Why does a cap vent?

[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?

I vented a cap recently - by pumping 20v into the 10v-rated cap on reversed polarity and the pic in my avatar shows the result. It 'exploded' with a load of smoke. The cap was very hot afterwards, so maybe it boiled?

[Topcat]

The theory is that of a pressure cooker. They heat, things expand, have no place to expand to, and poof.....they vent. They do it faster with shoddier brands, as the electrolyte is water-based, and evaporates at temps a lot lower than good ones.

[gonzo0815]

I think it is the H2, wich is generated by a chemicall reaction of H2O and AL if no inhibiting additive is in the elektrolyt. The H2 would be collected at the top of the cap, abd if the pressure is high enough the safety vent will burst.

[kc8adu]

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?

both. electrolysis starts the bulging.as it dries out esr rises and heating occurs.as the vent opens and more electrolyte is lost the cap looses capacitance and goes open.

[whiz]

some stupid questions

1)What is the "X" mark or the "T" mark on the top of the caps? is this the escape door?

2)The electrolyte is it corrosive for our skin? is it dangerous to smell it?

3)do we know the chemicals it contains?

4) The thin metal film inside the cap what is it? aluminium or some other metal?



thank you...

[PeteS in CA]

1)What is the "X" mark or the "T" mark on the top of the caps? is this the escape door?

It's an intentional "weak" spot in the can. It make it less likely that the whole can will launch like a little rocket. The scorings vent at a fairly predictable pressure - electrolyte vapor and/or hydrogen. BTW, electrolytes with significant water content are not peculiar to crap caps: e.g. all Rubycon low impedance caps have significant water content, but Rubycon (the other two 'cons and Panasonic also) knows what to add to stabilize the electrolyte.

2)The electrolyte is it corrosive for our skin? is it dangerous to smell it?

As a whole, no, though I wouldn't use electrolyte as soap or a beverage.

3)do we know the chemicals it contains?

The basic ingredients are known publicly.

4) The thin metal film inside the cap what is it? aluminium or some other metal?

Aluminum

[whiz]

Originally Posted by PeteS in CA

It's an intentional "weak" spot in the can. It make it less likely that the whole can will launch like a little rocket.

quite interesting effect if they could lift off...

thank you for the analysis...do you know any good sites so i can read
about the composition of electrolyte?

[VILE DISRUPTION]

probably google. try "capacitor electrolyte" or something.

[gonzo0815]

Think this i somehow a secret wich is kept closed to the public. As today to mutch well skilled competitors in low cost countries are waiting for the proper rezept.

[arneson]

The write ups Iv'e read on this go quickly over my head but detail the tweaking of test solutions. Adding stabilizers, nuetralizers and removing water.
Lots of graphs and math. I don't think the prefered formular is secret but any improvements would certainly be, unless HP taps the phone.

[PedroDaGr8]

Originally Posted by arneson

The write ups Iv'e read on this go quickly over my head but detail the tweaking of test solutions. Adding stabilizers, nuetralizers and removing water.
Lots of graphs and math. I don't think the prefered formular is secret but any improvements would certainly be, unless HP taps the phone.

Any chance that you could send me some of these, or the links / names etc. I am trying to find out as much of this as I can inorder to try and put together a plain english article on why capacitors vent (initially I was going to do some research but the Hillman et al. article is quite good on the research).

[davmax]

I have used thsi reference before: http://industrial.panasonic.com/www-...ABA0000TE4.pdfBasically the internal cap pressure is claimed to be caused by excessive hydrogen. Good electrolyte formulae absorb the hydrogen whilst the bad cap brands have used an incorrect formula and excess hydrogen is the result. Too much and pop goes the vent.

[PedroDaGr8]

Originally Posted by davmax

I have used thsi reference before: http://industrial.panasonic.com/www-...ABA0000TE4.pdfBasically the internal cap pressure is claimed to be caused by excessive hydrogen. Good electrolyte formulae absorb the hydrogen whilst the bad cap brands have used an incorrect formula and excess hydrogen is the result. Too much and pop goes the vent.

