by James Dreamer

Created on: August 24, 2010

Alternator diodes are necessary to convert the alternating current generated by the coils in the alternator into the direct current needed to charge the battery and supply the vehicle’s electrical system. They do not produce any power. They simply change one type of power into another. Here is an explanation of how they perform this task.

The diode is in a class of electrical components known as semiconductors. There are many ways to manufacture them, but the most common is to permeate a silicon substrate with impurities such as boron or phosphorous. Although this description sounds obscure, the result is an electronic junction that allows electrons to flow in only one direction. This is the simple function of the diode, to allow current to pass through it one way, but not the other. Think of it as a one way street for electrons.

Why are they necessary in an automotive alternator?

When a coil of copper wire passes through a magnetic field, electricity is produced. If the coil is connected to an electrical circuit, that power can be harnessed. As the magnetic field approaches the coil, the coil’s lead on one end becomes positive and the other negative. As the magnetic field departs from the coil the electrical flow reverses. The positive lead becomes negative and vice versa.

This is how the coils function in an alternator. The electricity produced is constantly alternating, that is, switching from negative to positive. This is referred to as alternating current or AC. While the appliances in our home are designed for AC, it is incompatible with the direct current or DC system in an automobile. In a DC circuit, positive is always positive and negative is always negative. The AC produced in the coils of the alternator must be converted to DC, or rectified, before it can be utilized by the rest of the vehicle’s electrical system.

A set of six diodes mounted in the alternator perform this rectification and in fact, a diode is sometimes referred to as a rectifier. When the coil is producing power that is the same polarity as the DC system, the diodes allow it to pass through. When the AC reverses however, they block the flow. This allows only compatible DC current to flow into the vehicles electrical system from the alternator.

Why not use a DC generator in the first place? While it is possible to manufacture a DC generator, and indeed, that is what was used in early automobiles, employing an alternator and rectifying the output using diodes has been found to be much more efficient, especially at low Rpm’s, such as when idling at a stoplight.

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