# How to go about building a Capacitor Bank for rail gun?

Hi,
For my physics 2 class our group has decided to build a rail gun. I went to my local Walgreen's and asked for used disposable cameras and the guy was glad to get rid of 20 and give them to me. I researched what "general" specifications for flash capacitors was and it was 330V and holds 270 uF. Before i go about building this bank I have a few questions.

1) From what I understand, you can add capacitors in series to increase the potential difference. ie. for 2 330V capacitors hooked up in series would be capable of 660V and would hold  135 uF (let me know if my math is wrong). My question is that I've heard to be careful and not exceed a capacitor's voltage rating. Is this considered "overloading" the capacitors voltage rating since the bank will be be trying to achieve a max of 660V?

2) Assuming that (1) will not overload the capacitors, what voltage is best to operate a rail gun at, Ive heard that "high-voltage" is best, and Ive seen most around 400V. It seems pointless to hook up 2 capacitors in series to gain this 70V of potential difference, would it be useful in trying to achieve a greater voltages (ie. 500 or 600)? and would I even want to get to 500 or 600 Volts.

3) My goal is to eventually get a semi-large bank with these capacitors having all 20 of them used in the bank. Ive thought of two configurations, one only being possible if (1) will not overload the capacitors.
1) (660V) to have 2 sets of 10 capacitors. Each individual set will be hooked up in parallel acting as a single capacitor, and then in series with each other. The outcome bank (correct me if I'm wrong) would be 660V and 1350uF.
2) (330V) to have All 20 capacitors hooked up in parallel. The outcome bank would be (again, correct me if I'm wrong) 330V and 5400uF.
Which would be more optimal? I will NOT go strait into building the full bank, will probably try 6 first and work my way up as I get more comfortable.
3) I'm not sure if all will be rated exactly the same. If some were rated at slightly different voltage ratings and capacitance ratings does this affect my configuration?
4) I was going to use a voltmeter to measure the Potential Difference when charging the capacitors, what is the best way to go about charging a bank like this? Ive heard that by using a singe camera circuit you can achieve this, but at slow rates... what would be a good (preferably cheap) way of going about this?
5) I've heard of capacitors sometimes discharging at the switch. How do I prevent this?
6) (MOST IMPORTANT) How do I safely discharge a bank? What important safety precautions should I take when building this.

5 years ago
Adding in series increases potential but reduces capacitance by 'inverse of the sum of the inverses'...so you're right

so if its 270uf caps at 300v
2 will be 600v
but capacitance will be
1/(1/270 + 1/270) = 1/0.0074074074074 = 135uF

...so its a direct tradeoff, usefulness for total storage;
I'd say stick with 'put them all in parallel with the largest bus wires you can find' and have the largest 350v bank you can build.

3) that rating is just 'when it will blow up' -- don't exceed the voltage.
4) get a 400v dc source and you can charge them. A camera circuit can do it - I've run camera chargers in parallel and it worked also.
5) there isn't enough insulation. Get a beefier switch (you're running a LOT of power for a very short time. You need a larger/better insulated switch.
6) Beefy resistors. The bigger the resistor, the more heat it can dissipate. Light bulbs are good for this (resistors designed to get hot)...just make sure you put multiple in series if your voltage is too high.
Always assume the bank is charged
Never touch it
Ever.
Don't think its funny to shock people with it. You can kill them.
Sparks are cool. Eyes are cool. Sparks in eyes are NOT cool. Get safety glasses.
Epond89 (author)  frollard5 years ago
awesome man, thanks for your help!

P.S. would you recommend building a coil-gun or a rail-gun? would one be more effective with this bank size? thanks!
5 years ago
Before you get going on it, take a look at what's already out there. The Navy has done a lot of research on it for ships weaponry and also future aircraft launchers for carriers (instead of steam)

I know we had some guys at the Naval Academy try and make a rail gun for a senior project. Do some blogosphere searching.

It's been a while since I took Physics, but check your math and formula approach for a second, taking a step back....Don't rail guns function based off of current and not Voltage?

http://en.wikipedia.org/wiki/Railgun

http://en.wikipedia.org/wiki/Coilgun

You need a lot of Amps to generate a strong magnetic field to launch a projectile (ie think more lead-acid golf cart or car batteries and less cameras) The Navy needs nuclear power to generate this kind of energy...

Capacitors are cool for generating a spark or a shock, but when it comes to making stuff move, it's the amps that kill.

Also, after you verify the formulas, begin thinking about materials and sturdiness. (The wires will attempt to separate from each other magnetically)

I'd think rail-gun would be far simpler than the coil approach, materially.

Perhaps a bit of copper pipe split in half or quartered lengthwise could be the two "rails", with a penny attached to a cork for a bullet maybe?

Maybe something as small as a staple for better velocity?

As a side note, don't get to upset if you find your rail gun doesn't have enough "ummph" (ie only pops a projectile a few inches). Gotta play it save with electricity.

Best wishes and have fun

-_______
+ ______ pew! ~ o

5 years ago
coil