Replacing Capacitors
Replacing faulty capacitors is one of the most common repairs in electronic equipment. Ageing electrolytic capacitors are a frequent cause of hum, unstable power supplies, startup failures and poor circuit performance. Choosing the correct replacement capacitor and installing it properly helps restore reliable operation and prevents further damage.
When Should a Capacitor Be Replaced?
- Bulging or swollen top.
- Leaking electrolyte.
- Cracked or damaged casing.
- High ESR.
- Capacitance outside the specified tolerance.
- Excessive leakage current.
- Visible overheating or burn marks.
- Intermittent equipment faults traced to the capacitor.
Choosing a Replacement Capacitor
| Parameter | Recommendation |
|---|---|
| Capacitance | Use the same value unless the circuit documentation specifies otherwise. |
| Voltage Rating | Use the same or a higher voltage rating. Never use a lower voltage rating. |
| Temperature Rating | Prefer 105°C capacitors for power supplies and high-temperature environments. |
| ESR | Use low-ESR capacitors where required, especially in SMPS circuits. |
| Ripple Current | Equal to or greater than the original rating. |
| Physical Size | Ensure the replacement fits the available PCB space. |
| Lead Spacing | Match the PCB hole spacing whenever possible. |
Understanding Capacitor Markings
Before replacing a capacitor, read the markings printed on its body.
| Example | Meaning |
|---|---|
| 470 µF | Capacitance. |
| 25 V | Maximum working voltage. |
| 105°C | Maximum operating temperature. |
| Low ESR | Designed for high ripple-current applications. |
Required Tools
- Soldering iron.
- Desoldering pump or solder wick.
- Leaded or lead-free solder.
- Flux.
- Side cutters.
- Digital multimeter.
- ESR meter (recommended).
- Safety glasses.
Replacement Procedure
- Disconnect the equipment from the power source.
- Discharge all high-voltage capacitors safely.
- Identify the faulty capacitor.
- Note the polarity before removal.
- Desolder the old capacitor carefully.
- Clean the PCB pads if necessary.
- Insert the new capacitor with the correct polarity.
- Solder the leads securely.
- Trim excess lead length.
- Inspect the solder joints before powering the equipment.
Capacitor Polarity
Electrolytic capacitors are polarised and must be installed correctly.
| Marking | Meaning |
|---|---|
| Stripe on capacitor body | Negative (-) terminal. |
| PCB "+" marking | Positive terminal location. |
| Long lead (new capacitor) | Positive terminal. |
Installing an electrolytic capacitor with reversed polarity may cause it to overheat, vent or fail.
Common Replacement Mistakes
| Mistake | Consequence |
|---|---|
| Using a lower voltage rating. | Premature failure. |
| Incorrect polarity. | Possible capacitor damage. |
| Using a general-purpose capacitor instead of a low-ESR type. | Unstable operation in SMPS circuits. |
| Poor solder joints. | Intermittent faults. |
| Overheating PCB pads. | Lifted tracks or damaged board. |
Application-Specific Recommendations
| Equipment | Recommended Replacement |
|---|---|
| Switch-Mode Power Supply | 105°C low-ESR electrolytic capacitor. |
| Audio Amplifier | High ripple-current electrolytic capacitor. |
| Speaker Crossover | Polypropylene film capacitor where appropriate. |
| Motor Run Circuit | Motor-run capacitor with the correct AC voltage rating. |
| Timing Circuit | Film or C0G/NP0 ceramic capacitor for better stability. |
After Installation
- Inspect all solder joints carefully.
- Verify the polarity of every electrolytic capacitor.
- Check for solder bridges.
- Measure the power supply voltages after repair.
- Monitor the replacement capacitor for excessive heating during initial operation.
Real-World Examples
| Equipment | Typical Repair |
|---|---|
| Computer Power Supply | Replace high-ESR output capacitors. |
| LCD Monitor | Replace bulging electrolytic capacitors. |
| Audio Amplifier | Replace dried filter capacitors causing hum. |
| LED Driver | Replace faulty output filter capacitor. |
| Television | Replace failed SMPS capacitors preventing startup. |
Repair Tips
- Replace all capacitors of the same type in a high-temperature area if several have aged similarly.
- Buy capacitors from reputable manufacturers to reduce the risk of counterfeit or poor-quality parts.
- Keep replacement leads as short as practical.
- Use fresh solder and flux to obtain reliable joints.
- If replacing large power-supply capacitors, ensure the replacement has an adequate ripple-current rating.
Key Points
- Always use the correct capacitance and an equal or higher voltage rating.
- Observe polarity when replacing electrolytic capacitors.
- Choose low-ESR and 105°C capacitors for power supplies where appropriate.
- Inspect your work carefully before applying power.
- Good replacement practices improve reliability and extend equipment life.