Alternator - BOSCH: Operation

With ignition on, rotor winding is connected to battery through a warning light and ignition switch. A small current flows through rotor to ground through a voltage regulator. This small current flow produces a weak magnetic field. When engine is started, it begins to build-up alternator output voltage through output diodes and field or exciter diodes.

As alternator voltage builds up, the same amount of voltage will be applied to both sides of warning light (12 volts from alternator, 12 volts from battery). At this point, warning light will go out. As alternator speed increases, charging current will flow through output diodes to battery. Action of regulator is similar to a vibrating contact type voltage control, except switching of rotor circuit on and off is done by transistors. When battery voltage rises to 14 volts, transistors located in voltage regulator, switch on and off very quickly in order to maintain control voltage at a steady 14 volts.