The present invention relates to an electric discharge machining power source of an electric discharge machine which performs machining by using electric discharge of a capacitor.
A power source circuit of an electric discharge machine which performs machining by using electric discharge of a capacitor conventionally uses a circuit as shown in FIG. 5. Referring to FIG. 5, reference symbol E denotes a power source; R1, a current-limiting resistor; C2, a charge/discharge capacitor; P, an electrode; W, a work; T2, a transistor serving as a switching element; and G2, its base. With this circuit, a pulse is input to the base G2 of the transistor T2 to turn it on and thus to charge the capacitor C2. The charge voltage of the capacitor C2 is applied between the electrode P and the work W. The discharge current from the capacitor C2 flows as an electric discharge between the electrode P and the work W, thereby performing electric discharge machining. In order to quickly charge the capacitor C2, the resistance of the resistor R1 must be set small. However, when the resistance of the resistor R1 is set small, the resistor R1 generates heat, resulting in a large energy loss.
In view of this situation, an electric discharge machining power source as shown in FIG. 6 is developed (Japanese Patent Application No. 59-35984). Referring to FIG. 6, reference symbol E denotes a power source; T3 and T4, charge and discharge transistors serving as switching elements; G3 and G4, bases thereof; C3, a charge/discharge capacitor; D3, a diode; P, an electrode; W, a work; and L3, an inductance.
The operation of this electric discharge machining power source circuit will be described. First, when a pulse is applied to the base G3 of the charge transistor T3 to turn it on, a current flows from the power source E to the capacitor C3, inductance L3, transistor T3, and power source E, and the capacitor C3 starts to be charged. At this time, the current linearly increases because of the impedance of the stray inductance L3. Then, the charge voltage of the capacitor C3 also gradually increases. When application of the pulse to the base G3 of the charge transistor T3 is stopped to turn it off, the current as the energy accumulated in the stray inductance L3 flows through the diode D3, thus constituting a so-called flywheel circuit to further charge the capacitor C3. In this case, the charge current and voltage to the capacitor C3 are determined by the value of the stray inductance L3 and the width of the pulse applied to the base G3 for turning on the transistor T3, as is apparent from the above description. Therefore, the charge voltage can be adjusted by adjusting the pulse width and the stray inductance L3.
In this manner, the capacitor C3 is charged, the pulse is applied to the base G4 of the transistor T4, and thus the transistor T4 is turned on. As a result, the charge voltage in the capacitor C3 is applied in the gap between the work W and the electrode P, the capacitor C3 starts discharging, and the discharge current flows. In this manner, since this electric discharge machining power source uses no power source limiting resistor, no energy loss is caused and good power source efficiency is ensured. However, since the discharge switching element T4 is used, the overall system becomes expensive. Particularly, in order to flow a large discharge current, a plurality of transistors T4 (only one is shown) serving as the switching elements must be connected in parallel with each other, rendering the electric discharge machining power source expensive.