Abstract: The invention relates to a high voltage AC power supply which can be operated as either a constant current source with overvoltage protection or as a constant voltage source with current limiting protection. The power supply includes a sinusoidal transformer having a high turn winding and a sampling circuit on its secondary side for sampling voltage proportional to the load current. A comparator compares the sampled voltage to a reference voltage and supplies an error voltage to a buck regulator. The buck regulator converts the error voltage to a variable DC voltage which is fed to an oscillator formed by the primary winding of the transformer.