# Capacitors and capacitance

this time I'd like to ask a few questions about capacitors, all noob questions again;

• What's a better idea, having one capacitor at 5x farads or five capacitors at x farads each? I want to get a certain amount of voltage and want to discharge the direct current as fast as possible.
• I'm trying to build an amateur railgun, and intend to shoot projectiles that weigh about 10 grams. Using the formula that someone helped me with in my other question, in order to give that a speed of 100m/s at a total of 20.000 uF assuming 2% efficiency I'd need a little less than 112V's. The question is, is this amount of voltage I need my capacitor or series of capacitors to provide for the duration of the shooting?
• What determines how much voltage a capacitor can hold? This may come off stupid as I am aware that there is a voltage rating when you buy them, but better safe than sorry.
• I was only assuming 2% efficiency, and depending on my rail length it could be higher. The question is, what determines how fast the capacitor will discharge and how it can be prevented from going above the rated voltage?
• If you are asking these questions, I would question the safety of doing this project. – Brian Carlton May 15 '13 at 19:23
• I'm trying to get started, why is that a bad thing? Thank you for your concern though. – lychnus May 15 '13 at 19:25
• a good way to learn is to blow things up, just don't do it stupidly. – placeholder May 15 '13 at 19:43
• @pjc50 suggested your 16,000uF bank of caps would need to be charged to 176V and be capable of discharging a current of 280A in order to get the energy. Then he quite rightly mentioned that perhaps only 20% of the stored charge would be useful and I suspect this is because of laws of diminishing returns - as you discharge the cap the current rapidly and progressively gets smaller due to the terminal voltage dropping. In short you may be looking at purchasing 250V rated electrolytic caps with a total capacity of over 50,000uF. I think this should be your starting point. – Andy aka May 15 '13 at 19:59
• I've seen this kmestore.com/470uF-250V-Electrolytic-Capacitor - it's a 470uF 250V cap and it costs \$10 - you'll need over one-hundred in parallel. Then you'll need to find something that can handle a 1000A switching that will connect this large bank of caps to the rails. Just one ohm impedance at say 200V will limit the current to 200A so you'll need something that is significantly less than 0.5ohm and preferably 0.1ohm - I don't know where you'd get one from. – Andy aka May 15 '13 at 20:05