# Electrolytic capacitor got hot, bulged or popped: polarity and voltage rating

> Why an electrolytic capacitor fails: reversed polarity, a voltage rating too close to the supply, or ripple current. Stop at once and replace it.

Canonical: https://roboticsengineering.in/fix/electrolytic-capacitor-reversed-polarity-bulge-hot/ | Published: 2026-11-03 | Updated: 2026-11-04

**Electrolytic capacitors have a positive and a negative leg.** Reversed, or over their voltage rating, they can heat, bulge or burst.

## What the message means

The stripe on the can marks the negative side. Reverse voltage damages the inside and a high voltage breaks the film.

## Fix it in this order

1. Disconnect power at once if a capacitor is hot or smells.
2. Check the stripe on the can marks the leg that goes to ground.
3. Choose a voltage rating well above the real supply, for example about double.
4. Replace a bulged or leaking capacitor. Do not reuse it.
5. Keep the part away from hot places such as a driver heat sink.
6. Use a low-ESR type near switching converters.

## Why it happens

An aluminium electrolytic capacitor gets its capacitance from a very thin oxide layer on the positive side. That layer is formed with the polarity in mind. Reverse voltage, or voltage above the rating, can break it down, so current rises, the part heats and gas builds up inside the can.

Many cans have a scored top that is meant to open under pressure, which is why a failing capacitor bulges first and may then vent. A bulged top or leaked fluid means the part is finished, whatever it measures afterwards.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Top of the can is bulged | Replace it. |
| Gets warm at normal use | Check rating and ripple. |

## Related

[check before power-on](https://roboticsengineering.in/fix/check-short-circuit-with-multimeter-before-power-on/), [buck converter sag](https://roboticsengineering.in/fix/buck-converter-output-drops-under-load/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

## Why this happens

An aluminium electrolytic capacitor gets its capacitance from a very thin layer of aluminium oxide on the positive foil, with a liquid electrolyte as the other plate. The oxide is built up with a definite polarity. If the voltage is applied the wrong way, the oxide stops blocking current, leakage current rises, and the electrolyte starts to break down into gas. The same happens when the voltage is above the rating, because the oxide layer can only withstand a limited field.

Heat adds to the damage. Current through the capacitor, such as the ripple current from a switching converter, dissipates power in its internal resistance. A part that runs warm ages faster, and a part that runs hot dries out. The scored top of the can is a deliberate weak point so the gas escapes there instead of bursting the side, which is why a bulge on top is the sign to look for.

## How to confirm the cause

- Read the can. A stripe, usually with minus signs, marks the negative leg. On the board, the silkscreen marks the positive side with a plus sign or a shaded half circle. Compare them.
- With the power off and the capacitor discharged, use the multimeter continuity mode to confirm that the negative leg goes to the ground rail, and the positive leg to the supply rail.
- Read the voltage printed on the can, and measure the highest voltage the rail reaches in real use, including battery full-charge voltage and motor spikes if you have an oscilloscope. A margin of about twice is a common rule of thumb.
- Look at the top of the can under a good light for a dome shape, brown residue on the board, or a smell. Any of these means the part has failed, even if a meter still shows some capacitance.
- After a few minutes at normal load, switch off and check the can with a fingertip or an infrared thermometer. A can that is clearly hotter than its neighbours is under too much ripple current or sits next to a heat source.

## Worked example

Choosing the rating for a pack that is nominally 12 V but charges higher is simple arithmetic. The values below are an illustration, not a recommendation for a specific design:

```
3S Li-ion pack, nominal 11.1 V, fully charged 3 x 4.2 V = 12.6 V
Margin of about 2x: 12.6 x 2 = 25.2 V
Next standard rating at or above that: 35 V (a 25 V part has little margin)
Ripple heating, example only: I_rms = 1.0 A, ESR = 0.1 ohm
P = I^2 x ESR = 1.0 x 1.0 x 0.1 = 0.1 W (check ESR and ripple rating in the datasheet)
```

## Common mistakes

- Trusting leg length. Legs get trimmed, so rely on the stripe on the can.
- Fitting a 16 V part on a 12 V rail that sees spikes from motors or a fully charged battery.
- Reusing a capacitor that was reversed once. Its oxide is damaged even if it looks normal.
- Using a general-purpose capacitor in a switching converter that asks for low ESR.
- Soldering the capacitor tight against a hot heat sink or driver chip.

## If this did not fix it

- The supply connector itself may be reversed, for example a battery plug or barrel jack with the wrong centre polarity, which reverses every capacitor on the board at once.
- Inductive spikes from motors or relays can exceed the rating briefly. A diode or a snubber at the load may be missing.
- The converter’s ripple current may be higher than the capacitor can take; check the part’s ripple rating and ESR in the datasheet.