JPS6435897A - Induction plasma generator - Google Patents

Abstract

PURPOSE:To avoid the breakage of a vacuum tube by invariably monitoring the balance state between the power source current or the vacuum tube plate current or the high-frequency current flowing in an RF coil and the vacuum tube plate voltage and stopping or squeezing the oscillator output when the balance is lost. CONSTITUTION:The three-phase AC input is controlled by a thyristor circuit 21 phase-controlled by the control signal from a current control circuit 13, the alternating current, with its amplitude partially cut, is transmitted to the secondary side via a transformer 2b and converted into the direct current by a succeeding rectifying circuit 2c. The direct current is then converted into the high-frequency alternating current by a reverse converting section 3 and fed to an RF coil 4, the circuit 13 is negative-feedback-controlled so that the power source current is invariably kept constant during this period, the voltage applied to a torch 5 is set to the preset level for the plasma ignition. In case of misfiring due to some causes in this constitution, a transistor 14b in a plasma misfiring detecting circuit 14 is turned on by the output D1 of a current detecting circuit 11 and the output D2 of a plate voltage detecting circuit 12, a relay 14 is operated, and the circuit 2a is turned off.