Capacitor Faults
Capacitors are generally reliable components, but they can fail because of ageing, excessive temperature, overvoltage, high ripple current, mechanical damage or manufacturing defects. Faulty capacitors are among the most common causes of failure in power supplies, audio amplifiers, televisions, motherboards and industrial equipment. Recognising the symptoms of capacitor failure is an important step in diagnosing and repairing electronic circuits.
Why Capacitors Fail
- High operating temperatures.
- Ageing and electrolyte drying.
- Excessive ripple current.
- Overvoltage.
- Incorrect polarity.
- Mechanical stress or vibration.
- Poor manufacturing quality.
- Moisture or corrosion.
Common Capacitor Faults
| Fault | Description |
|---|---|
| Open Circuit | The capacitor no longer stores charge effectively. |
| Short Circuit | The terminals become electrically connected internally. |
| Reduced Capacitance | The capacitance value falls below specification. |
| High ESR | Internal resistance increases due to ageing. |
| Leakage Current | Excessive DC current flows through the dielectric. |
| Dielectric Breakdown | The insulating material fails under high voltage. |
Visual Signs of Failure
| Observation | Possible Cause |
|---|---|
| Bulging top | Internal gas pressure caused by overheating. |
| Leaking electrolyte | Seal failure or ageing. |
| Split safety vent | Excessive internal pressure. |
| Discoloured sleeve | Overheating. |
| Burn marks | Electrical overload or short circuit. |
| Cracked ceramic body | Mechanical stress or PCB flexing. |
Symptoms in Electronic Equipment
| Equipment | Possible Symptoms |
|---|---|
| Audio Amplifier | Hum, weak bass, distortion or protection mode. |
| Switch-Mode Power Supply | Failure to start, unstable output or chirping. |
| Television | No power, repeated restarting or distorted picture. |
| Computer Motherboard | Boot failure, crashes or spontaneous resets. |
| LED Driver | Flickering or unstable brightness. |
| Motor Controller | Erratic operation or shutdown. |
Electrical Tests
- Measure capacitance with a capacitance meter.
- Measure ESR using an ESR meter.
- Check for leakage current where appropriate.
- Use an ohmmeter to detect open or short circuits.
- Observe ripple voltage with an oscilloscope.
Testing Results
| Measurement | Likely Condition |
|---|---|
| Correct capacitance and low ESR | Capacitor is likely healthy. |
| Correct capacitance but high ESR | Capacitor should usually be replaced. |
| Very low capacitance | Capacitor has deteriorated. |
| Zero resistance | Possible short circuit. |
| Infinite resistance immediately | Possible open circuit. |
Common Causes by Capacitor Type
| Capacitor Type | Typical Failure Mode |
|---|---|
| Aluminium Electrolytic | Drying electrolyte and increased ESR. |
| Tantalum | Short-circuit failure after overstress. |
| Ceramic MLCC | Mechanical cracking. |
| Film Capacitor | Rare failure, usually overvoltage related. |
| Supercapacitor | Capacity loss due to ageing. |
How to Prevent Failures
- Choose the correct voltage rating.
- Use 105°C capacitors in high-temperature equipment.
- Select low-ESR capacitors for switching power supplies.
- Keep components away from heat sources.
- Avoid reverse polarity on electrolytic capacitors.
- Do not exceed ripple-current ratings.
Repair Tips
- Replace leaking or bulging capacitors immediately.
- Always match the original capacitance value unless the circuit design allows otherwise.
- Use an equal or higher voltage rating.
- Install electrolytic capacitors with the correct polarity.
- After replacement, inspect solder joints and clean any flux residue.
Common Mistakes
| Mistake | Consequence |
|---|---|
| Replacing with a lower voltage rating. | Premature capacitor failure. |
| Ignoring ESR. | Power supply instability. |
| Reversing capacitor polarity. | Possible venting or explosion. |
| Using a general-purpose capacitor in an SMPS. | Overheating and shortened service life. |
| Ignoring visible damage. | Further equipment failure. |
Real-World Examples
| Equipment | Typical Capacitor Fault |
|---|---|
| Desktop Computer | Bulging motherboard capacitors causing boot problems. |
| Audio Amplifier | Dried filter capacitors causing audible hum. |
| LCD Monitor | High-ESR capacitors preventing startup. |
| Television | Failed SMPS electrolytic capacitors causing repeated restarting. |
| Industrial Power Supply | High ripple due to ageing filter capacitors. |
Key Points
- High ESR is one of the most common capacitor faults.
- Visual inspection often reveals obvious failures such as bulging or leaking electrolytic capacitors.
- Testing should include capacitance, ESR and, where appropriate, leakage current.
- Correct replacement parts improve reliability and service life.
- Proper capacitor selection helps prevent premature failures.