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Charged capacitor + piece of aluminum = very large flash

  1. Aug 1, 2012 #1
    I have heard of a project where the military hooks up many large capacitors (with short discharge time coefficients) to a bar of aluminum and the end result is an extremely bright explosion.
    Could someone expand on exactly what is happening here?
     
  2. jcsd
  3. Aug 1, 2012 #2

    Averagesupernova

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    I hadn't heard of any specific stories about the military but what you describe sounds like charging up a series of capacitors and discharging them in one quick event. In school we had 100,000 uF defib capacitors that we would charge up to about 400 volts and discharge with alligator leads. Sounded about like a 12 gauge shotgun.
     
    Last edited: Aug 1, 2012
  4. Aug 1, 2012 #3
    If you crowbar a big bank of capacitors, bad things usually happen whether you use aluminum bar, steel bar or gold bar. As the charge in the caps get discharged in matter sub nano second. Can you imagine the amount of current it pass through the bar.

    Just use a small cap of 1uF charge to 1V. There is 1X10EE-6 coulomb. Say it discharge in 1nS (1X`10EE-9).

    [tex]Q=It\;\Rightarrow\; I=\frac Q t= 1000A[/tex]

    Of cause any resistance will limit the current flow and takes longer to discharge. But if you charge to say 100V with a bank of capacitor of 10,000uF............You get the drift?

    Bad things usually happen when passing that kind of current if you are not careful. In that short duration of discharge time, there is no time for heat to conduct away, tremendous hot spot create and vaporize anything with slightest resistance.

    I worked in the other extreme. I worked with high voltage. We used Kilovac HV relay for switching the HV lines. We always use precaution to power down the voltage before switching, but sometimes an arc somewhere cause the relay to chatter and it burn the relay very often. We make every precaution to limit the current. But just discharging the capacitance of the coax cable cause the fuse of the relay contact.

    Even you say that's only 200pF in the coax. But if you charge to 5KV, discharge in 1nS. Use the formula I gave, you'll find it's still a lot of amps going through. We had to switch to relays with tungsten contacts. Still it can only take so many hits before it fail also.

    Not all capacitor can produce tremendous current. I was a photographer for a while, I have those really powerful flash units that produce big flashes. They have to use special capacitors, I think they are the low loss or something. Usually they are short and stubby. They are similar to those used in switching power supply. Those can produce more current when discharged as they have lower parasitic resistance.
     
    Last edited: Aug 1, 2012
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