Alternator Light and Voltage Regulator Operation

by William Warburton

I was wondering if someone can explain to me how a voltage regulator works.

A quick summary: The alternator's generating coils are wired to the charging system via diodes to convert AC to DC. These coils are "fixed" (unlike a generator, which uses brushes to tranfer large currents to the car), and this accounts for the alternator's improved capacity. Output current is generated by passing a field current through coils in the rotating part of the alternator. The field current is controlled by the regulator and is relatively small (a few amps), so the brushes that carry it last longer.

The regulator senses the output voltage and cycles the currrent through the field coil by switching it rapidly, if the average current is high, we get high output, low for low output. Electronic (solid state) regulators have no moving parts and can switch faster and are therefore more accurate and more reliable.

Another thing I don't understand is when terminal D+ is live and when it is not, or asked differently, how can the alt light on the dash be off while the aux. fan relay (or the aux. battery charging relay for that matter) are powered: all three being connected to D+.

The alt-light is effectively connected *between* battery (+) and alternator output, so it indicates when either side is low, though not both(!). Hence it's on when the ignition is on but before the engine is started and also if the engine is running but the battery getting low or the alternator isn't producing output. It actually performs an important job in providing the initial excitation current to the alternator's field windings.

See the Virtual Mechanic for the gory details of the charging system.

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