Alternator diode bridge: configuration, operation principles, features

Diode bridge

Dc alternators in cars stopped being installed a long time ago. All modern manufacturers produce vehicles equipped with AC alternators, as such alternators have the same power with less weight and size.

What is a diode bridge for and how does it work?

A car's alternator produces three-phase alternating current. At the same time, all consumers in the car are supplied with DC voltage. In order to obtain it, a diode bridge is used in the design of the alternator. The diode bridge principle of operation is based on the diode's ability to allow electric current to flow in one direction only.

The name of the node reflects its design. A diode bridge consists of a group of power diodes connected in a bridge circuit. The diode bridge design includes two aluminium heat sink plates which are connected to each other via dielectric bushings. One plate of the diode bridge is connected to the generator housing and the other to the terminal through which the direct current is supplied to the consumers. On each plate is located from three to four diodes, depending on the design of the generator Diode bridge for six diodes is used for generators in which the stator winding is connected according to the scheme «delta». And eight diode bridges can be used with «star» stator winding.

To make the generator work more stable and efficient, three additional low-current diodes are connected to the main diodes, which are responsible for rectifying the current, to supply the voltage regulator and the field winding.
Diode bridge design

1, 2, 3 – diodes connected to the positive plate; 4, 5, 6 – diodes connected to the negative plate. 

Features of modern diode bridges

The amperage at the alternator output depends on the engine crankshaft speed. When conventional diodes are used in a diode bridge, voltage spikes of up to 20V are possible, which is harmful to consumers. In order to keep the output voltage as stable as possible, modern diode bridges use avalanche diodes instead of conventional diodes.

A avalanche diode –is a type of stabilitron, which when voltage is applied to it will not pass current until the voltage value becomes higher than the specified value for which it is designed. This property allows a stable voltage to be maintained at the «+» terminal of the generator.

How to diagnose and repair a diode bridge?

As a rule, a visual inspection of the diode bridge does not allow you to determine whether it is in good working order or not. And due to the fact that the design uses powerful diodes designed for high currents, it is impossible to accurately determine the state of the diode bridge with a multimeter.

That's why auto repair shops use specialised testers to diagnose diode bridges. MSG Equipment's line of equipment includes two testers that can detect diode bridge failure –MS014 and MS021. The MS014 tester detects:

• the breakdown of one or more diodes;
• breakdown of one or more diodes;
• short circuit between heat sinks.

And the MS021 tester can detect the following in addition to the above mentioned failures:

• diode type (normal diode, avalanche diode);
• diode degradation (increased forward voltage drop);
• back breakdown voltage of avalanche diode;