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Registered Member #1005
Joined: Sat Sept 15 2007, 09:55PM
Location:
Posts: 1
For a school project that I'm finishing up this weekend, a couple of my friends and I decided to build a coilgun. We ended up basing it around a 76000 MFD 15 VDC capacitor I found in my garage, and it worked to some extent (it averaged about 0.22% efficiency but at the same time we barely knew what we were doing)
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Higher voltages are often found in induction type coilguns where you want the fastest pulse and largest current. Several thousand volt capacitor banks are not uncommon with these, and when you get that high all you need is a spark gap to trigger so less cost/complexity on the switch (well depending on a few things).
Registered Member #1327
Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38
I'm still tying to learn the math involved. I built my first coilgun with 3 27000 mfd caps @ 14.8 volts. I found that each time I added a cap the projectile went a little further. Still with only 3 caps each @ 27000 mfd @ 14.8 vdc my projective only traveled a few inches. I assume the more mfds, the more power, thus high voltage = more power with less mfds. Again, I am still trying to learn the math and a lot more. D.L
Thanks to a formula found on Barry Website, "W = V2 * C / 2" "W = energy in joules (watt-seconds) V = voltage in volts C = capacitance in farads" and Google is my best friend.
Registered Member #1208
Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
in general, people use more voltage to increase the energy as opposed to capacity. this shortens the pulse to the duration where the pulse is gone by the time the projectile is in the center of the coil.
assume: .1 ohm total resistance, 10v, 1 farad( 50 joules) 1KW for .05 seconds
.1 ohm total resistance, 10000uf 100v (50 joules)
100KW output for .0005 seconds
1000v 100uf (still 50 joules) .1 ohm =>
10 MW for 0.000005 seconds.
the faster the pulse the faster the projectile speed before suckback
but if you go overboard on voltage, your projectile saturates and any extra magnetic energy goes toward keeping your projectile from moving. try the gausspistol.com calculator, and keep max flux density under 3 teslas...
Registered Member #1386
Joined: Tue Mar 11 2008, 08:16AM
Location:
Posts: 13
The electrical reason to include with ramses good input is resistance. You can only push a finite amount of current through a resistor. You need that high voltge to overcame the DC winding resistance of the coil to get that large current spike regarless of capacitance and projectiles. If the average hobby coilgun had coils down into an amazing .1 ohm winding resistance at 25 degrees C using the same coil geometry and wire length of a 0.5 ohm coil, then 66V would accomplish the same current pulsing work as 330v.
Yeah, you can use low voltage but remember power in is power out minus loss. Same reason why cars use above 1.5 litters of displacement. I bet a .1 litter car would be very efficient but would it cary is own weight plus passanger upto the rated highway miles per hour? No.
Registered Member #1208
Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
also keep in mind that my calculation COMPLETELY ignored inductance, which is usualy a huge factor in the pulse width. and .1 ohms seems massive, i don't see how you could consider it an extreme low. my multimeter's self resistance is ~.3ohms, so i can't measure accurately way down there.
and the reason i changed capacitance 1- to keep energy constant and 2- is because i can find 1 farad, 12v caps, but would anyone like to sell me a 1 farad, 1kv cap? 1 MJ would be sweet...
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low voltage coil guns?
