PURPOSE: To suppress switching surge voltage by connecting the base resistor of a power transistor to an auxiliary output of a rectifier.

CONSTITUTION: A power transistor 307 is connected at its base to a resistor 306 that is connected to an auxiliary rectifier output 202. When the voltage to the rectifier output 202 is cut off, the base current is not instantly cut off because a negative-feedback capacitor 305 discharges a gradually decreasing current to the base. Therefore, the power transistor is gradually brought into the cut-off state, resulting in delay of decrease in the initial excitation current from a battery 4. In other words, the switching rate of the power transistor is decreased to supply a field coil 102 with an intermittent initial excitation current.