What Are the Functions of a Diode in an Alternator?

by David Sandoval
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Rectifier diodes are electronic devices that allow electrical current to flow in one direction only. Because of this electrical property, diodes are used to convert alternating current electrical energy into direct current electrical energy. In an automotive electrical system, diodes can be found in several places, including the alternator. Alternator diodes perform three critical functions for the automotive charging system.

Conversion from AC to DC Power

An alternator converts mechanical energy into electrical energy by creating a variable electromagnetic field. This field is induced across alternator windings, and creates an alternating current signal. However, the electrical devices in a car require direct current electrical energy.

The electrical energy passes through a device called a rectifier, which is made up of six or more diodes. The rectifier converts the alternating current energy produced by the alternator into direct current energy the automobile can use.

Feedback Preventer

At times, some electrical parts fail. If a part fails suddenly, there is typically a short burst of energy fed back into the electrical system. A fuse typically protects most electrical components from feedback; however, the alternator is not typically protected by a fused circuit.

Since the diodes will only allow electrical current to flow in one direction, the energy feedback travels as far as the diodes, but not into the alternator itself. The alternator will then be isolated from any energy returns which can damage the alternator.

Proper Polarity for Charging a Battery

Since diodes allow electrical current to flow in one direction only, the polarity of the direct current electrical energy at one end of the diode chain is always positive, whereas at the other end of the chain, the polarity is negative.

For a battery to charge while an alternator is in operation, the energy the battery receives must be of matched polarity (positive to positive, negative to negative). The diodes in a rectifier serve as a one-way check valve so that the polarity stays consistent while charging the battery.

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