Abstract: The invention relates to a high voltage electrical power supply device for an auxiliary circuit CA of a motor vehicle.The device comprises an alternator 1 having two identical stator windings 11 and 12, each connected to a rectifier bridge 13 and 14 supplying a rectified alternator voltage on a terminal B+1, B+2 an excitiation regulator 2 enabling the regulation of the excitation current of the inductor winding of the alternator and means for parallel-series/series-parallel switching of the stator windings. Alternator output terminals B+1, B+2 are provided for generating a nominal value voltage or a high voltage. A control logic is interconnected with the excitation regulator 2 and the switching means 3 to ensure the starting of the high voltage operation conditionally with respect to status parameters of the vehicle such as the engine speed, the clutch position, the temperature of the vehicle, and the supply voltage of the device.