Alternator Rectifier Bridge

Alternator cutaway.

A rectifier bridge can contain 3, 6, or sometimes 8 diodes, depending on its design. Diodes are like one-way check valves that allow current to flow in only one direction. A positive-biased diode allows only positive voltage to pass through, and a negative-biased diode allows only negative. The bridge's job is to convert AC voltage to DC voltage. This process is known as rectification. It allows the positive voltage to pass onto the Bat+ terminal and the battery while allowing the negative voltage to pass to the ground. Most alternators have 3 or 6 diodes. If one of these fails, it will reduce alternator output by approximately 1/3.

Rectifier bridge.

The bridge is located in and fastened to the alternator case.

Test a rectifier bridge or a diode trio with an ohmmeter. A diode should allow voltage to pass in only one direction. Place the positive probe onto the diode and the negative to ground to test a forward-biased diode. The meter should read continuity. When the probes are reversed, the meter should read infinity. The opposite is true for a reverse-biased diode.

AC DC Rectification

Rectifiers receive voltage from the stator, typically in one of two configurations. The stator in the diagram is configured in a Wye design. It has three leads; each connects to and passes voltage to a diode on the rectifier. It also has a ground lead. Delta configurations only have three leads and run in parallel instead of in series, like the Wye configuration.