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In the answer to another question, I read that it's bad to short-circuit electrolytic capacitors. Why is that? What happens if I do?

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    \$\begingroup\$ To be specific, it's only bad to short--circuit charged electrolytic capacitors. \$\endgroup\$ – Connor Wolf Apr 18 '11 at 19:05
  • \$\begingroup\$ @Fake: obviously.. \$\endgroup\$ – Federico Russo Apr 11 '12 at 17:43
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Electrolytic capacitors may become permanently damaged by excessive peak currents, which will definitely occur during short-circuit events. The reason is that (a) the internal resistance will cause a momentary, but large power dissipation (heat!) and (b) the distribution of the current spike inside the capacitor will not be formed evenly across the large area of the aluminum foil and hot spots may occur. The electrolyte may vaporize along these small zones and damage to the insulating aluminum oxide layer may occur as well.

If you're lucky, the capacitance will decrease just a bit or the top of the can may change its shape into something like a dome. If you're very unlucky, the cap may fail and heat up quite a bit (and eventually blow).

With very large currents, e.g. during inrush events into the primary caps of switching power supplies, you can actually feel how the caps heat up in an unhealthy way (don't touch live circuits or charged caps!!!). Such inrush events may be viewed as the opposite of a short circuit condition, just that the current flows in the opposite direction ("into the cap").

BTW: Also true for other capacitors like ceramics. I've seen ceramics explode, too, when they were subjected to rapid discharge events. The ceramic dielectric material changes its shape just a tiny bit when the electric field varies. If this happens fast, enough force may be created for the capacitor to blow. Disc capacitors will withstand some abuse, ceramic multilayer capacitors (MLCCs) are quite sensitive.

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  • \$\begingroup\$ even discharge rates that are less than short-circuit can be hard on a capacitor, if they happen frequently enough. learned this lesson by replacing the capacitor in a strobe light with an ordinary electrolytic - it got REALLY hot. \$\endgroup\$ – JustJeff Apr 18 '11 at 22:20
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    \$\begingroup\$ OT, but I just wanted to mention that discharging lead-acid batteries with a short circuit is even worse... Don't test car batteries with a crow bar! \$\endgroup\$ – Toybuilder Apr 21 '11 at 0:18
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    \$\begingroup\$ Another way to put things is that if a cap holds a certain amount of energy, shorting the cap through a resistance will cause all of the energy in the cap to get converted to heat somewhere. The ratio of the cap's internal resistance to the external shorting resistance will determine the fraction of heat dissipated inside the cap. Electrolytics have enough internal resistance that the ratio can be very high. \$\endgroup\$ – supercat Jan 29 '14 at 21:21

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