What are markings on top of electrolytic capacitors?

I often see a marking like the following pattern on the top of radial PTH elcos.

What does it mean? Is it an identification of the manufacturer?

I believe they are vents when the pressure inside a capacitor would build up due to heating of some sort. It will pop open instead of just blowing up.

• Not really vents so much as scored lines that rip open. On the other side the elastomeric seal vents hydrogen – Nick T Dec 16 '10 at 12:56
• yup - Since electrolytic caps that malfunction release hydrogen. If you had one of the ill fated capasitor rot capasitors, this would fail, and you'd have somenasty gunk on top – Journeyman Geek Dec 16 '10 at 14:33
• If they don't have vents, the capacitor explodes violently and can launch the casing at your forearm hard enough to draw blood. :D – endolith Dec 16 '10 at 15:19
• @endolith, is that a personal experience? – vicatcu Dec 16 '10 at 15:58
• @vicatcu: How could you tell? :) – endolith Dec 16 '10 at 16:20

Here's a (somewhat artistic) video of those features in action.

The first two events (0:00, 0:03) are what happens when the stamped features do not work, but all the subsequent electrolytic capacitor failures (0:14, 0:16, 0:18, 0:34, 0:36, 0:39...) rip the case open in a relatively safe and controlled manner, notably not causing shrapnel to fly around.

• I liked the "MOSFET = Metal Oxide Semiconductor Fire Emitting Transistor" comment by kkristss. – Kevin Vermeer Dec 16 '10 at 16:37
• @Kevin Yeah... and you of course know why the MOSFET is in the circuit? – ChrisR Jan 22 '16 at 12:04
• @Kevin ... its to protect the fuse. – ChrisR Jan 22 '16 at 12:04

They are strategically weakened parts of the casing which will bulge upwards and help the capacitor release hydrogen and/or electrolyte. They work quite well. Don't try this at home, but if you blow up an old capacitor with no release vents it will explode violently sending electrolyte and alfoil flying everywhere. The vents are designed to prevent such a catastrophic failure from happening.

It's interesting to see how different manufacturers have different vent patterns. It might be due to cost - most have four vents in a cross/plus sign fashion, but some cheaper ones have only two or three. It might save some money on the production line to score only two or three times. And some smaller ones have no vents at all. That's probably another question, though.

• Don't try this at home... Too late :-) I once had one such an elco wrongly connected and it exploded while I was bent over the board. The cap of the cap :-) only missed my left eye by a few centimeters and left a dent in the ceiling. And it smelled! Under more controlled circumstances we held contests at school: shoot the cap as far as possible. – stevenvh Dec 16 '10 at 15:58