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6 Farad Coilgun Answered

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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Patrik
Patrik

13 years ago

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...

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adrewmc
adrewmc

Reply 13 years ago

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

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Patrik
Patrik

Reply 13 years ago

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.

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Kiteman
Kiteman

Reply 13 years ago

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.

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Patrik
Patrik

Reply 13 years ago

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.

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adrewmc
adrewmc

Reply 13 years ago

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

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adrewmc
adrewmc

Reply 13 years ago

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.

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westfw
westfw

Reply 13 years ago

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?

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guyfrom7up
guyfrom7up

13 years ago

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

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adrewmc
adrewmc

Reply 13 years ago

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

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guyfrom7up
guyfrom7up

Reply 13 years ago

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

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guyfrom7up
guyfrom7up

13 years ago

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

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westfw
westfw

13 years ago

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...)

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adrewmc
adrewmc

Reply 13 years ago

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