The diodes for rectification are mounted on heat sinks, in packs of three. One heat sink has three positive diodes and the other has three negative diodes.
The positive diode heat sink is insulated from the frame , and is connected through the output terminal to the battery positive or B plus terminal.
The negative diode heat sink is connected to the frame, and this allows the return circuit, via the battery negative terminal to be completed.
Construction
A rectifier assembly is in a 3-phase format to work with the stator windings. The assembly consists of 6 diodes each of which works like a valve to electric current, letting it pass in one direction but not the other.
It basically converts A.C., the current produced by the stator that alternates from going in one direction to next going in the other, to a current which is unidirectional, called D.C. The rectifier stack supplies the required D.C. current from the stator to the battery and accessories.
The field diodes do an identical job to the rectifier assembly, except these smaller diodes only have to provide sufficient current for the alternator to supply its own field current. This system is called "self excitation".
Source: CDX Global & Wikipedia - en.wikipedia.org
