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An ambitious atempt

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Rob11095

Sat Oct 15 2011, 10:31PM

Registered Member #4140 Joined: Sat Oct 15 2011, 05:37PM
Location:
Posts: 3

Well, it actually isn't very ambitious for what I've seen on here before, but for me and my friends, its pretty insane. One day at lunch we all decided "hey lets build a coilgun" 3 weeks of procrastination later, here i am. Most of us are fairly new at any sort of amateur electrical engineering projects, but we have somewhat of an understanding of the science behind it(many thanks to barry and his site). We are still in the planning stages, but i figure i might as well share what we are planning on doing. I have my eye set on 4 400v 3900uf caps that will be our bank. I also bought a 2 ft piece of plumbing conduit for a barrel, with inner diameter at about 16mm, and an outer diameter of 22mm. I also bought the projectile, which is a 4 inch long piece of... well, im not sure, but it is magnetic and was the closest thing i found that nearly matched the diameter of the barrel. Now for the questions.

1) with the power source, i was thinking of some sort of 12 volt battery, any suggestions? I'm also aware i need to step up the voltage to that of the caps, which is 400. for this i was looking into a 12v to 120v step up converter, and a 4 stage voltage multiplier. Now, is there any way i can buy these cheaply, or do we have to make them our selves? And if there is an easier way to do this, please tell me.

2)for the coil itself, well, im dumbfounded, sorta. How long should the coil be? I've read mine should be about 6 inches (3/2 ratio with projectile) also, what gauge magnet wire should i use with my power? i saw 500' of 22 on amazon for like 18$, a good deal, or would i be wasting my money? And how many turns should i have? is it trial and error, or is there some sort of equation i can use to get the perfect amount of layers.

3) scr... what? could someone explain to me in layman's terms how to hook one up? i know what it does, just not understanding how to wire it.

4)Is external iron really as simple as it sounds? put the coil in an iron pipe and BAM it is slightly more efficient.

We'll definitely have more questions as we progress on our daunting task, but if you guys can explain these concept questions to me it would be awesome.

P.S. Its my first time posting, be gentle :)

P.S.S.

here is everything we have planned out so far, not much and it may seem VERY basic (and messy) but its what we got so far.

Turkey9

Sat Oct 15 2011, 11:20PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

For the projectile, I'd cut it down to 2 in. If the projectile is too long, it seems that the force from the coil goes way down. This isn't from any theory, just what I've observed through experimentation.

For your power supply, are you against using outlet power? If not, use a voltage doubler on the outlet voltage and that will give you about 330V DC. You don't want to use 400V because that is the maximum rating of your capacitors. If you go above that, there will be a risk you'll damage them. 330V will give you plenty of power for your first coilgun.

For the coil, I would recommend 14 awg solid core wire. This wire is the same stuff that is used in household wiring. It can be found at any hardware store. It has pretty thick insulation but will not really matter for your first gun. It's cheap and will handle a lot of current. To start off with, make your coil 3/2 projectile length (3 inches) and give it eight layers. Eight layers may be too many but that is a good thing as you can just remove turns when you start tuning.

SCRs are super easy. Hook one end of your coil to the positive terminal of the capacitors, the other end to the anode of the SCR. Then hook the cathode of the SCR to the negative terminal of the capacitor bank. To trigger the coil, connect the negative terminal of a 9V battery to the negative terminal of your capacitors and the positive terminal in series with a 1K ohm resistor to the gate of the SCR. The coil will fire when the circuit is completed. If you want to shoot the gun more than once, you'll also have to add a diode to the coil to protect the SCR and capacitors. Connect a diode rated at 600V and around 600A in anti-parallel with your coil. This means that the cathode goes to the positive of the capacitor bank and the anode goes to the anode of the SCR.

External iron will indeed increase the efficiency. Cut the pipe to the length of the coil for best results.

Rob11095

Fri Oct 21 2011, 11:48PM

Registered Member #4140 Joined: Sat Oct 15 2011, 05:37PM
Location:
Posts: 3

Thank you for all the advice. the week has been hectic with all my schoolwork, so i haven't made much more headway with the project. Regarding your suggestion to using outlet power, we were looking to make the gun portable, so i don't think we could use outlet power. The a stationary gun takes away all of the coolness factor of our gun, and who would want that? Also, the capacitors i found were sort of a scam, so we've decided to change to 6-8 1800uf 450v caps for $13.50 a piece. A problem i encountered is making our voltage multiplier. i'm not sure what our diodes ad capacitors should be rated at to create it. Also, where does everyone go to get their diodes, capacitors, scrs and other components they need. And is 14 awg really OK? would going for a smaller wire really be a bad decision or does it not matter. And does the pipe have to fit perfectly around the coil for external iron or can there be some space in between?

rp181

Sat Oct 22 2011, 04:14AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Buy some of these from the forum:

Thomas W

Sat Oct 22 2011, 10:43AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

i can sell you some capacitors

-inverter grade
-450v 3300uf
-400v 4700uf
both low ESR
madce by Itelcond

Inducktion

Sat Oct 22 2011, 05:29PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

As far as I know, one of the best ways to make a coilgun portable is to get a battery pack of some kind. Lithium Ion's are the most lightweight, and contain the most energy of rechargeables I know. They're rather finicky, though, and don't like to be discharged past a certain voltage, and charged past a certain voltage.

Lead Acid batteries are considerably heavier, (well, they have lead in them after all) but they are the easiest battery to charge.

NiCd's and NiMH batteries are the hardest to charge because they're a weird way to tell how they're done. BUT, they are the most rugged battery type from my understanding. They don't mind being completely discharged, at all. Plus, they maintain the same voltage for a long time (as opposed to other batteries, whose voltage slowly goes down under load....for Nickel type batteries, it barely changes)

Then once you have your battery pack decided, you would need to put together some sort of voltage booster.

Boost converters are rather popular, but they're somewhat inefficient, and waste a bit of heat. But they are relatively easy to put together.

ZVS drivers - People use the famous mazelli flyback driver, and wind their own transformers on flyback cores. ZVS drivers are extremely efficient, and waste very little heat, and generally charge a lot faster than boost converters. (only issue is winding the transformer, which can take quite a bit of time)

Dr. ISOTOP

Sat Oct 22 2011, 05:48PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Inducktion wrote ...

Boost converters are rather popular, but they're somewhat inefficient, and waste a bit of heat. But they are relatively easy to put together.

A proper boost converter can be very efficient, though it requires the use of a high-voltage battery back.

earth3

Tue Oct 25 2011, 04:52PM

Registered Member #3721 Joined: Sun Feb 27 2011, 04:43AM
Location:
Posts: 40

you shouldnt use an AC inverter and a cockroft walton multiplier to charge caps just use a boost converter or uzzors 50W charger

earth3

Tue Oct 25 2011, 04:53PM

Registered Member #3721 Joined: Sun Feb 27 2011, 04:43AM
Location:
Posts: 40

what do you mean by external iron anyway?

Inducktion

Tue Oct 25 2011, 06:15PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

earth3 wrote ...

what do you mean by external iron anyway?

It's some strips of iron on the outside of the coil that helps focus the magnetic field inwards towards the projectile, making more energy actually go into the projectile.

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