An improved power supply unit for a vehicle electrical system of a motor vehicle, which has an a.c. generator, that provides a phase signal, having an excitation coil, a field regulator assigned to the excitation coil and a rectifier having rectifier elements for the rectification of the generator voltage supplied by the a.c. generator. The field regulator has a voltage recording range for the evaluation of the phase signal having a minimum value and a maximum value, the maximum value being adjusted to the nominal voltage of a power supply unit of the motor vehicle.