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Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
You might be neglecting the coil's inductance, which will probably be a large amount for your coil. It takes about 140 meters of wire, if you have 11 ohms of 24 awg. Anyway you build it, this will have a lot of turns and high inductance, probably 30 mH or more.
Try the RLC simulator page and dial in V = 50 volts, R = 11 ohms, C = 1.4 millifarads, and L = 30 mH. This will simulate what happens in your circuit. The inductance slows down the discharge time and lowers the peak current. I'm seeing a peak of 3.5 A that lasts about 50 msec.
Coilgun efficiency suffers from any resistance. This means the best coilguns are built from fewer turns of heavy wire, just one or two layers, and high peak current of hundreds or thousands of amps.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
ahh thanks. That's very helpful. Also, do I have to take into account the fact that (according to This chart a 24 AWG wire can only pass 3.5 A of current?
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Yanom wrote ...
Do I have to take into account the fact that (according to This chart a 24 AWG wire can only pass 3.5 A of current?
Not exactly. The chart represents the save average long-term safe current-carrying capacity.
You can at least double their figures for current that lasts only a very short time.
The real limiting factor for coilguns (with a current pulse of 20 msec or less) would be the "fuse" limit. That is, what amount of current for a few milliseconds will melt the wire? I don't have a chart handy for that, so do some web searches.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Barry wrote ...
Yanom wrote ...
Do I have to take into account the fact that (according to This chart a 24 AWG wire can only pass 3.5 A of current?
Not exactly. The chart represents the save average long-term safe current-carrying capacity.
You can at least double their figures for current that lasts only a very short time.
The real limiting factor for coilguns (with a current pulse of 20 msec or less) would be the "fuse" limit. That is, what amount of current for a few milliseconds will melt the wire? I don't have a chart handy for that, so do some web searches.
Cheers, Barry
i dont have the formulas memorized, but its just a function of the energy in the discharge and the mass of the wire. the limit is the melting point of the insulation if you want more than one go at it. i did something similar with induction heating.
edit: the specific heat of copper is .38gJ/g which means for every joule of energy asserted on one gram of copper, its temperature rises 1 degree C. an example would be 1kJ through 10g of copper wire would raise the copper 10 degrees. according to wikipedia, common temperatur classsifications for copper wire are: 105° C, 130° C, 155° C, 180° C and 220° C. find out what yours is, them calculate how hot you want it to get.
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is my math right?
