PURPOSE: To reduce an overcurrent flowing through a shunt diode by connecting a protective resistance in series to and a protective capacitor in parallel to the shunt diode.

CONSTITUTION: Between shunt diode 4 and shunt resistance 5, protective resistance 10 is interposed and in parallel to shunt diode 4 and protective resistance 10, protective capacitor 11 is connected. An overcurrent flowing through a shunt current flows into protective capacitor 11 until a value where it is clamped by protective resistance 10, so that the overcurrent through shunt diode 4 will lessen. Further, protective capacitor 11 protects shunt diode 4 from an overcurrent due to a surge voltage from the thyratron 6 side.