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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
I tried googling around but didn't find any evidence of big electrolytic capacitor explosion when used with coilgun or similar.
It could happen like this: Some guy has large electrolytic capacitors, lets say 20x 3900µF / 450VDC in parallel (78000µF / 7.8kJ) He overcharges them or doesn't have protective diode to kill negative currents -> one capacitor has already minor damage and gets stressed faster than others. Then @ full voltage that damaged capacitor fails completely and gets internal short circuit.. and all remaining energy from 19 capacitors will make damaged one explode violently.
But how violently, lets imagine that our unlucky (more like stupid..) guy is standing near by the bank measuring charge voltage, would he get injured?
Im going to make experiment with this subject, i have few dead capacitors (3300µF/350V) and 9.2kJ capacitor bank made out of same kind of caps. My plan is to get human analog (hopefully big piece of pig - mythbusters style) and i will place it next to capacitor and see what happens when over 9kJ is applied to that capacitor. Reason why i do this is that i want build my big bank as safe as possible, for operator and for other capacitors. If one fails, i want to minimize the damage to capacitors near by. To make it possible i have to know the failure mechanism when large amounts of energy is present.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
7.8 kJ is close to what you'd get out of a firecracker containing about one gram of flash powder. The old "M-80" firecrackers in the U.S. (which have been illegal for many years) had about 3 grams of powder, IIRC, so your hypothetical capacitor explosion would be about 1/3 as powerful as an "M-80" firecracker explosion. I wouldn't want to be there when it happened, but unless you got hit by shrapnel in the wrong spot and bled to death or something, I think you'd survive.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Whilst a wet electrolytic capacitor may suffer no permanent injury if actual scintillation occurs, a dry electrolytic capacitor may be permanently short-circuited under similar conditions.
When potentials, in excess of the formation voltage of the anodic film, are applied to a dry electrolytic capacitor, there is not a uniform increase in leakage current concentration over the surface of the anode foil. In fact the leakage current tends to concentrate at certain minute areas, resulting in the generation of hydrogen and oxygen, by electrolysis, at the cathode and anode foils respectively, and if the separator medium is not impermeable to the passage of these gases, small pockets of the highly explosive mixture of hydrogen and oxygen will be formed.
The increased current concentration is of such magnitude that a material rise in temperature occurs at the point of current concentration which in turn lowers the work function for emission from the ions of the electrolyte still further until the dielectric actually ruptures and a spark occurs. This spark ignites the small pocket of the explosive mixture of gases and a miniature explosion occurs. As a general rule, this explosion is of sufficient force to disrupt either the anode or cathode foils and form small crater-like holes, the projections of which are forced through the separator material causing actual electrical and mechanical contact between anode and cathode foils.
If sufficient current is available, an arc may form at the point of the short, resulting in the production of super-heated steam and catastrophic over-pressure inside the capacitor case.
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Capacitor explosion - How lethal it really is?
