When an aluminum electrolytic capacitor fails in the field, the visible sign—bulging, electrolyte seepage, or an open terminal—is often the strongest clue available before you order a replacement. Each symptom points to a different stress pattern, and each stress pattern changes the parameters that matter in the replacement part. This selection guide defines where the rules apply, sets measurable thresholds, and gives a seven-step sequence for retrofit procurement.
Application Boundary and Failure Mode Identification
The guidance in this article is written for radial-lead, snap-in, and screw-terminal aluminum electrolytic capacitors used in AC-DC power supplies, DC-link sections, motor drives, and inverters. It does not cover ceramic capacitors, film capacitors, or supercapacitors, which fail by different mechanisms. Where you see the following three symptoms on an aluminum electrolytic, the responses below apply.
Bulging (Domed or Cracked Vent)
Bulging is internal gas pressure from electrolyte decomposition. Common causes are sustained ripple overload, applied voltage above rating, reverse polarity, or end-of-life aging. A vent that has opened or cracked means the capacitor has exhausted its pressure-relief capability and must not be energized again. Confirm with an LCR measurement: capacitance typically falls more than 20% below nominal and ESR rises above the initial limit.
Electrolyte Leakage at the Seal
Wet or crystallized residue around the rubber seal or terminals is physical electrolyte leakage, not electrical leakage current. It usually follows seal degradation from thermal cycling, high core temperature, or storage far outside recommended conditions. The leaked electrolyte can corrode PCB traces and neighboring parts, so plan a board-level cleaning and inspection step before installing the replacement.
Open Circuit
No continuity across the terminals, or an intermittent capacitance reading, indicates a broken internal tab connection rather than a dielectric fault. Vibration, repeated thermal expansion, and handling stress during wave soldering are typical causes. Confirm with a handheld multimeter on the terminal-to-terminal path; a capacitor that reads open on every scale requires a mechanical assessment of the mounting arrangement, not just a change of brand.
Decision Thresholds for Replacement Sizing
- Capacitance: Measure with an LCR meter at 100/120 Hz. Replace when the unit reads more than 20% below its marked nominal value (typical tolerance is ±20%). Choose replacement capacitance equal to the marked value; do not increase it unless a circuit design review allows it.
- Rated voltage: Size so normal DC bus voltage does not exceed 80% of the rating, and transients do not exceed the absolute rating. If bus overvoltage or inrush caused the failure, step up one voltage class (for example, 35 V to 50 V) and re-verify case size and ESR.
- ESR (at the datasheet frequency, temperature-corrected): End of life is reached when ESR exceeds roughly two to three times the original datasheet maximum, in line with IEC 60384 criteria. For high-ripple duty, act when ESR reaches 1.5 times the initial limit.
- Ripple current: Calculate the worst-case RMS ripple in the actual circuit. The replacement’s rated ripple current at the operating temperature must exceed that value, with additional derating above the capacitor’s rated temperature.
- Lifetime rule: For every 10°C reduction in core temperature, expected life approximately doubles. Start from the capacitor’s rated lifetime at its rated temperature, then apply the core temperature estimate from your thermal measurement.
- Fit and mounting: Match case diameter, height, lead spacing, pinout, and polarity. Leave at least the vendor-specified clearance near the vent for pressure relief.
Failure Mode, Root Cause, and Sourcing Priority
| Failure mode | Visible / measured sign | Typical root cause | Confirmation check | Replacement priority |
|---|---|---|---|---|
| Bulging | Domed case top or open vent | Ripple overload, overvoltage, reverse polarity, aging | Capacitance drop >20%; ESR >2× limit | Ripple current rating, lower ESR, temperature class |
| Leakage (electrolyte) | Residue at seal/terminals, PCB corrosion | Seal aging, thermal cycling, high core temperature | Visual inspection; ESR rise; pin-to-pin contamination | Higher temperature rating, seal construction |
| Open circuit | No continuity, intermittent reading | Tab fatigue, vibration, soldering stress | DMM continuity across terminals; repeat at higher temperature | Terminal type, mechanical robustness, mounting support |
Note: electrical leakage current (the parameter measured in microamps under rated voltage) is a distinct characteristic and is not treated as a visible failure mode in this guide.
Seven-Step Selection Procedure
- Record operating context: applied DC voltage, ripple waveform (RMS and peak), ambient temperature, airflow, orientation, and target lifetime.
- Confirm the failure mode by inspection and LCR/DMM measurement. Discard vented or leaking units; they must not be re-energized, and they should not travel in a shipment with good production stock because electrolyte can damage other products.
- Set minimum requirements: capacitance equal to nominal (within tolerance), rated voltage at least 125% of maximum bus voltage, ripple current rating above measured RMS ripple, and ESR not exceeding the original datasheet limit.
- Check the circuit, not only the part. If overvoltage or ripple excess caused the failure, identify the upstream cause before buying replacements; otherwise the new capacitor will fail the same way.
- Select the product family: radial, snap-in, or screw-terminal type that matches the mounting hole pattern, lead pitch, and case envelope. Request RoHS/REACH declarations, lot code listing, and a stated shelf-life limit for the electrolyte type.
- Run a pilot batch: measure ESR and capacitance at 20–25°C, then run the unit under full load in the equipment and measure case temperature rise. Adjust the selection if core temperature exceeds the estimate used in the lifetime calculation.
- Confirm logistics for the order: lead time, MOQ flexibility, and packaging orientation for electrolyte-filled parts. Keep the anode-up orientation in storage and transport to reduce seal stress.
