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I'm a beginner at electronics, and I'm wondering slightly...

I've bought electrolytic capacitors, and I'm attempting to charge them with a 9v battery. They seem to be running quite hot, too hot to touch.

Am I just overloading them with current? I didn't see any max current listed.

Also, I'm having trouble using the electrolytic capacitor afterwards. Does charge run from the positive or negative lead once I use it to, say, make LEDs shine?

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  • \$\begingroup\$ Can you give us some details as to the voltage rating of the capacitor you are testing with? Also, did you observe the capacitor's polarity (positive vs. negative) when connecting it to your 9V battery? \$\endgroup\$ – HikeOnPast Dec 18 '12 at 17:51
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    \$\begingroup\$ Charge runs between the poles when you connect them with a circuit, or leaks out slowly through parasitic internal paths. Don't expect to light an LED very long, though there are configurations where a few seconds are possible. Of course they should not be self-heating, nor is it good for them to be subject to substantial heat from other sources. \$\endgroup\$ – Chris Stratton Dec 18 '12 at 19:20
  • \$\begingroup\$ The current will go to zero shortly after connecting the capacitor to DC. You are not exceeding the current rating. The common way to exceed the current rating is high-frequency AC, such as a switchmode power supply. A 9V battery should be no problem. \$\endgroup\$ – markrages Dec 18 '12 at 20:06
  • \$\begingroup\$ Reverse biasing polarized capacitors is just the warm-up (literally) to explosions and fire. See eevblog.com/2009/11/04/… \$\endgroup\$ – JYelton Dec 18 '12 at 22:18
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Electrolytic capacitors should not get too hot otherwise they'll have a tendency to vaporize the electrolyte. This can lead to spectacular results such as the capacitor exploding. Some electrolytic capacitors have notches in their casing to create a controlled explosion, though any explosion will render the capacitor useless.

Most likely you've hooked the electrolytic capacitor in the wrong polarity. Electrolytic capacitors only function correctly when hooked up with the correct polarity (higher voltage on the positive lead). If hooked up backwards, the capacitor will act more like a short circuit and get hot.

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  • \$\begingroup\$ Hmm.. Okay. The short lead is the negative pole, right? Should this hook into the + or - of a battery? \$\endgroup\$ – DLA Dec 18 '12 at 18:44
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    \$\begingroup\$ It varies by type, manufacturer, and even date of manufacture. Post a picture and I (or someone else) could tell you. Make sure you get the full can in the photo and both sides of it (so that we can see all the markings). \$\endgroup\$ – DrFriedParts Dec 18 '12 at 19:16
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    \$\begingroup\$ @DLA - "Short Lead" ??? There are 1000's of styles of capacitors. If yours has a short lead how would we know which was the positive or negative lead? You need to check either the polarity marking on the capacitor or consult the device datasheet. Note that proper orientation of the capacitor is + lead to the more positive voltage and - lead to the lower level voltage. If commecting right across the battery put the cap + to the battery + and the cap - to the battery -. \$\endgroup\$ – Michael Karas Dec 18 '12 at 19:18
  • \$\begingroup\$ Most can-type electrolytic will have a column of negative signs on the wrapper. The bare-metal type with a surface mount base attached usually have it printed with other information on the top. \$\endgroup\$ – Chris Stratton Dec 18 '12 at 19:19
  • \$\begingroup\$ @DLA Generally the short lead is the negative side also, so that should be hooked to the - side of the battery. But as they have pointed out, that is not always true. \$\endgroup\$ – Kortuk Dec 18 '12 at 19:44
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In general, things get hot when current flows through them. A properly-connected capacitor shouldn't have current flow in a DC circuit, so it should not warm up. So as others have pointed out, your capacitor is most likely connected backwards, and you should disconnect it immediately. Lucky you didn't use a tantalum, that would probably have just exploded! The side with the stripe on it is the negative pole, which should connect to the - lead of a battery.

You may wish to do some more basic reading about electronics before getting too much further. You could burn yourself pretty unpleasantly.

As a point of general reference, it is possible for an electrolytic capacitor to heat up even during normal operation, if the capacitor is exposed to ripple currents. This is a situation where the capacitor is rapidly charged and discharged, either partially or completely. For example, on the output of a rectifier, or in a switching power supply. Electrolytic capacitors have ripple current specs for just this reason.

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