Here’s a visual comparison between a brand-new electrolytic capacitor and one that has failed badly enough to bulge. I’ve got two 680 uF, 4 V capacitors here, one in rather better condition than the other. It’s quite interesting seeing them side by side because the damage becomes obvious once you know what to look for.

Some aluminium electrolytic capacitors have a scored pressure vent on the top. If internal pressure builds up under abnormal conditions, the weakened section is designed to open in a more controlled fashion. These particular capacitors have a different construction, though, and the failed one has started bulging around the bottom instead.

The rubber seal and plastic sleeve are being pushed out, and the capacitor isn’t sitting the way it should. Thankfully it hasn’t done anything especially spectacular, but it’s certainly ready for replacement.

One important catch is that a capacitor doesn’t have to bulge before it’s bad. Electrolytics can lose capacitance, develop excessive losses or leakage, or dry out without presenting an obvious visual clue. A swollen capacitor makes the diagnosis easy; it’s the innocent-looking ones that can be considerably more annoying.

The replacement is actually the same brand and rating as the failed capacitor. Normally I’d have a look at the alternatives, but in this case it was the only suitable part I had available. At least this time I didn’t need much convincing that the old one had to go.

Sources