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600j coilgun

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LAN_403

guitardude012

Wed Oct 22 2008, 09:58PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

IGBT's(usually negative thermal coeff) don't generally share current as well as mosfets and SCRs(positive thermal coeff.)

The coil sounds length doesn't seem right. You will probably want at least twice that length. I try to stay somewhere in the length = 5xdiameter range.

El_Roberto

Wed Oct 22 2008, 11:12PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Ok thanks ill try and get the calculator working so i can look at various coil lengths (your 5x1.4=7cm seems good), i want the igbt's for a higher efficiency 10-18 stages 1kj coilgun. It would use 18 250v 60j capacitors and i would aim for about 100amps per coil therefore i could switch it easier using igbt's in parralell (5 per coil means 100 igbts for 18stage= $300 :0, i think ill look for higher power cheaper ones...) I would use a microchip to switch the igbt's off when the projectile reached the centre of each coil using an optical sensor. The backemf would be routed into more caps to recharge the whole system

guitardude012

Thu Oct 23 2008, 12:56AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

If you are only switching 60J per coil you can probably get away with a beefy to-220 igbt or something like that..... it would be much cheaper. Keep in mind that you will probably want to fire the cg at least 10% under the capacitor's rated max voltage most of the time.

I have used IGBT driving optoisolators to protect the microcontroller (though i was driving SCR banks with them). You may start planning on something like that if you are going to use a microcontroller.

El_Roberto

Tue Oct 28 2008, 07:37AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Well Ive succesfully made a test Coilgun using one of my 100j capacitors;
http://i224.photobucket.com/albums/dd239/bob538/IMG_0110.jpg
The coil sucks though, I cant find a store that sells copper wire in any gauge bigger than 16awg (I want 12awg). Also I need a bigger diode because after successfully firing 2-3 shots at 180v (using reccomended 90% max charge of cap) I decided to try one at 200v, this fired it across my room into a wall (around 5m, from a height of 4cm) and the diode exploded! My dad (who gave it to me) said it was good for 100amps and my electornics teacher said that the back emf wouldnt be that powerful (guess he was wrong!). So does anyone know where i might find a supplier of copper wire in New Zealand?

aonomus

Tue Oct 28 2008, 03:15PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

From the picture (which the link is mostly broken,

) I can see that you are using fairly thin copper wire. For my 400J gun I used 18AWG, and made a much larger coil with a larger internal diameter. More windings = higher inductance = lower current pulse for longer duration, and less chance of things going explodey.

Another problem is the whole thing being cobbled together with alligator clips, you are likely introducing one heck of a bottleneck around the alligator clips....

There is a strong emphasis on doing the inductor simulation followed by the LCR simulation to see how the current pulse behaves, and what kind of back EMF you get.

Backyard Skunkworks

Tue Oct 28 2008, 06:33PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

As far as the diode(s), I'd reccomend huge bundles of 1N4007 diodes; they are quite cheap general purpose high voltage diodes each rated for 30A peak. I used a bundle of 10 for my 70J pistol, and I'll be using a bundle of around 50 for my current CG carbine project. You should also have the diodes in series with a very low resistance resistor (few ohms), this helps keep the peak current through your diodes lower.

You also may want to look into a bigger projectile. When I first starting playing with CGs last year, I had almost the exact same setup and the results were dismal, horrible suckback and generally weak shots. After I upgraded to 9x50mm 20g iron rods, the power jumped significantly.

guitardude012

Tue Oct 28 2008, 08:22PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

I agree with Backyard Skunkworkds, you will get better results with a larger projectile diameter. Also, either the coil is too short or the projectile is too long; I would decrease the projectile length.

El_Roberto

Wed Oct 29 2008, 02:38AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Yes I talked to my electronics teacher today and he also said these exact same things. I am going to make a better coil but for now that is all the wire i have and i dont think it would be a good idea to rewind it because ive allready used it for allot of coils so it is getting a bit brittle. As soon as i can find some 12 gauge wire im going to make a coil that will be about 7cm long and (i think, cant remember exactly what i worked out) 113 turns with a 9mm projectile.

EDIT: Well I managed to get some 18awg wire from one of the younger electronics pupils in my class and made a better coil, 264 turns (66 per layer 4 layers, around 220uH) and I bought 5m of 9mm id plastic tube. For a projectile im using a bolt I shaped into a bullet. With two capacitors (at 180v parralell for 191J) I can shoot through a corrugated cardboard box. Im giving up on the idea of a single coil massive energy coilgun and going straight to a 20 stage coilgun with 55j per coil for about 1.1kj total. I will make a new Thread once more parts arrive, and include pics etc.

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