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A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 20, 2007 3:29 am

I doubt a 9 volt battery is going to generate the sort of power needed in this case.
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Unread postAuthor: Ragnarok » Thu Dec 20, 2007 8:41 am

No, Alkaline or Zinc Carbon PP3 batteries (a correct use of the word battery, as 9V batteries are made from a chain of 1.5V cells) are useless for higher power applications.

You want at a higher current solution like a high capacity NiMH, NiCd, NiFe (hmm... Nickel seems to be popular for rechargeables) or Lead Acid battery pack, to at least about 12V, preferably 24V.

That should put out the required power.
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Unread postAuthor: jon_89 » Wed Dec 26, 2007 9:27 pm

I have also been looking at the idea of a coilgun? Anybody built one?
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 26, 2007 10:09 pm

jon_89 wrote:Anybody built one?


yes.
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Unread postAuthor: jon_89 » Wed Dec 26, 2007 10:37 pm

Thanks Jack for the info.jk I have already looked at the ones on this site and others, but how powerful are they? Can you have multiple stages just by using several camera flash circuits? For example 3 stages with 3 charge circuits and 6 capacitors per circuit. I was reading about someone using a relay to help with power and effiency is this true?
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Unread postAuthor: Ragnarok » Thu Dec 27, 2007 10:27 am

jon_89 wrote:I have already looked at the ones on this site and others, but how powerful are they?

If you are looking at a well made coilgun, think more in terms of typical air rifle power than average spudgun power because of their limited size, but they are very good at putting a colossally high force onto a small projectile - consider that an air rifle will use thousands of psi.
For each cubic inch of barrel volume, a coilgun can more power than just about any spudgun made today. The problem is that they have low efficiencies (at low velocity), and to get much energy into a projectile you would need a pretty large capacitor bank, which is impractical.
However, there is one coilgun I can think of which has a muzzle energy closing in onto .22 LR, although it uses a high calibre to do that - don't expect to build that with out a lot of research and experience.

Can you have multiple stages just by using several camera flash circuits? For example 3 stages with 3 charge circuits and 6 capacitors per circuit.

Yes, but you shouldn't. If you're getting into multiple stages, you need to really be handling things with proper resources, not bits stolen from disposable cameras.

I was reading about someone using a relay to help with power and effiency is this true?

Sort of. A relay is better than a manual switch, but if you really want to do it properly, you need SCRs, IGBTs or triggered spark gaps. IGBTs are the best choice because of the way they switch, but they have quite low peak current capacity for their cost.
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Unread postAuthor: jimmy101 » Thu Dec 27, 2007 12:46 pm

Like Jack said, a single 9V will probably be to wimpy for decent performance.

Several 9V's in parallel would be better.

Usually, for a disk flinger (like a penny or a CD) you gear up the wheels so they are spinning as fast or faster than the motor. It will take a while for the motor to get the wheels up to speed but that's OK. The energy for the launch comes mostly from the rotational energy stored in the launch wheels and not directly from the power being generated by the motor during actual the launch sequence. Basically, you use fairly massive wheels and store energy in their spinning. To get this to work both wheels must be powered otherwise ~half the energy ends up going into accelerating the unpowered wheel.

Often, two wheel flingers for disk shaped ammo will have one wheel spining a bit faster than the other. This gives the projectile spin which greatly increases its stability in flight, kind of like a frisbee.

Disk flingers that use a throwing arm are also usually designed to give the ammo some spin when it is released from the arm.
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Unread postAuthor: jon_89 » Thu Dec 27, 2007 12:59 pm

is a long coil or a short fat coil better?
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Unread postAuthor: rp181 » Thu Dec 27, 2007 1:07 pm

It all depend on your projectile, and the type of wire, it is best to expierement. The good thing about coil guns is that the projectile shoots down the center of the barrel, so there is no friction resistance.

look at 4hv.org, in the electromagnetic projectile accelerators, there's some good ones, the old archives have more. To charge your caps, you will probably want a boost converter, for that look at anothercoilgun site. It's 555 based.

I think multi stage would get you the most power, with optical triggering, pretty much a nightlight circuit hooked up to a SCR.
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Unread postAuthor: jon_89 » Thu Dec 27, 2007 1:49 pm

What do you think is the biggest capacitor that can be charged with a camera flash circuit? I am not going for all out power here, but I would like to achieve alot by just using camera flash.
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Unread postAuthor: rp181 » Thu Dec 27, 2007 2:28 pm

When i did mine, i got 3 250v 100uf caps in parrallel, it charged in ~ 10 sec. If you insist on sing camera flash, get a couple and put them in parrallel, ule get more current, wich means faster charging. With that setup, i got a bb to go about 20ft, its not that great. It would be worth it o go with boost converter and decent caps. Boost converters use solid state components, camera flash uses a transformer, so its not that powerfull. Mine gave out 220V on a fresh battery, it drained it fast too. Boost converter can give up to 500 volts easy. Google buck boost converter.
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Unread postAuthor: jon_89 » Thu Dec 27, 2007 8:02 pm

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Unread postAuthor: POLAND_SPUD » Thu Dec 27, 2007 11:36 pm

using steam seems like a good idea. GGDT shows that you can get about 30-40% increase in power when using steam... maybe it's worth trying you know - it gives much better performance than air and high pressure can be produced using just a source of heat.... of course this would need completelly new technology, for example sealing materials for temperatures above 150 C or insulating material (so that it won't burn you)..i mean different than what is used now in spudguns
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Unread postAuthor: Ragnarok » Fri Dec 28, 2007 7:20 pm

The optimal coil size depends on many variables.

In a good coil gun, the coils are as long as the projectile, no more, no less.
An important thing is that spheres are very poor coilgun projectiles. If you want some proper power, you need a cylindrical projectile - but most people will round or point the forward end and sacrifice some power for better air resistance.

It is important to realise that camera flash coilguns will never be impressive unless you have an boringly long charge time, and are unlikely to go over 1 joule of kinetic energy.

A camera charger will put a weedy 1 or 2 watts of power out into the capacitors, which will result in long charge times on larger capacitor banks, which ARE needed to get any respectable power.

If you build a charger, expect 20 watts from a low power one, and 100 watts from a really meaty one - that will cut charge times unbelievably. Those are the portable ones. Mains based chargers are easily over 1kW of output.

I'm going to be trying to make myself a coilgun that will be able to compete on muzzle energy with an air rifle sometime soon, but I don't expect more than a couple of dozen joules from it, and I'm going to be using more voltage than most hobbyist coilguns do to increase the field strength.
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Unread postAuthor: Hotwired » Fri Dec 28, 2007 8:02 pm

Rag likes the idea of potential :wink:

The reality is that no hobby coilgun has been made that even gets on the same map as a pneumatic air cannon.

The least hobby-ish hobby ones I've seen top out at a 3KJ coilgun using 948V punching through a drinks can and knocking over the plastic shield behind it.
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