How to Test a Diode Bridge with a Multimeter: Step-by-Step

A diode bridge (bridge rectifier) is a configuration of four diodes arranged to convert AC to DC. Testing a diode bridge ensures it is functioning correctly by verifying each diode’s conduction in the forward direction and blocking in reverse.

⚠ Important: To get accurate results, desolder or remove the bridge from the circuit, as other components can skew the readings.

1️⃣ Understanding the Diode Bridge

A four-diode bridge has:

1. Two AC input terminals (for the AC supply)
2. + output terminal
3. - output terminal

Each internal diode must be tested for forward conduction and reverse blocking.

Common Pinout (Reference)

$$\text{ AC } \leftrightarrow \, \text{~} \\ \text{ AC } \leftrightarrow \, \text{~} \\ \text{ Positive } \leftrightarrow \, (+) \\ \text{ Negative } \leftrightarrow \, (-)$$

2️⃣ Required Tools

• Digital multimeter (with diode test or resistance mode)
• Soldering iron (if removing the bridge from PCB)
• Test probes

3️⃣ Testing a Known-Pin Bridge

If you already know the pin assignment (marked on the module or datasheet), checking is straightforward:

Step 1: Check Each Diode in Forward Direction

1. Select the diode test mode (or a high-resistance range if not available).
2. Place the red probe on the diode’s anode and the black probe on the cathode.
3. Read the display:
   • A typical forward voltage drop ($0.5-0.7V$) indicates a good diode.
   • If the meter shows OL, 1, or infinite resistance, the diode may be open.

Step 2: Reverse the Probes

1. Switch probe polarity (red probe on the cathode, black probe on the anode).
2. The meter should now show OL or no conduction.
   • If it shows a voltage drop, the diode is shorted (faulty).

Step 3: Repeat for All Four Diodes
Make sure each of the four internal diodes passes forward current in one direction only and blocks in the other.

4️⃣ Testing an Unknown-Pin Bridge

If the pin assignments are unknown:

Step 1: Identify the AC Input Pins

• Check pairs of pins with both polarities of the multimeter.
• AC pins will show open (OL) in both directions because in the bridge, two diodes are in series (opposing directions) at those points.

Step 2: Find + and – Outputs

• Once AC pins are located, test the other two pins.
• Typically, one pin will always conduct from each AC pin in one direction → This is the (+) output.
• The other pin will conduct to the AC pins in the opposite direction → This is the (–) output.

📌 Note: If you find a diode is shorted or open, the bridge is faulty.

5️⃣ Results Interpretation

1. Forward conduction (~0.5–0.7V) → Diode is good.
2. No conduction in both directions → Diode is open (faulty).
3. Conduction in both directions → Diode is shorted (faulty).
4. AC pins should show open in both polarities.

6️⃣ Common Faults & Symptoms

1. Open Diode – The rectifier fails to provide DC output or reduced voltage.
2. Shorted Diode – Typically blows fuses or triggers overcurrent protection.
3. Leakage in Reverse – May cause low DC output or heat issues.

7️⃣ Practical Tips & Safety

• Desolder or remove the bridge rectifier before testing.
• Use diode test mode for best results.
• If testing in resistance mode, choose high-resistance range.
• Always ensure the circuit is unpowered and capacitors are discharged before touching any component.

8️⃣ Conclusion

Testing a diode bridge with a multimeter ensures each diode is functioning (forward conduction, reverse blocking). Key steps include:

✔ Identifying AC input and DC output pins.
✔ Checking forward and reverse conduction for each internal diode.
✔ Verifying OL or no conduction in reverse.

A faulty bridge rectifier can cause low DC output, blown fuses, or overcurrent issues—making regular testing essential in power supplies and other AC-to-DC circuits.