Why does a cap vent?

[ben7]

Originally Posted by Hondaman

Maybe the power was on, I'm not sure. The teacher gave the impression that "it wasn't supposed to happen", so I assumed the power was off and the cap was discharging. Please wear safety glasses around high voltage. And don't wear jewelry (wedding ring) which could cause a short or get tangled up with parts when working on stuff.

Maybe the startering contactor was stuck ON.

[Sparkey55]

Originally Posted by Tom41

Like the title suggests, what exactly causes a cap to vent? Does it heat up and 'boil' the electrolyte? Does electrolysis occur and produce gas?

The pressure inside the container is greater than the pressure on the outside of the container.

To boil anything requires heat to be applied.

Without expanding gases or fluids there would be no pressure exerted.

[tecmax]

Answer
Vented gas caps work by a pressure activated one-way release valve. When pressure builds to a certain point it opens the valve. This allows a small amount of air to be released into the gas tank line.

[TELVM]

^ I'm afraid we're talking electronics cap-acitors here, not vented fuel tank caps .

[jts1957]

They discovered that venting caps were preferred over the exploding ones.

[SteveNielsen]

The only time I saw a cap explode was back in the 80s when building a 286 pc clone. They were into using little tantalum caps all over the board then, not all the little cans we see now. As soon as I applied power to the board one of those little puppies went off like firecracker! Sure glad the top didn't hit me because it ricocheted all over the shop like a tiny bullet! Smelled like hell too!

[SteveNielsen]

That is not to say I have not seen exploded caps after the actual explosion, that is the only time that I saw one in the act of exploding.

I had a blonde Fender Tremolux Amp that the big power supply caps blew out after a very hot and long day in the back window of a VW microbus and being set up on a very hot outdoor stage and powered on. I heard and smelled those caps blow but couldn't see them. It was a mess. That is why they make them with vents now.