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Capacitor Shorting over aluminum.

  1. Apr 6, 2012 #1
    I had a 4700uF-35v capacitor that I connected in parallel with about 18v. I would short it over some aluminum foil and being that it was connected in parallel, each slight disconnection with the foil would result in a recharge. Probably not worth mentioning, I definitely saw some sparks. The foil would actually deteriorate (holes would be made.)

    My question is, what exactly is happening to the aluminum? And what "are" these sparks that are being seen?

    Also, do I need to take safety precautions when doing this (other than the needed precaution for the possibility that the capacitor may explode if not handled properly)?
  2. jcsd
  3. Apr 6, 2012 #2
    First, stop what you are doing. It is potentially dangerous.

    By shorting the capacitor you are dumping the accumulated energy very rapidly, resulting in potentially large currents. The sparks are electric arcs eroding holes in the thin foil.

    What are you trying to accomplish?
  4. Apr 6, 2012 #3
    Nothing specific that I am trying to accomplish. Just general experimenting. I figure that large currents are resulting do to the very low internal resistance of a capacitor. That along with the low resistance of the foil. Though that being said, are these large currents capable of being driven though me considering I have much more resistance than foil? I know I am safe in touching the wires of a capacitor as the potential is low, but in terms of an already high current through foil, I am not sure.

    Care to elaborate how these large currents can be dangerous at a low potential?

  5. Apr 8, 2012 #4


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    Staff: Mentor

    "Sparkling" white sparks are hot aluminium particles burning up in oxygen. Blue sparks are the result of ionized air (ionized nitrogen and oxygen atoms releasing energy after absorbing it from the electrical discharge).

    High discharge currents can overheat the capacitor; if the moisture boils the can will rupture, the foils short circuit, and all remaining energy in the capacitor will erupt in one giant bang. Protective eyewear is mandatory.
  6. Apr 8, 2012 #5
    Thanks. I figured the blue were from ionized air, but I wasn't sure about the white sparkling.

    Ah, yeah. I guess that makes sense considering there is some resistance within the capacitor and I am sure the charge discharge frequency is high when sparking in parallel with the battery. Either I should limit the time that this goes on, or remove the battery resulting in only single discharges.

    I have had eyewear on the whole time. Not that it would necessarily help for an explosion, but I have also been covering my face with my shirt. Keeps sparks away from my face, anyway.

    So do I need not worry about currents considering the low voltage? Of course other than the fact given by the high current that touching the foil could result in a burn by direct heat transfer.
  7. Apr 8, 2012 #6
    When I said, "potentially", I meant that your haphazard manner of experimenting often leads to scaling up to "see what happens", which can lead to bad things.
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