Distended Capacitors: Visual Signs, Root Causes & Safe Replacement
A bulging or distended capacitor is a clear warning sign. The electrolyte inside is decomposing and generating gas. Left unchecked, it can leak, vent, or rupture. Here's how to identify the problem, understand the root cause, and pick a reliable replacement.
Visual Signs of a Failing Capacitor
Electrolytic capacitors have a built-in safety feature: score lines (K-shaped cuts) on the top of the aluminum can. When internal pressure builds up, the top bulges before rupturing along these lines. But bulging is just one sign — here are all the visual indicators:
Bulging / Domed Top
The flat aluminum top is curved outward. This is the most common and recognizable sign. The K-score lines may be visibly stressed or partially split.
Failed — replace immediatelyElectrolyte Leakage
Brown, oily fluid leaking from the base seal or the top vent. The electrolyte is corrosive and can damage the PCB traces underneath.
Failed — replace immediatelyTilted or Pushed Off Base
The capacitor is leaning or the rubber base seal has been pushed out by internal pressure. Often accompanied by leaked electrolyte.
Failed — replace immediatelyBurn Marks / Discoloration
Black soot on terminals or PCB, or yellowed/browned plastic sleeve. Indicates excessive heat or arcing from a short circuit.
Failed — replace and inspect circuitCorroded Leads / Terminals
Green or white buildup on the leads. Caused by leaked electrolyte reacting with the metal. The capacitor may still measure OK but should be replaced.
Suspect — test and replaceNo Visible Signs
The capacitor looks perfect but the circuit is malfunctioning. Capacitance has dropped or ESR has risen beyond acceptable limits. Only detectable with test equipment.
Possible hidden failure — test with meterRoot Causes of Capacitor Bulging
Excessive Operating Temperature
Every 10°C above the rated temperature cuts capacitor life roughly in half (Arrhenius equation). Capacitors near heat sinks, power transistors, or in poorly ventilated enclosures fail first.
Prevention: Use 105°C rated caps. Improve ventilation. Add spacing from heat sources.
Ripple Current Overload
AC ripple current flowing through the capacitor generates internal heat via ESR (I²R losses). If ripple current exceeds the rating, the electrolyte boils and gas pressure builds up.
Prevention: Check ripple current specs. Use low-ESR caps. Parallel caps to share current.
Voltage Stress / Spikes
Voltage spikes from inductive loads, switching transients, or lightning can exceed the rated voltage and puncture the oxide layer, causing localized heating.
Prevention: Use caps rated for 20–50% above expected peak voltage. Add TVS diodes for spike protection.
Age / Electrolyte Drying
Over years, the liquid electrolyte slowly evaporates through the rubber seal. ESR rises, capacitance drops, and internal heating increases — accelerating the remaining electrolyte loss.
Prevention: Proactive replacement every 7–10 years in critical applications. Use long-life series (10,000+ hours).
Defective Electrolyte (Capacitor Plague)
A well-documented manufacturing issue from 2000–2007 where stolen electrolyte formulas were copied incorrectly, leading to premature failures in millions of capacitors.
Prevention: Replace all suspect-era caps. Use reputable manufacturers (Nichicon, Rubycon, Panasonic).
How to Choose a Reliable Replacement
When replacing a bulging capacitor, don't just match the value — upgrade the specs to prevent recurrence:
| Parameter | Minimum Match | Recommended Upgrade |
|---|---|---|
| Capacitance | Same value (µF) | Same value or slightly higher |
| Voltage Rating | Same or higher | 20–50% higher than original |
| Temperature Rating | 85°C minimum | 105°C (almost always worth it) |
| ESR | Same or lower | Low-ESR series if available |
| Ripple Current | Same or higher | Higher ripple rating = runs cooler |
| Lifetime (hours) | 2,000 hours | 5,000–10,000 hours for reliability |
| Physical Size | Must fit PCB | Same diameter, can be slightly taller |
Bulging Capacitor FAQ
Why do capacitors fail?
What causes capacitors to blow?
Is a bulging capacitor dangerous?
Can a capacitor be bad without bulging?
What causes a capacitor to bulge?
Should I replace all capacitors if one is bulging?
Can I replace a bulging capacitor with a higher voltage rating?
What is the "capacitor plague"?
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