Abstract: Power supply of the saturable reactor type is disclosed. The improvement is the inclusion of a variable impedance network, such as a pair of opposed semiconductor controlled rectifiers, in series with the current winding of the saturable reactor. Also included is an electronic circuit for changing the impedance of the variable impedance circuit, such as a control circuit for monitoring an operating parameter and for enabling or disabling the firing of the semiconductor controlled rectifiers in response to a change in the operating parameter. A preferred embodiment of the power supply also includes a transformer and a rectifier for supplying high-voltage, direct current to a sputtering apparatus.