The rectifier (diode bridge) converts the three-phase AC produced in the alternator’s stator into DC for the battery and electrical system.
How does it work?
A typical rectifier has six main diodes (three positive, three negative) forming a full-wave bridge; some designs add exciter or star-point diodes for 8, 9 or 12 in total. The diodes also stop battery current flowing back into the alternator.
Failure signs
- Undercharging or a faint charge warning light
- Reduced alternator output
- Battery drain while parked (leaking diode)
- Flickering lights, radio whine (AC ripple)
- Alternator overheating
Diodes are tested with a multimeter in diode mode, an oscilloscope, or a dedicated tester such as MSG Equipment MS021.
Frequently asked questions
Can a bad diode drain the battery?
Yes, a leaking diode can let current flow from the battery into the alternator while the car is parked.
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