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coil gun s

hay guys, im thinking of making a coil gun, and im just woundering What capacitors are best?

the coil gun is just meant to be small not that powerfull.

also this is what im thinking for the setup - battery's power a dc/dc converter which, charge capacitor banks which is then dumped in to the coils ( a piaxe controls the timing of the coils )etc .

an comments are appreciated 



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If you want it as cheap as possible and still be good if done properly, get disposable photo cameras, the one I made shot the "bullet" 20 meters, and broke beer bottles :) only one coil to! so no MCUs or anything used, just a switch...

thanks for your input guys,

@ amando, really 20 meters thats impressive :)

Use some kind of solid state control system, an SCR, IGBT, MOSFET, something that isnt a mechanical switch. I have problems with the contact melting, debounce, etc. You'll want a system that'll control charging, firing, and ensure that you have safeguards in place in case, such as bleeder resistors, and heavier gauge wiring. 24 gauge wire isnt going to like a massive surge of power running through it. Also use plenty of diodes to prevent power from surging back into the coil.

Power supply can be the hardest thing to deal with at times. Since you are going for low power, it is common to use camera flash circuits paralleled together. The next hardest part can be the firing coil. It is difficult to find the right number of windings, size, etc. Just buy lots of wire (18 gauge for smaller coilguns, 24 is too small) and determine how big the projectile to coil size is. More coils doesnt always mean better, as resistance can get in the way. Too many coils, too much resistance.

i was planing on useing  2  capacitor banks, one primary ( for the first coil ) and one secondary ( for second smaller coil ) .

i was thinking (for primary capacitor bank)  6 or 8 x 25v 2200uf electrolytic low ECR caps  and 4 for the secondary bank

( a dc/dc converter would charge these )

for firing control i was thinking a picaxe 14 or 18 , with a line sensor hooked to the barrel between coils so i can tell when to turn on the second coil. and just using the picaxe to control a mosfet to switch on coils,

i would also use picaxe to control battery monitoring and a servo to reload , etc 

Don't build a low voltage coilgun, they have issues with discharging fast enough. Higher voltage=better performance in the long run. You will thank me later when your coil takes such a long time to discharge. You wouldn't need a DCDC converter to charge 25V caps, just A SLA battery or a lot of AAA batteries. 25V capacitors is a very low rating anyways. It is common to see coilguns operating in the 300V+ range. 450VDC is common. The formula for calculating Electrical Joules is 0.5*C*V^2. Therefore it is more reasonable to increase Voltage rather than capacitance for more energy. If you've never built a coilgun, start with a basic flash circuit coil gun for experimentation. Go to your local walgreens, rite aid, etc and ask for the camera flash Circuits. Parallel several of them together (power input and power output) and that should be a decent enough power supply until you become more acquainted in electronics, as the more power you deal with, the greater harm you can cause on yourself and others...

Another bit of advice when dealing with voltages over 450V (dielectric breakdown) always treat the system like it could be on. Never touch exposed bits without any doubt that it could be energized, such as an capacitor buss rail or wire. If you have to bump or tune something, always one hand behind your back, that way you dont complete the circuit with both hands and several hundred volts runs through your heart, possibly causing DEATH. I've been shocked 3 times in my entire life, and I was following the one hand rule all 3 times. 25KV at 30mA is very dangerous, but because I was shocked with only one hand, it had no reason to complete the circuit by traveling through my heart. 120VAC is very unpleasant, threw me across the room and etc. ONE HAND RULE.

Go for at least twice the voltage rating that what your power supply can output on your caps. Use the highest quality wire you can find and make sure your coil is wound correctly. Install protection diodes. Use a thyristor (SCR) rather than a relay as your firing mechanism. They are faster, more durable, safer, can be triggered by lower voltages than a relay. Start with a single coil gun because timing is a nightmare!