A power supply and timing circuit for an X-ray tube capable of maintaining constant X-ray exposure values and stable peak plate voltage despite relatively large variations in the voltage of the source from which the power supply is fed. The cathode-to-plate current in the X-ray tube is varied responsively to changes in the voltage of the source, thereby varying the load on the high voltage transformer and holding its output voltage at a constant value despite changes in the source voltage. Changes in the cathode-to-plate current may be compensated for by the use of a milliampere-second integrator to time the exposure.