Hitachi Automotive Systems Espelkamp GmbH | Eugen-Gerstenmaier-Str. 8 | 32339 Espelkamp


Rectifiers are also important electronic components of an alternator. Both regulators and rectifiers are critical components that must work properly in order to ensure that the alternator itself functions.

Faulty regulators and/or rectifiers can cause overvoltage, which may eventually harm the electrical equipment installed in the vehicle. On the other hand, a low current might not sufficiently charge the battery, causing problems when starting the car. Ultimately, it can destroy the battery, especially during cold winter temperatures.

Function of the rectifier:
Alternators generate an alternating current that cannot be used by the battery, or by the other electric components and controls in the vehicle. The rectifier converts the current into a direct current (dc). The most critical components in rectifiers are the diodes. Performance of the diodes and thus the rectifier itself mainly depends on the quality and the capacity of these diodes.

Most alternators have six diodes, 3 positive and 3 negative, and some with additional diodes in a sixplus bridge circuit (B6U) rectification (KL30 and KL31). The diodes also ensure that no current flows from the battery, only to the battery.

In some rectifier types three further and somewhat weaker diodes are fitted (Diode-Trio or trigger diodes). These serve as part of the regulation circuit, generate a direct current (Kl. D+) and also serve to trigger the voltage regulator or the load control lamp in the car’s dashboard.

We distinguish between the following different types of rectifiers:

Number of diodes:
The minimum amount of diodes is 6 diodes, 3 positive and 3 negative diodes. Especially within heavy-duty alternators (IN=100A) it’s necessary to switch then diodes in parallel to increase the total output current. The electric current generated by the alternator is thus distributed evenly between the parallel diodes.

Alternators with Zener Diodes:
Zener diodes are especially used on applications with sensitive electronic components. These can prevent major damage caused by voltage peaks due to sudden discharges. In 14V systems, Zener diodes with a voltage range 24V - 32V are used and in 28V systems the range is 36V - 44V.

Additional use of trigger (or field) diodes:
Depending on the regulator used, the rectifier may be fitted with trigger diodes. These trigger diodes generate a direct current for the regulator and the charging control lamp in the dashboard.

CT connection:
Some rectifiers have a CT (center terminal) connection. This can be the case in certain alternators with a special star connection of the stator winding, which results in an increased performance of about 10 %.

W terminal:
Some rectifiers have an extra W terminal whereby the rotational speed
of he alternator can be determined. This can be used for example on diesel cars for connecting with the rev counter to measure engine rpm, or in the regulation circuit where there are no trigger diodes.

Check all products, technical data and further information
via our online catalogue

Auto Care Association