How the diode bridge converts an alternator’s three-phase AC into DC, why modern bridges use avalanche diodes, and how they are diagnosed.
Modern vehicles use AC alternators because they deliver the same power with less weight and size. Since on-board consumers run on DC, the three-phase output is rectified by a diode bridge, whose diodes let current flow in one direction only.
- The bridge consists of three or four power diodes on each of two aluminium heat-sink plates separated by dielectric bushings.
- Six-diode bridges suit delta-wound stators; eight-diode bridges can be used with star windings.
- Three additional low-current diodes supply the voltage regulator and field winding.
Because conventional diodes allow voltage spikes of up to 20 V, modern bridges use avalanche diodes that block current below a set threshold. Visual inspection cannot confirm a bridge’s condition, so equipment that tests under load is required.