An X-ray unit is provided having a self-contained high voltage supply including a pair of capacitors charged through diodes from a pair of transformer secondary windings and connected in series through the windings to the anode and cathode of an X-ray tube. Switching means are provided for operation either in a first mode in which the alternating voltages across the secondary windings are in phase and in which the output voltage is equal to the sum of the DC voltages across the capacitors to be substantially constant, and a second mode in which the alternating voltages across the secondary windings are out of phase and in which they are added to produce a pulsating output voltage having a peak value which is approximately double that of the substantially constant voltage obtained in the first mode of operation.