Capacitors Fail Too
Like any component, aluminum electrolytic capacitors have predictable failure modes. Knowing them helps you design around them and troubleshoot when things go wrong.
Common Failure Modes
1. Electrolyte Dry-Out
The number one failure. Electrolyte gradually evaporates through the rubber seal. Capacitance drops and ESR rises. The rate roughly doubles every 10 C. A capacitor rated 2,000 hours at 105 C might last 32,000 hours at 65 C. Running hot kills capacitors faster than anything else.
2. Vent Rupture
The pressure relief vent on top of the can is a safety feature. If internal pressure builds from overheating, reverse voltage, or excessive ripple current, the vent opens to prevent explosion. A bulging top or split vent means the capacitor has already failed or is about to.
3. Short Circuit
The oxide dielectric breaks down if the capacitor sees voltage spikes exceeding its surge rating, or reverse voltage. Once the dielectric is punctured, current flows freely — a dead short. This is why electrolytics have a surge voltage rating (typically 10-15% above rated voltage).
4. Open Circuit
Vibration, thermal cycling, or poor soldering can break the internal connections between lead wires and foil tabs. An open capacitor provides zero capacitance.
5. Parameter Drift
Normal aging: capacitance gradually decreases and ESR increases. End-of-life is typically -20% capacitance or +100% ESR from initial values.
How to Spot a Failing Capacitor
- Visual: Bulging top, leaking electrolyte (brown crust near seal), split vent
- Electrical: High ripple on the power rail, unexpected resets, intermittent operation
- Thermal: Capacitor runs hotter than neighboring components — classic sign of high ESR
- Measurement: Capacitance below -20% of rated, ESR above 2x datasheet spec
Design to Avoid Failures
- Derate voltage by at least 20%
- Keep ambient temperature below rated — ripple current adds internal heating
- Mount away from heat sources
- For long-life products, use capacitors rated 5,000-10,000 hours at 105 C
