The field circuit, which is controlled by the voltage regulator, might be one of two types. If it is an A-circuit voltage regulator, it is positioned on the ground side of the rotor. Battery voltage is picked up by the field circuit inside the alternator. The regulator turns the field circuit off and on by controlling a ground. A B-circuit voltage regulator is positioned on the feed side to the alternator. Battery voltage is fed through the regulator to the field circuit, which is then grounded in the alternator. The regulator turns the field circuit off and on by controlling the current flow from the battery to the field circuit. Most voltage regulators are mounted either on or in the alternator. Some charging systems might have a regulator mounted separately from the alternator. A typical electronic regulator is an arrangement of transistors, diodes, zener diodes, capacitors, resistors, and thermistors.
Most alternators with internal regulators use current generated by the alternator stator to energize the field circuit. The regulator controls a ground to turn the field circuit off or on. Because the stator generates AC current, a set of three diodes is used to rectify the current to DC. These three diodes are often referred to as a diode trio. A separate sensing circuit delivers battery voltage to the regulator to control the on/off cycle of the alternator.
Battery current is delivered to the regulator through the sensing circuit. While the battery voltage is low, it turns on transistor 3 (TR3), which then turns on transistor 1 (TR1). When TR1 is on, current flows from the stator, through the diode trio, through the field coil, through TR1, and to ground.
When battery current builds to approximately 13.5 to 14 volts, the zener diode (D1) trips to switch transistor 2 (TR2) on. This diverts current away from TR3, which then turns off. When TR3 turns off, TR1 must turn off also. This blocks the flow of current through the field circuit. The magnetic field of the rotor collapses and the alternator ceases to generate current. The battery then begins to discharge until the voltage level drops to a point when the zener diode turns off and stops current flow to TR2. TR3 then turns TR1 on to allow current flow through the field circuit, energizing the rotor coil. This process repeats itself hundreds of times every second.