Overview
The Bridge Rectifier (or Diode Module) converts AC input voltage to DC voltage to supply power to the inverter circuit.
This test confirms diode integrity and ensures proper AC-to-DC conversion within both single-phase and three-phase systems.
Safety Warning
⚠️ High Voltage Hazard
Ensure power is completely isolated before performing any resistance or diode test.
Failure to disconnect power can result in severe injury or component damage.
Testing Overview
Testing can be performed under two conditions:
Power Off (Diode Mode Test) – Verifies internal diode function.
Power On (Voltage Measurement) – Confirms AC input and DC output voltages.
1. Power Off Test (Diode Mode)
Tools Required
Digital multimeter with Diode Test Mode
Procedure
Disconnect power and all control harnesses.
Set the multimeter to Diode Test Mode.
Measure between terminals according to the tables below.
Compare readings to expected voltage drops (approx. 0.4V for diodes).
Single-Phase Bridge Rectifier Test
| BLACK LEAD | RED LEAD | EXPECTED VALUE |
|---|---|---|
| + → ~ | 0.4 V | |
| + → ~ | 0.4 V | |
| ~ → - | 0.4 V | |
| ~ → - | 0.4 V | |
| + → - | 0.7 – 0.9 V | |
| - → + | O/L |
Terminal Reference:
+ (Red) – Positive DC output
– (Black) – Negative DC output
~ (Orange/Brown) – AC input lines
Three-Phase Bridge Rectifier Test
| BLACK LEAD | RED LEAD | EXPECTED VALUE |
|---|---|---|
| + → AC1~ | 0.4 V | |
| + → AC2~ | 0.4 V | |
| + → AC3~ | 0.4 V | |
| AC1~ → - | 0.4 V | |
| AC2~ → - | 0.4 V | |
| AC3~ → - | 0.4 V | |
| + → - | 0.7 – 0.9 V | |
| - → + | O/L |
Terminal Reference:
L1, L2, L3 (AC1~, AC2~, AC3~) – AC input phases
+ (Red) – DC positive output
– (Black) – DC negative output
2. Power On Test (Voltage Measurement)
Tools Required
Digital multimeter with AC/DC Voltage Range
Procedure
Reconnect AC power.
Measure between the following terminals:
| BLACK LEAD | RED LEAD | EXPECTED VALUE |
|---|---|---|
| AC1~ → AC2~ | 400–415V AC | |
| AC2~ → AC3~ | 400–415V AC | |
| AC1~ → AC3~ | 400–415V AC | |
| AC1~ → Neutral | 230–240V AC | |
| AC2~ → Neutral | 230–240V AC | |
| AC3~ → Neutral | 230–240V AC | |
| – → + | 560–600V DC |
Diagnosis and Interpretation
| Observation | Diagnosis | Recommended Action |
|---|---|---|
| All readings normal | Rectifier operating correctly | No action required |
| 0V or inconsistent diode readings | Internal short | Replace bridge rectifier |
| Infinite (O/L) in all directions | Open circuit | Replace bridge rectifier |
| Low DC output (Power On) | Faulty rectifier or capacitor | Check smoothing capacitor and DC bus |
| No AC input voltage | Supply or fuse issue | Check upstream power supply |
Technical Notes
Always verify the smoothing capacitor after rectifier replacement.
Confirm all wiring connections are tight and free of corrosion.
For three-phase units, check for balanced AC supply before diagnosis.
A rectifier short circuit often results in input fuse failure or breaker trip.
