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  • If I discharged my caps on thick bronze rails connected to the caps by copper wires and nothing else, would that be dangerous? I'd like to safely discharge the caps as fast as possible for the highest amount of current I can get.
  • What's the best method to use multiple capacitors in a way to maximize the current safely while minimizing power loss?

Bonus Question: I said bronze, but I need the highest amperage possible while being sturdy. What would be a good material? Could copper be better?

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    \$\begingroup\$ You could always try a current shunt made of pure silver: It has (I believe) the highest conductivity among metals. Copper comes a bit lower, and lower still is gold. "Sturdy", on the other hand, is a bit undefined. \$\endgroup\$ – Anindo Ghosh May 18 '13 at 16:15
  • \$\begingroup\$ When you discharge capacitors, the energy stored in them has to go somewhere. In this case I would expect it to be converted to heat. You may need to make sure that the heat can be 1. removed from places where it is likely to cause damage (e.g. electrolyte) and 2. dissipated adequately to avoid setting fire to things. \$\endgroup\$ – RedGrittyBrick May 18 '13 at 16:47
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    \$\begingroup\$ youtube.com/watch?v=EoWMF3VkI6U \$\endgroup\$ – RedGrittyBrick May 18 '13 at 17:10
  • \$\begingroup\$ As fast as possible? Are you trying to build one of those can-crusher devices? If so, the metal doesn't matter so much as the cross section area. Copper tubing works well. And if it's fast discharge times you're after, be aware that electrolytics are rather sluggish compared to other capacitor types. \$\endgroup\$ – JustJeff May 18 '13 at 17:43
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    \$\begingroup\$ "...would that be dangerous?" Um. You're making a railgun. On purpose. Would that be dangerous? Um, yeah. That would be dangerous. Don't do it. \$\endgroup\$ – HikeOnPast May 19 '13 at 3:03
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You're going to run into two dangers doing this.

1) The spark. Depending on the voltage on your caps, there could be a very large spark when you suddenly short the terminals. That will need to be contained. Repeated applications could lead to damage of the terminals or the discharge bars. I'd recommend using a contactor to make the actual connection. Depending on the voltages and currents, that could get pricy.

2) The current. Again depending on the voltage, the ESR of the cap, and the resistance of the discharge path, you could be talking about kiloamps of current. It's a very short pulse, which thick wire and bar and good connections should be able to handle, but the instantaneous thermals might damage the capacitor. You should consult the datasheet, or else the manufacturer, to find out the instantaneous pulse current the cap can supply. My guess is that most aluminum capacitors wouldn't support that kind of behavior for large numbers of repetitions. And if they failed after a few shots, it might be pretty sudden.

As for maximizing the current and mimimizing the power loss, you'll want low ESR caps, lots of them, in parallel. Without specs it's impossible to make more concrete recommendations.

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    \$\begingroup\$ 3) exploding-wire phenomenon. If you try to discharge a 20uF 10KV capacitor bank with some #10 gauge wire, you'll get a face full of copper shrapnel. \$\endgroup\$ – wbeaty Mar 16 '14 at 4:20
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Copper is usually good. You'll be wiring your capacitors in parallel for effective discharge. The limiting factor is the equivalent series resistance of the capacitors, which will be shown as ESR in the datasheets. The datasheet might also describe how fast they can safely discharge.

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  • \$\begingroup\$ The datasheet will show a "ripple current" which is the highest continuous current the capacitor can take without internal heating destroying it, assuming certain parameters. For a single discharge with a long cooling period, it would be very hard to blow a capacitor that's not charged beyond its rating. The ESR will be the limiting factor in how much current you will get out of it. \$\endgroup\$ – Jon Watte Jun 17 '13 at 18:42
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Trying to destroy your capacitors?

It's not possible to give a good answer without knowing what kind of capacitors: half-inch filter caps, or 300LB laser caps?

High-volt caps have a limited lifetime for direct-discharge (shorting.) Even the ones designed for this (defib, laser, or "energy storage" caps) can be progressivly ruined, then finally fail open. Direct shorting can also trigger a discharge inside the dielectric layer.

So, if you're building a capacitor-discharge rig, DON'T waste your capacitor lifetime by connecting with a shorting bar, use a resistor bank and a HV relay.

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