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I recently came across 4 1.5Farad capacitors and made them into a capacitor bank, but I just want to get a general idea what would happen if I made a 6 farad coilgun...would it go through walls or anything?

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Internal resistance may play an important role as well. R times C determines the time constant for charging and - more importantly - discharging the caps. If it takes a few seconds to discharge the energy stored in the caps, it won't do you all that much good for a coil gun...

I believe the ESR is .002 Ohms...does that help?

That sounds reasonable. So your time constant would be on the order of 1.5F x .002 Ohm = 3 milliseconds.

At 20V, the energy content of one cap would be 1/2*C*V2 = 300J - not bad!

This site claims a single 12J capacitor should be enough for a simple coil gun. So yes, in theory you should be able to use these caps to build a coil gun. However, most coil guns are built using lower capacitance, higher voltage caps (e.g. flash caps from disposable cameras), so it's unlikely you'll be able to use an existing design.

With the higher capacitance, you'll get a longer pulse to discharge through the coil, but a higher peak current, so at the very least that will affect the design of your coil. Intuitively, I would guess you'll get a design with a longer coil (longer time pulse) made out of thicker wire (higher amperage), suitable for accelerating a large mass (higher energy content), but at a lower speed (again, longer time pulse).

The link above has some really good information, plus additional links in case you want to design your own coil gun using those caps.

As a guide to the sort of damage 300J can do, most steel toe-caps in work boots are designed to protect from 200J impacts.

Heh - interesting factoid!

As another point of comparison - muzzle energy for real bullets range from 500J (.45 ACP or 9mm Parabellum) to 1000J for rifles (.22) and upwards. Here's a little list.

and your math was with one cap...if I did it multistage I could get pretty high J's

wow, thats awsome...I got a area thats abandoned for four miles in every direction for testing, some concrete walls, and a portable generator (just in case) lined up for when I final do build it.

6F at 10 to 20V could be interesting; worth experimenting with, anyway. You may run into inductance issues as well a ESR issues, but you won't know till you try.
So these are those big caps I sometimes see advertised for improving something in fancy car audio systems?

make sure youo use diodes, you wouldn't want the voltage feeding back and recharging it backwards!

any links for diodes that can handle the voltages in play?

for you voltage isn't a problem, amps is, I'd ask someone else, I'm mediocre at semiconductors

generally you want higher voltage at lower farads because there's less internal resistance and you can pack more power in a shorter blast.

What voltage? The energy content is what matters, and that's 1/2*C*V2
That means 6F@5V is "only" 75J, while if you had 300V (typical photoflash voltage) and "only" 2000 MICRO Farads, you'd have 90J. (The latter is pretty lethal, electrically, while the former might have problems pushing current through your coil...)

hmm...well the specs I can get of the packaging that have to deal with voltage are as follows: VDC: 20 Max 24v surge ESR: 0.002 Ohm or less