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Registered Member #1465
Joined: Sun May 04 2008, 10:41AM
Location:
Posts: 3
Hello all, I have recently delved into the wonderful world of coil guns, as I'm sure many of you have also done. I built it as a single stage, with two 330v 80u F Capacitors hooked in parallel. It can shoot a piece of nail cut to proper length about 20-30 feet, and I am quite proud of myself :D .
Anywhom... Here's if the problem for which I require your assistance. I added an additional capacitor 330v 160uF in parallel with the other two. When I fire the gun with the additional capacitor the range is DRASTICALLY decreased. The projectile has perhaps only half of the velocity. This seems contradictory to what I expected. I have no Idea why this effect is perhaps some of you know the cause of this...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The problem is called 'suckback'...
You can find more about it in the wiki, but the main problem is that the capacitors are taking to long to discharge, and the projectile is getting sucked back into the coil as it is trying to leave.
You can probably fix it by removing turns from your coil, or possibly using bigger wire. What is the coil made of right now?
Registered Member #1465
Joined: Sun May 04 2008, 10:41AM
Location:
Posts: 3
The coil is made of 26 gauge magnet wire ( I think its 26) quite a few turns I would guestimate 300 or so perhaps more. It's very fine wire... Hard to do soldering on haha. Thank you "..." and electroholic for your responses I think I will construct a new coil made of thicker magnet wire. Cheers!
Registered Member #1208
Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
if you got the magnet wire at radio shack, it also included some 20 gauge, you should use that, and lay off the turns. I wouldn't put them in series if your power supply can't charge to 600v, because that will reduce your capacitance to 40uf, the capacitance of a single capacitor.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
The pulse width in the coil is the important part. That is, the time it takes the capacitor(s) to discharge should match the time it takes to move the nail from outside to fully inside the coil. Then the current must remain zero to allow the nail to coast out the other side. A too-long pulse width results in the dreaded suck-back effect.
How does pulse width depend on the capacitance C and inductance L? I'm glad I asked that! When resistance is low, the time is proportional to the LC time constant.
When you double the capacitance (by placing another in parallel) then you double the pulse width. This is certainly the cause of your slower performance.
To compensate, you could use a heavier nail that accelerates more slowly. Or you can remove turns to reduce the inductance by half. But wait ... don't remove half the turns!
Inductance is proportional to the square of the turns. To reduce the inductance by half, you must divide the existing number of turns by the square root of two. Or multiply by 0.707, same thing. For example to cut inductance in half:
If you have 100 turns now, remove 29.3 turns, leaving 70.7 turns.
If you have 300 turns now, remove 88 turns, leaving 212 turns.
Cheers, Barry This life is a test. It is only a test. If this were a real life, you would be told where to go and what to do.
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First coil gun... Capacitor issues.
