Pressurised caps?

[Tom41]

I recently took apart a couple of capacitors from the failed PSU on my work PC. They were the large black ones nearest to the power input sockets. After getting the black plastic off, I saw vents at the top. Knowing that some of these caps can be phoney (there was a post on these forums some time ago showing a small cap inside a big can), I decided to puncture a hole and see if the cap was genuine.

Turns out it was genuine, I could see (and smell) the electrolyte and paper inside. But when I punctured the hole, there was a small hiss as if gas was escaping. Which leads me onto my question - are caps supposed to be pressurised inside like that? Or does high pressure inside mean electrolysis has occurred and the cap is failing?
If so, it could be a good way of testing for bad caps - though you'd ruin each cap you tested that way

[arneson]

Re: Pressurised caps?

Well this could be a marketing thing, Prevented caps, has a nice ring to it.
We blow them before you do.

[stretch0069]

Re: Pressurised caps?



as far as the OP....I'm not sure about your situation. How long had it been de-energized before you poked it?

I've had caps explode, but that was energized. We had caps in a TPN-30 PS that we called "shotgun caps" because if they blew, they exploded like one of those party do-dads.

[tiresias]

Re: Pressurised caps?

Hi Tom,

I just peeked under the hood of my wife's Enermax PSU - sorry about the picture quality, she took the Nikon SLR with her to work!

Looks like Hitachi's new HP3s are actually quite high-tech after all.

[arneson]

Re: Pressurised caps?

Wait a minute here..

[Elitist]

Re: Pressurised caps?

Capacitors bulge and/or explode due to electrolysis with build up of hydrogen gas. Ancient units were fitted with a pressure relief valve in the base. Nowadays, a correct electrolyte formulation should include hydrogen scavengers and components that accomodate overcharge (this is not the right palce for a full discussion - refer to any good technical manual on batteries, for example) by parasitic electrode reactions that are not gas generating. A bulged capacitor would normally expect to dissipate its payload of pressurised hydrogen by diffusion (it is the lightest, smallest gaseous element!) through the base rubber seal. However, I've noticed that sealing technology has improved so much that pressure can remain for weeks or longer inside the can. If you want to test the state of the internals in an exciting and potentially hazardous manner, try re-flatting the bulge by smashing it down on a flat surface such as a kitchen worktop. Usual disclaimers, of course, as, if you guess wrong, you could lose an eye, blow your face off or, if there's a flame around, burn the house down. Notwithstanding, it works for me and rarely, the cap can even be restarted!

[trodas]

Re: Pressurised caps?

Geeee, is nto better to replace the cap with a new one?

In case the pressure remaining, the base rubber could blew off and... the cap is gone totaly

[Rainbow]

Re: Pressurised caps?

If the cap is bulged, it's already bad. And it cannot be "fixed". The electrolyte is partially converted to gas.

[arneson]

Re: Pressurised caps?

I tried to role my own.
Pulled the cap apart, put new paper between the foils rolled it up medium tight and sealed with glue on bottom.
If the Chinese caps are going to burst maybe this is is the answer...ahem,huh?

[999999999]

Re: Pressurised caps?

No they should not be pressurized, they were progressively failing.

On some poor caps, there isn't any electrolysis needed for a gas buildup. Just sitting around they will build up pressure until the electrolyte has decomposed entirely or it vents. I had a bunch of old GSC caps that did that, some had failed on motherboards so I replaced the whole lots of them and left them sitting in a jar. Later even the ones that hadn't bulged yet, started to and kept going till the tops and bottoms were blown.

Unfortunately many of these HV caps have a plastic cover on the top that starts out looking a little bulged from the way the shrink-sleeve compressed them (or they really were designed as curved plastic pieces, I don't know which). You have to push down on it to know if the top of the cap is flat or not.

[yanz]

Re: Pressurised caps?

Hi Tom,

I just peeked under the hood of my wife's Enermax PSU - sorry about the picture quality, she took the Nikon SLR with her to work!

Looks like Hitachi's new HP3s are actually quite high-tech after all.

bbfhh..i spilled my coffee

[yanz]

Re: Pressurised caps?

Yup, under the plastic cover they are flat; I have to use my sharp knife to make sure it is.

[Elitist]

Re: Pressurised caps?

Jeez, 99etc., you're a mine of info. I even printed out your recent piece on Hot caps to send around to friends.
It is very difficult for an outsider to comment on the exact chemistry of electrolyte formulation - the most highly guarded secrets of all; guess we wouldn't be here if details were in the open literature. However, there are a number of points to bear in mind. In the aulden dayes, it was just a question of adequate buffering (borates are self-buffering, anyway) and avoidance of overvolting. Things moved on. Now there are organics in there and, apart from ptfe, we all know that organics spontaneously decompose, a little faster as the temperature rises (actually, T is in the exponent, too) and in the presence of catalysts. For organics, almost any metal substrate will do! Notwithstanding, that barrier film is extremely coherent. It has to be, because aluminium is one of the most reactive elements known. But there are always impurities on both sides of the interface just waiting to catalyse a catastrophic failure should any breach occur.
So, yes, you are correct - gas build up is inevitable. Keeping some charge on the cap may help hold the film integrity by electrolytic enhanced self-healing, although the oc at the film interface should be well removed from the pzc?