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Re: Best way to blow up capacitors - constant current source?

F

Fritz Schlunder

Jan 1, 1970
0
Al Borowski said:
G'day,

For a statistics class, I have to preform some kind of experiment and
analyize the result - this is an extremely open ended project. I'm
thinking about analyising capacitor failure on overvoltage conditions
(EG getting a heap of caps of different values, voltage ratings, and
seeing what voltage they blow up at).

I have an 80 volt power supply lying around, and I'd like to create a
circuit to continually ramp up the voltage accross the capacitor until
it fails (pref. with a large BANG!). I'm thinking about creating a
simple constant current source (say 1 ma or so) out of the 80 volt
supply. I'd then see how long it takes for a capacitor to explode. I
should then be able to work out the voltage when the cap failed.

My question is... Will a capacitor explode if only a small charging
current is used? Or will the cap only fail shorted/open circuit, but
with no visible failure?


The answer will of course depend on exactly what type of capacitors you plan
to destroy, but for all normal capacitor types I'm familiar with what you
propose simply will not work. In order for them to explode violently, they
must very rapidly develop high internal pressure. That pressure is normally
provided through heating and vaporization of internal material. Since
normal capacitors aren't very flammable (a limited exception perhaps for
Tanatalum electrolytics), all of the energy required to produce the heat and
rapid internal pressure build up must be provided electrically from your
power source. In your proposed scenario you plan to use an 80V (max
compliance) 1mA constant current supply, which works out to be at the very
most 80 milliwatts of juice. 80 milliwatts isn't enough energy to make
anything get hot (even an eighth watt resistor would handle this with ease),
much less make anything explode violently.

The key to making capacitors explode (usually people use aluminum
electrolytics) is to rapidly apply a substantial overvoltage condition
(especially in the opposite polarity for polarized capacitors) from a very
stiff voltage source powersupply. If the voltage source is too low, or not
stiff enough, the capacitor will merely get hot and eventually slowly vent
or slightly rupture releasing the built up pressure with no catastrophic
explosion.

Something to realize about aluminum electrolytic capacitors in general is
that they behave quite similarly to crappy zener diodes when you abuse them
with improper voltages. If you apply more voltage than rated they
eventually just start getting "leaky." That is the leakage current through
them starts increasing substantially at some value above the rated voltage.
If you apply reverse voltage to the electrolytic it will get substantially
leaky at a reduced voltage (quite possibly less than the rated forward
voltage). Assuming that you prevent the capacitor from getting very hot and
building up any substantial internal pressure the capacitor will not be
particularly harmed by your voltage abuse (at least on a short term basis).
 
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