How to Test Thyristors (Silicon-Controlled Rectifiers)?

Necessary Tools and Preparations

1. Digital Multimeter (DMM): Equipped with ohmmeter (20kΩ–200kΩ ranges) and diode test mode.

2. Test Leads with Alligator Clips: For secure terminal connections.

3. Isolated Power Supply (for In-Circuit Tests): To safely apply bias voltages.

4. Breadboard or Test Socket: For off-circuit evaluations.

Thyristors typically have three terminals: for SCRs, these are anode (A), cathode (K), and gate (G); TRIACs feature main terminals MT1, MT2, and gate (G). Always refer to the datasheet for pin configurations to avoid misconnection.

Off-Circuit Testing: Component-Level Evaluation

Testing a thyristor outside the circuit eliminates parallel component interference, providing accurate results.

1. SCR (Unidirectional Thyristor) Testing

• Anode-Cathode (A-K) Junction Check:

• Healthy Reading: High resistance (∞ or OL) in both directions, as the SCR is non-conductive without gate trigger.

• Fault Sign: Low resistance in either direction indicates a shorted A-K path, a common failure from overcurrent.

• With the gate floating, measure resistance between A and K in both directions.

• Gate-Cathode (G-K) Junction Test:

• Healthy Reading: High resistance (∞). Low resistance signals a damaged G-K junction.

• Healthy Reading: 100Ω–1kΩ (ohmmeter) or 0.6V–1.0V (diode test), reflecting the PN junction’s forward voltage drop.

• Forward bias: Red probe to G, black probe to K.

• Reverse bias: Swap probes (red to K, black to G).

• Triggering Test:

• Healthy Response: Resistance drops to <100Ω, indicating the SCR conducts after triggering. Disconnecting the resistor should leave it conducting (latching behavior); it only turns off when A-K current drops below the holding current.

• Connect a 10kΩ resistor between A and G (to simulate a trigger pulse), then measure A-K resistance.

2. TRIAC (Bidirectional Thyristor) Testing

• MT1-MT2 Resistance:

• Healthy Reading: High resistance (∞) in both states, as the TRIAC is off without gate activation.

• With G floating, measure resistance between MT1 and MT2 in both directions.

• Gate-MT1/MT2 Junctions:

• Healthy Reading: 200Ω–2kΩ (ohmmeter). Reverse bias (swap probes) should show high resistance.

• Forward bias: Red probe to G, black to MT1 (or MT2).

• Bidirectional Triggering:

• Healthy Response: Low resistance in both directions after triggering, confirming bidirectional conduction.

• Connect a 10kΩ resistor between G and MT1, then measure MT1-MT2 resistance in both voltage polarities.

In-Circuit Testing: Practical Troubleshooting

Testing on a populated PCB requires caution to avoid damage from residual voltages or parallel components.

1. Power Off & Discharge:

• Disconnect the circuit supply and discharge all capacitors to prevent electric shock or false readings.

2. Preliminary Short Check:

• Measure A-K (for SCR) or MT1-MT2 (for TRIAC) resistance with the gate unpowered.

• Fault Sign: Low resistance (<1kΩ) suggests a shorted device; isolate by lifting one terminal from the PCB for confirmation.

3. Triggering Under Bias:

• Apply a small voltage (e.g., 5V) to the gate through a 1kΩ resistor (relative to the cathode/MT1).

• Healthy Response: Voltage across A-K (or MT1-MT2) drops from supply voltage to <1V (saturation), indicating proper conduction.

• Fault Sign: No voltage drop or excessive voltage (>2V) signals a failure to trigger, possibly due to a broken gate or internal junction damage.

4. Commutation Check (for GTO/IGBT Thyristors):

• For gate-turn-off thyristors, apply a negative gate pulse after triggering to test active turn-off capability. A successful test shows a return to high resistance state.

Common Failure Modes and Detection

• Shorted Junctions (A-K/MT1-MT2):

• Caused by overcurrent or thermal runaway. Detected via low resistance in off-state measurements.

• Open Gate Junction:

• Results from electrostatic discharge (ESD) or soldering damage. Shows infinite resistance in G-K/G-MT1 tests.

• Latching Failure:

• The thyristor fails to conduct after triggering. Check gate voltage and resistor values in the drive circuit.

• False Triggering:

• Occurs in TRIACs due to voltage transients. Use an oscilloscope to monitor gate signals for unwanted noise.

Safety and Best Practices

• Always test high-voltage thyristors (>500V) with caution; use insulated probes and ensure the circuit is fully de-energized.

• For power devices, verify heat sink attachment and thermal paste integrity, as overheating is a primary failure cause.

• When replacing a faulty thyristor, match voltage (VDRM, VRRM) and current (IT(AV)) ratings per the datasheet to avoid premature failure.

Conclusion

Testing thyristors requires methodical checks of their terminal junctions and triggering behavior, distinguishing between unidirectional (SCR) and bidirectional (TRIAC) types. Off-circuit tests with a DMM’s ohmmeter and diode mode provide precise component-level diagnosis, while in-circuit tests validate functionality within the system. By following these steps, engineers can identify shorts, opens, or triggering failures early, ensuring reliable operation in high-power applications. Whether maintaining industrial equipment or troubleshooting renewable energy inverters, thorough thyristor testing is key to preserving circuit integrity and safety