Swollen or Leaking Electrolytic Capacitor: Diagnosis and Replacement
One of the most common causes of failure in electronic devices is electrolytic capacitors that tire over time. The liquid electrolyte inside them dries with heat and time; the capacitor loses capacitance, its internal resistance (ESR) rises and the circuit starts to work unstably. The good news: this fault is one of the easiest both to diagnose and to repair. In this article we go step by step, starting from the symptoms through diagnosis, choosing the right replacement and safe swapping.
Symptoms: when to suspect it?
The following symptoms often point directly to a failed supply capacitor:
- The device won't turn on at all, or takes seconds to start.
- Flickering on the screen, ripple in the image, or a hum in the audio system.
- The device works for a while then freezes, resets or turns off by itself.
- Crackling, heating or a smell from the power supply.
These symptoms are common especially in power supplies, motherboards and LED TV/monitor supply stages.
Visual diagnosis
Open the device safely and inspect the capacitors by eye. A healthy electrolytic capacitor's top cap is flat. The following signs indicate a fault:
| Sign | Meaning |
|---|---|
| Bulging (doming) of the top cap | Internal pressure has risen, the capacitor has failed |
| Brown/dark dry residue on top | Electrolyte has leaked out |
| Leakage at the base, swollen solder | Electrolyte has leaked from below |
| Split along the top vent line (X/K/T) | The safety vent has triggered |
If there's visible swelling, the diagnosis is clear; but a capacitor can look fine externally and still be electrically failed.
Electrical diagnosis: ESR and capacitance
A capacitor can be tired without swelling. Here the most reliable indicator is the ESR (equivalent series resistance) value: in a tired capacitor the ESR rises markedly. An ESR meter or capacitance meter shows how far the capacitor has degraded relative to its rated value. For general capacitor measurement methods, our how to test a capacitor with a multimeter article offers a detailed start. Tip: you can often do the ESR measurement without removing the part from the board, but for a capacitance measurement, removing the capacitor gives a more accurate result.
Choosing the right replacement
When replacing, you need to choose the right part, not a "similar" one. Watch this order:
- Capacitance (µF): choose the same value; a small upper tolerance is no problem.
- Voltage (V): it can be equal or higher, never lower.
- Temperature grade: don't replace a 105 °C part with an 85 °C one; 105 °C is preferred in supply stages.
- Low-ESR type: in switch-mode power supplies, always use a capacitor marked "low ESR"; an ordinary type heats up and fails again. In very demanding spots, a polymer capacitor is a more durable alternative.
- Size: make sure the diameter and height fit the board.
Safe replacement steps
Before the repair, unplug the device and make sure large capacitors — especially in power supplies — are discharged (discharge them through a suitable resistor if needed). Then:
- Note the old capacitor's polarity: the striped side on the body is the negative (−) leg.
- Heat the legs with a soldering iron and remove the old part; mind your soldering technique.
- Install the new capacitor with the correct polarity: an electrolytic capacitor installed backwards heats up and can explode.
- Check the solder joints, trim the excess legs and verify the circuit by eye.
Common mistakes
- Reversing the polarity: an electrolytic capacitor is polarized; connected backwards it explodes.
- Choosing a lower voltage: a part below the rated voltage fails quickly.
- Using standard ESR: an ordinary capacitor where low-ESR is needed heats up and repeats the fault.
- Skipping discharge: an undischarged large capacitor carries an electric-shock risk.
- Looking only at the swollen one: all capacitors of the same series may be tired; evaluate all same-age parts in the supply stage together.
Frequently asked questions
Is a non-swollen capacitor healthy?
Not always. A capacitor that looks fine externally can be failed in terms of ESR; a definitive diagnosis requires an ESR/capacitance measurement.
Can I fit a higher capacitance?
A small increase is usually no problem, but large deviations can change the supply timing and current draw; where possible, stick to the rated value.
Should I replace just the one bad capacitor?
Same-age capacitors on the same board tire similarly; if one has failed, checking the others prevents a re-open.
Conclusion
An electrolytic capacitor fault is an economical repair that can bring many devices back to life with the right diagnosis: recognize the symptoms, do the visual and ESR-based diagnosis, choose a replacement matched in capacitance/voltage/temperature/ESR, and swap it minding the polarity. You can review supplier offers for the capacitors you need with search and the comparison tool.