Testing Inductors
Testing an inductor is an important step when repairing or designing electronic equipment. A faulty inductor can cause excessive ripple, poor filtering, power supply failure, distorted audio, unstable oscillators or complete circuit failure. Depending on the equipment available, inductors can be tested using a digital multimeter, an LCR meter, an oscilloscope or specialised test instruments.
Visual Inspection
Always begin with a careful visual inspection before making electrical measurements.
- Look for cracked ferrite cores.
- Check for burnt or discoloured windings.
- Inspect solder joints.
- Look for loose or broken leads.
- Check for signs of overheating.
Testing with a Digital Multimeter
A multimeter cannot directly measure inductance unless it has an inductance measurement function, but it can detect common faults.
| Test | Expected Result |
|---|---|
| Continuity | Low resistance. |
| Open Circuit | No continuity. |
| Short to Core | No electrical connection to the core (unless designed that way). |
Most inductors have a low DC resistance because they are made from copper wire.
Testing with an LCR Meter
An LCR meter is the preferred instrument for testing inductors because it can measure several important parameters.
| Parameter | Purpose |
|---|---|
| Inductance | Verify the correct value. |
| Q Factor | Measure efficiency. |
| DC Resistance (DCR) | Detect excessive winding resistance. |
| ESR (where applicable) | Evaluate losses in some specialised components. |
Testing with an Oscilloscope
An oscilloscope allows you to observe the behaviour of an inductor during operation.
- Check current rise and decay.
- Observe switching waveforms.
- Detect voltage spikes (back EMF).
- Measure ripple current.
- Identify oscillations or ringing.
Testing in Circuit
Some inductors can be tested without removing them from the circuit. However, nearby components may affect the measurements.
If readings appear incorrect, remove one terminal from the circuit and test the inductor again.
Common Faults
| Fault | Symptoms |
|---|---|
| Open Winding | No continuity. |
| Shorted Turns | Reduced inductance and overheating. |
| Core Saturation | Poor performance under load. |
| Cracked Core | Reduced inductance and audible noise. |
| Loose Winding | Buzzing or vibration. |
| Overheated Coil | Discolouration or burnt insulation. |
Troubleshooting Guide
| Symptom | Possible Cause |
|---|---|
| Power Supply Ripple | Damaged filter inductor. |
| Weak RF Signal | Incorrect inductance. |
| Poor Audio | Faulty crossover inductor. |
| SMPS Overheating | Saturated or damaged power inductor. |
| Relay Driver Failure | Open relay coil. |
Safety Tips
- Disconnect power before testing.
- Discharge capacitors in power supplies before handling the circuit.
- Use insulated test probes.
- Avoid short circuits while measuring.
- Use instruments rated for the voltage being tested.
Recommended Test Equipment
| Instrument | Typical Use |
|---|---|
| Digital Multimeter | Continuity and resistance. |
| LCR Meter | Inductance, Q factor and DCR. |
| Oscilloscope | Waveform analysis. |
| Function Generator | Frequency response testing. |
| Vector Network Analyser (VNA) | RF inductors and impedance measurements. |
Interesting Facts
- A multimeter cannot detect all inductor faults.
- Shorted turns may show normal resistance but significantly reduced inductance.
- Cracked ferrite cores can greatly reduce inductor performance.
- High-current power inductors often fail due to overheating or saturation.
- Many RF inductors require specialised instruments for accurate testing.
Key Points
- Always begin with a visual inspection.
- Use a multimeter to check continuity.
- An LCR meter provides the most complete measurements.
- An oscilloscope is valuable for observing dynamic behaviour.
- Test the inductor out of circuit if the results are uncertain.