Nice article, covers a wide range of failure modes.
The mode of failure for the tiawanesse capacitors was investigated by Hillman et al. Article It appears to be more complex than this. The good electrolytes actually prevent the H2 generation in the first place. It seems that the bad electrolytes are usually marked by pH>=6 while the good ones are have a pH of around 2-4, bad electrolytes also seem to be lacking in phosphate ions compared to the good electrolyte. The reason that the pH is important, is that it degrates the aluminum oxide dielectric layer. Which I explain the mechanism in detail in this thread http://www.badcaps.net/forum/showthread.php?t=3047
The dielectric degredation is characterized by a gradually rising capacitance value (as the dielectric layer gets thinner and thinner) until there is a sudden spontaneous drop (dielectric layer is gone, now have highly conductive electrolyte bridging to plates).

[mbutts]

Very informative thread!

on a side note

Eons ago when I was in college I was doing a lab project. I had an axial cap on a breadboard in a circuit and was attempting to take a voltage reading. The DVM counted down toward zero volts. Just be for it hit that magical number there was a very loud pop followed by small quantities of white hair like debris floating in the air. I had inadvertently put a cap in backwards and it blew up. One end of the cap was still on the breadboard, the other had shot to the ceiling and bounced back down on the bench top. Needless to say I got the attention of the rest of the lab. I picked up the piece off the bench and burnt the heck out of my fingers. Since then, it has been down hill with my love affair of bad caps and I replace them every opportunity I get.

[PCBONEZ]

Bit of caution as to:
>>>>>>>>>>
3)do we know the chemicals it contains?
<<<<<<<<<<

The answers I see are pertaining to the low ESR caps this site is about.
I agree, those aren't especially nasty chemicals.

But other types of caps indeed have very some nasty stuff inside.
[off the top of my head.]
- The electrolyte in Tantalum foil capacitors may be as much as 40% sulfuric acid.
- Old oil filled caps may contain PCB's.

.

[PCBONEZ]

>>>>>>>
by pumping 20v into the 10v-rated cap on reversed polarity
<<<<<<<

Most "on site topic" caps are rated for 1v max on reverse polarity.
-
Reverse polarity strips the oxide layer off the aluminum.
(Due to electrolysis in the wrong direction.)
The oxide layer is the bulk of the dielectric (insulator) between the plates.
When it's gone the cap is effectively shorted internally. (Or partially so.)
More current. More heat. More gasses?. Thermal expansion?(electrolyte)
~Pop~
.

[DRHCREEK]

Years ago while working on tube television chassis inside a wooden console, I saw an aluminum can electrolytic launch the can into the top of the wooden console. It planted itself into the wooden top enough to cause the top of the furniture to require refinishing.
Caps in those televisons were 400+ uf's at up to 600 volts. They became grenades if installed incorrectly. Be very careful about ploarity when installing any cap.

[Hondaman]

Seven years ago, my vocational school teacher had his own HVAC business in the daytime and taught us at night. He told us a story once about a capacitor that blew on a rooftop air conditioning unit. He bent down to pick up a tool and heard it explode. It is a good thing he bent down.

(I don't know why a capacitor the size of a 330 or 354 milliliter cola can would be mounted on its side, 5 or 6 feet above the surface of the roof. And I presume the power was off when it blew. What could cause such a time delay before the explosion?)

Anyway, the top blew. The contents struck the cover panel of the neighboring air conditioning unit (because it was laying on its side). It was a hot mess.

(Why would the capacitor blow after power was removed?)

It is a good thing my teacher bent down when he did!

(He also told us about the time he was working on a rooftop unit at a small corner food store [Dayton Ohio, probably late 1970s]. He came down from the roof to take a break and discovered that someone had brutally murdered the two workers and put their bodies in the freezer while he was working up there.)

Anyway, be safe out there!

[Uniballer]

Originally Posted by Hondaman

And I presume the power was off when it blew. What could cause such a time delay before the explosion?

I would assume the power was on. There are plenty of things an HVAC guy may want to do to a rooftop compressor with the power on (e.g. measuring refrigerant pressures).

[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.