A power supply apparatus, in which the three phases of an alternating-current main are connected each through rectifier elements connected in antiparallel to the primary winding of a three-phase transformer. The secondary windings of this transformer are connected through rectifiers to a load device, such as an electron beam gun or other such device which produces short-circuits while in operation. Measuring sensors are provided on the primary AC side which, in the presence of an excess current on the primary AC side, block controlled rectifier elements. The blocking takes place very rapidly because GTO thyristors are used as rectifier elements. The decay of the magnetic energy in the transformer is achieved by the fact that the primary side of this transformer is connected in delta.