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Comment by bestouff | Hacker News Reader

They say in the intro that capacitors don't explode. My past time as EE student tends to prove otherwise.

Usually a cap blows it's top if you either exceed it's voltage rating or it's polarized and the voltage on the pins is the wrong polarity.

Outside these two conditions, the only other thing is age, other than that, they tend to not explode.

Or it was made with a stolen formula....

Tantal capacitors can actually start burning

source: https://www.vishay.com/docs/49268/tn0003.pdf other source: at $WORK we are having some problems with igniting tantal caps

Tantalum capacitors break to form a short while essentially any other option breaks and becomes an open circuit.

A short dumps a pile of current across the capacitor. an open circuit usually just means your circuit doesn't work well.

Don't use tantalum without a careful plan to deal with this issue... why are you forced to use them instead of a modern electrolytic?

I am in the lucky position to develop the firmware for the device / not being the electronics guy

The reason is a mismanaged project. Power budget on a CR2032 is tight and initially a supercap with much bigger capacity was planned. The company producing those went out of business and people were forced to somehow get it to market, working. We did, but now those new tantal cap problems arrived.

That used to be a good source of income. Buy up old analog synths, categorically replace all the caps and they'd usually work again.

I got this great memory from electronics class in high school, we were measuring some circuit which included an electrolytic capacitor, using an oscilloscope. All of a sudden I hear a panicked voice from the back of the class saying "it's growing, it's growing... turn it off... turn it off!" and my other classmate replying "but it _is_ off!" which was immediately followed by a very loud bang.

I turn around and see my two class mates looking shocked and confused, covered in fine fur all over their upper bodies, along with the desk.

Never got around to understanding just how they managed it, but they most certainly did manage to blow up that capacitor in a spectacular way.

The fun is that capacitors work even when power is off, so if they had several caps together and one was the wrong way round or the power supply was off but not disconnected OR the capacitor was already charged beyond a point of no return, it'll blow.

While they look fairly harmless, one should also never forget that capacitors are the most dangerous part of any electric circuit; they don't become safe if power is off and they have no limit to their current other than "what the wire can do", which is usually northwards of "the wire evaporates".

I've experienced various caps burn their legs and solderpoints in less than a millisecond without warning once a short was realized. And on top of that, the huge currents running through wires not intended for that will usually cause quite a bit of inductive backlash in nearby components.

I've worked with capacitors that were designed to dump their charge in a few us. You really do not want to mess with that stuff without proper precautions, nothing scares me more than a couple of hundred volts power source with an Ri too low to measure. You need to forcibly restrain your cabling or it will jump all over the place during a discharge.

Heh... Recalling a demonstration by an EE prof way back when. He had the bank of capacitors charging up on his desk, and was writing on the blackboard the equations of how much energy was stored. Hand raised in the back of the room. "Uh, Dr. Miagawa, I think you slipped a decimal point there." Prof looks over his work again, says "Oh, dear" unplugs the charger and backs away. He tapes two meter sticks together with a screwdriver on the end to discharge his little electrobomb. WHAMO, screwdriver vaporizes.

I'm pretty much convinced this was not a mistake, it was a "Be really CAREFUL with this stuff!" object lesson.

And of course you can harness this all to shrink coins [1].

[1]: https://www.youtube.com/watch?v=tCh8UoLY-ow

So, aside from that, howe was the theatre Mrs Lincoln?

and then there's the large caps on my AC compressor that tend to go out every few years.

Those dry out. They're there to help a tri-phase motor run on single phase power and that's massively abusive to the caps. They tend to get hot enough that the electrolyte slowly evaporates in spite of a lot of effort to seal them. If you look carefully you'll find a little 'x' somewhere in the plastic bottom, that's an overpressure relief. If it is distended outward that cap is about to die.

You can blow them quite nicely if you manage to create a magnetic field inside the capacitor that exceeds the strength of the beaker it is contained in. That's a lot of bang for your buck, please don't try this at home. The good thing is most capacitors have an internal resistance and inductance high enough that this isn't a problem. But with capacitors that are made for very fast discharge (such as the ones used in old school xenon flash equipment for studios) you may be in for a surprise.

Purposefully weakening the container can bring this sort of behavior out at lower levels.

this is definitely something I want to try at home

5x the rated voltage the wrong way across the terminals does a good job.

Start small. 1000 uf and work your way up from there. Stay away from anything that looks like it is mechanically reinforced to make the housing stronger for mounting purposes or mechanical stability. Those are bombs. Don't come crying ;)

To be useful in cars, they also need to not explode when physically damaged.

You mean like gas tanks?

Capacitors generally release all of their energy all at once if you short them, which is very bad.

Gasoline is flammable, but it needs the right fuel air mixture to actually explode. Car fires are relatively common, but as far as I know car explosions are not.

So wait ... we need something that can rapidly absorb power, and rapidly disperse power. And the criticism is that this can explode ? An explosion is simply rapidly dumping energy in something that's a gas or about to become a gas. Not that other forms of rapidly dumping power are any less damaging, btw.

So, yes, they can explode. That's actually pretty much the point.

Be reasonable. Correctly designed power devices are made to control the rate. Its considered a no-no to go out of control.

One of the primary arguments that I hear for them is how much safer they are than Lithium Ion batteries. Being able to explode (even if this is actually unlikely) greatly weakens that purported advantage.

(I do not believe this will actually be a primary impediment to adoption of either Lithium Ion or Super Capacitors)

That definition of “explosion” also includes correctly functioning rockets. Are you sure it’s a useful definition?

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