 33a to 33e, is connected in series with the power supply 5. The resistance values of the current limiting resistors 33a to 33e different from each other are designed to be a power of two, i.e., once, twice, four times, etc. When a rectangular wave having a constant current value and a duration t.sub.p as shown in FIG. 61 is to be supplied, some of the switching devices 32 are switched on by their corresponding drive circuit 30 to cause current to flow through the corresponding current limiting resistors 33. When the discharge is started, a machining current is supplied to the machining gap through selected resistors 33. A difference voltage between the output voltage of the main power supply 5 and the discharge voltage generated at the machining gap between the electrode 1 and the workpiece 2 is applied to each current limiting resistor, thereby determining the current flowing in the current limiting resistor. Since the discharge voltage is generally a constant value, the machining current is determined uniquely by the selection of the current limiting resistors.

Further, as shown in FIG. 62, the rising speed of a current waveform can be controlled. By switching the switching devices 32 on/off continuously after the discharge current has risen up to a point indicated by 48 in FIG. 62, the current can be further increased but can be raised with its slope further reduced. Such intentional control of the discharge current waveform is often exercised to provide finer control of the machining operation.

Finally, there is a circuit arrangement for a fourth conventional power supply apparatus shown in FIG. 63, which is disclosed, for example, in the specification of U.S. Pat. No. 4,306,135. In this drawing, 49 indicates a fixed current limiting resistor, 50 denotes a semiconductor amplifier such as an FET, 51 designates a switching device for switching the semiconductor amplifier 50 on/off to turn a discharge pulse on/off, 52 represents a digital signal which specifies the current waveform shape of the discharge pul