Electronic – 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?

Best Answer

  1. Having a bunch of caps in parallel will add their capacitance but also puts all their internal resistances in parallel, giving you less overall internal resistance and more current dumping capability.
  2. Capacitors don't hold voltage, they hold charge. You can take a set amount of charge and discharge it in a certain time period. You need to rethink of your problem in terms of energy you need and discharge rate of that energy.
  3. Caps have a maximum rated voltage above which they'll be damaged. However caps store charge, not just a certain "voltage". They hold energy. 20 uF capacitor can be charged with a 3V rail, or a 6V rail or any rail up to it's maximum rated value. You put a certain amount of charge into it and get a certain amount of charge out of it. The useful thing here is that caps don't like the voltage across them to change quickly. They exhibit inertia when it comes to voltage whereas inductors exhibit inertia when it comes to current. When you discharge a cap you are draining power out of it very quickly. You are dumping out a lot of "energy"/"power". Since P = IV will always be true and caps don't like sudden change of voltage across them they will instead dump a lot of current -- i.e. show sudden change in current to keep the V part of that equation dropping smoothly (or following a discharge curve).
  4. A cap's current capability is determined by it's internal resistance + resistance of the load.