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Njubster

Sat Apr 20 2013, 07:26PM

Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40

I was bored yesterday, so I knocked together an adjustable capacitor charger that can fill the capacitor with 200V-500V (

). I want to make a coilgun with it, but I'm not sure about few things...

The coil - how do I wind it? Can I just randomly make turns, or should they be carefully arranged like windings on the flyback's secondary? And how many turns and layers should suffice? And what should the wire diameter be? And should it be wound around a metal tube, or plastic tube is good alone?

My cap is 100u 400V, extracted from an old TV, will that work? I know people make coilguns with camera flash capacitor, and this is not too different. I would buy a big one, but the prices here are ridiculously high.

Also, if i remember correctly, when inductor is placed across a charged capacitor, it makes a damped oscillation untill everything is dissipated. That means that the positive voltage will come to the negative end of the cap, and electrolytes do not like that, right?

Sigurthr

Sat Apr 20 2013, 09:05PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

You would use a diode of sufficient current and voltage rating between the anode of the electrolytic and the inductor to prevent polarity reversal being applied to the cap. An anti-parallel diode across the cap is never a bad idea either.

I've never made a coilgun but I have dumped a few joules from caps in to coils/wires to watch them explode.

Njubster

Mon Apr 22 2013, 02:02PM

Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40

Destroying caps is fun, and I love the smell. I love this machine. Thanks, makes sense...

Anyone have any tips about the coil?

Barry

Tue Apr 23 2013, 03:50AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Njubster wrote ...

The coil - how do I wind it? Can I just randomly make turns, or should they be carefully arranged like windings on the flyback's secondary? And how many turns and layers should suffice? And what should the wire diameter be? And should it be wound around a metal tube, or plastic tube is good alone?

Almost any kind of coil winding method will work, as long as all the turns go on in the same direction. Random is fine; the magnetic field comes from "amp turns" and doesn't care much about the shape of the turns.

That said, a neatly wound coil will take a little less wire and therefore have a bit lower resistance. It will also have a lower inductance which is usually helpful, too.

Generally speaking, coils tend to work better with heavier wire (lower resistance) and a small number of layers, maybe two or three layers at most. Use whatever insulated wire you can get. Magnet wire has a advantage in its thin insulation allows more turns to be packed more tightly. But again, use whatcha got, even hookup wire or speaker wire.

Tube should be non-conductive to avoid eddy currents. A thin-walled plastic tube is great; a drinking straw is a reasonable choice. Don't use a glass tube; the coil will produce forces that can break glass during firing.

The coil length should be about the same length as the projectile, for best efficiency. So, this usually means you choose what you want to shoot first, then design the firing tube and coil around it.

Njubster wrote ...

My cap is 100u 400V, extracted from an old TV, will that work?

You should compare capacitors by calculating its stored potential energy.
PE = 1/2 * C * V^2
In your case, PE = (1/2) (0.000 100) (400^2) = 8 joules at full charge

Imho, a small starter coilgun should plan to have 10 to 100 joules of stored energy. You can get fun results in this range. Although 8 joules can be made to do something, it will be frustrating to have nothing happen until things are tuned up just so.

If you can, get two of those old TV capacitors, preferably three or four.

Njubster wrote ...

Also, if i remember correctly, when inductor is placed across a charged capacitor, it makes a damped oscillation untill everything is dissipated. That means that the positive voltage will come to the negative end of the cap, and electrolytes do not like that, right?

True, exactly so. It will shorten the life of an electrolytic capacitor if it receives a reverse charge like this. Now, the real question is, how much shorter, and do you care.

Personally, I don't have protection diodes on any of my coilguns. If their life has been shortened, I haven't noticed it yet. But then I also don't charge them to maximum rating. If your caps are cheap, maybe you don't care if they eventually fail. Just be aware that the failure mode may include getting hot, expanding and leaking acid. So keep them in some kind of protective cover. And in the meantime, have fun!

Cheers, Barry
Success is stumbling from failure to failure with no loss of enthusiasm. - Winston Churchill

Njubster

Wed Apr 24 2013, 03:49PM

Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40

Wow, thanks a lot! That is very thorough!

I'll be getting some PC power supplies soon so I'll take the caps out of them.

I'm gonna be boring for a bit more...

Switching - if I was to use 10-20 joules and not more, could I use a TV power transistor for a switching device? Would it die soon?

I never released how important that part is till I got there, bah...

Maxwell

Wed Apr 24 2013, 06:21PM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74

It's not necessarily the total energy stored that is the issue - it is the current. You could have a gigajoule of energy released over an hour and never blow the transistor (exaggeration) - 277Watts.

However, 100 joules released in 0.01 seconds would be 10,000Watts.

Transistors typically aren't the answer. You'll have much better luck with a Cheap SCR. IF you go with a small SCR I personally recommend a protection diode (@Yandersen claims that location of the SCR doesn't matter but I found that my small SCR's blew in the back-current when placed in front of the coil).

Njubster

Sat Apr 27 2013, 06:28PM

Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40

oh... I won't do it with transistor then, thanks :) I guess I'm equipped with all the knowledge I need to build my weapon now lol :)

Thanks everyone! :D

Will bump when I get stuck somewhere probably though.

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