How to Test the Quality of a Darlington Transistor?

Necessary Tools for Testing

1. Digital Multimeter (DMM): Equipped with ohmmeter and diode test functions.

2. Test Clips or Breadboard: To securely connect the transistor during off-circuit tests.

3. Isolated Power Supply (Optional): Required for biasing the transistor in in-circuit tests.

Most Darlington transistors feature three primary terminals: base (B), collector (C), and emitter (E). Some power variants may include a fourth terminal for heat sinking, but testing focuses on the three core terminals.

Off-Circuit Testing: Accurate Component-Level Evaluation

Testing a Darlington transistor outside the circuit eliminates interference from adjacent components, providing definitive results. Use the ohmmeter (start with the 10kΩ range) or diode test mode for precise measurements:

1. Base-Emitter (B-E) Junction Inspection

• Forward Bias Test: Connect the multimeter’s red probe to the base (B) and the black probe to the emitter (E).

• 0Ω or near-zero resistance indicates a shorted B-E junction, typically from overcurrent.

• Infinity (OL) suggests an open junction, often caused by thermal stress or voltage surges.

• Healthy Reading: A resistance of 1kΩ–10kΩ (ohmmeter) or a voltage drop (Vf) of 0.7V–1.2V (diode test), reflecting the forward-biased PN junction of the two cascaded BJTs.

• Fault Signs:

• Reverse Bias Test: Swap the probes (red to E, black to B).

• Healthy Reading: Infinity (OL), as the reverse-biased junction should block current.

• Fault Sign: Low resistance signals junction damage.

2. Base-Collector (B-C) Junction Check

• Forward Bias Test: Red probe to B, black probe to C.

• Healthy Reading: Similar to B-E: 1kΩ–10kΩ (ohmmeter) or 0.7V–1.2V (diode test).

• Fault Indications: Shorts (0Ω) or opens (OL) signal a defective junction.

• Reverse Bias Test: Reverse probes (red to C, black to B).

• Healthy Reading: Infinity (OL). A low reading indicates a compromised junction.

3. Collector-Emitter (C-E) Conductivity Test

• With no base connection, measure C-E resistance in both directions (red to C/black to E and vice versa).

• Healthy Reading: Both directions should show infinity (OL), as the Darlington transistor is non-conductive without base bias.

• Fault Sign: Low resistance in either direction means a shorted C-E path, a common failure from overload or thermal runaway.

In-Circuit Testing: Practical Troubleshooting in Assembled Boards

Testing while the transistor remains soldered requires caution due to parallel components. Follow these steps:

1. Power Disconnection: Always turn off the circuit supply and discharge capacitors to avoid damage.

2. Preliminary Short Check: Measure C-E resistance with the base disconnected. A low reading may indicate a short, but confirm by lifting one terminal from the PCB to isolate the transistor.

3. Biased Conduction Test: Apply a small base current (e.g., via a 10kΩ resistor from B to Vcc) and measure the voltage across C-E.

• Healthy Response: Voltage drops from near-supply voltage (cutoff state) to ≤0.5V (saturation state) when biased, indicating proper switching.

• Fault Sign: No voltage change or excessive drop suggests gain degradation or open junctions.

4. Thermal Stability Check: If the transistor overheats during operation despite passing resistance tests, its thermal performance may have degraded—replace it to prevent catastrophic failure.

Common Failure Modes and Remedial Actions

• Open Junctions (B-E or B-C): Caused by prolonged overvoltage or mechanical stress.

• Solution: Replace the transistor and install transient voltage suppressors (TVS diodes) to absorb voltage spikes.

• Shorted C-E Path: Typically results from excessive current beyond the rated I₀ or inadequate heat sinking.

• Solution: Upgrade to a Darlington with higher current/voltage ratings (check datasheet for I₀, VCEO) and ensure proper heat sink attachment.

• Reduced Current Gain (β): Occurs due to aging or repeated thermal cycling, leading to insufficient load driving.

• Solution: Use a transistor tester to measure β; replace if it falls below 50% of the datasheet value.

Conclusion

Testing Darlington transistors requires methodical checks of their internal junctions to identify shorts, opens, or performance degradation. Off-circuit tests using a multimeter’s ohmmeter or diode mode offer the most accurate results, focusing on forward/reverse biases of B-E and B-C junctions and C-E isolation. In-circuit testing demands caution to avoid false readings from parallel components, prioritizing bias-based conductivity checks and thermal evaluations. By following these steps, engineers can ensure these high-gain devices reliably amplify signals and handle heavy loads, safeguarding the integrity of electronic systems. Whether qualifying new components or troubleshooting failures, this testing framework provides a robust foundation for maintaining Darlington transistor functionality