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# Would a microwave capacitor be a good choice for a coilgun design? Answered

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If neither is good then can you recommend some that would be ideal?  I already know hat disposable camera capacitors can be good.

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## 2 Replies

the mechanical engineer (author)2011-02-14

Theres really only one way to find out: try it yourself. If it doesn't work then just replace the microwave caps with photoflash caps and you'll be just fine. i've seen working coilguns with microwave caps, but im not sure how well it will work. a lot depends on the ratings. but really, coilguns are very simple and it's easy to interchange parts, so if it dosent work the only thing your out of is 5 minutes of work and a little bit of solder.

Jack A Lopez (author)2011-02-10

There are a couple of reasons why the capacitor from a microwave oven is NOT a good choice for a coilgun capacitor bank, and these are: (1) this capacitor stores less energy than a typical disposable camera capacitor, and (2) the capacitor in a microwave oven is typically manufactured with an internal resistor, for safety purposes, to safely discharge it in some number of seconds.  Note that this resistor does not interfere with the operation of the capacitor in the oven's voltage doubler circuit, because the time it needs to hold a charge in that case is much shorter, i.e. for a half cycle of the power line frequency, 1/100, or 1/120th of a second.

But how do I know this?  Well, when I was little, in physics class, they taught us some simple and useful formulas regarding capacitors.

Energy stored in a charged capacitor is

U = (1/2)*C*V2
http://en.wikipedia.org/wiki/Capacitor#Energy_storage

For example, a typical capacitor in a camera-flash circuit has a capacitance of 300 uF, and is charged to a voltage of about 300V.  Thus the amount of energy stored in this charged capacitor is ;

U = (1/2)*(300*10-6 F)*(300 V)2 = 13.5 J

By comparison the amount of energy stored in a typical microwave oven capacitor is about 1 uF at 2000 V or so, which is:

U = (1/2)*(1*10-6 F)*(2000 V)2 = 2.0 J

For a decent sized capacitor bank you're probably going to want energy storage in the range from 10 to 1000 joules.

Regarding the question of what voltage you want to charge the capacitor bank to, I would guess something in the range from 300 to 1000 volts.  That guess is based mostly on typical values I have seen looking at other people's coilgun projects on the web.

If you want an answer for "what voltage" that is less anectdotal, and  more grounded in the basic physics, consider that making the capacitor bank larger is actually making it slower,
http://en.wikipedia.org/wiki/RC_circuit
http://en.wikipedia.org/wiki/RLC_circuit
and you want large peak power which means you want your capacitors to discharge quickly, and to make that happen you choose large V, rather than large C,  in the equation (1/2)*C*V2, but there are probably practical limits to how large you can make V.  E.g. if it was millions of volts, it would probably just discharge itself through the air surrounding the capacitor.