SpudFiles

Well i was thinking, MRI machines are just huge@ss magnets,
and my friend just built a gauss gun, we got some big caps...
I wannna Make an uber-electromagnetic-propulsion potato cannon

wadda ya think?
oh and this link might give you some ideas:coilgun.ru
yeah that tank thing up top...

Potatoes aren't made of a ferrous material, last time I checked.

Sorry for my post, I was in a pissed off mood due to the fact that my air cannon failed. Anywho, sorry Vov, best of luck to you.

wow SNM way to be a smart @ss. I believe what he meant was in the style of a potato gun. anyway vov what were you thinking?

Yes MRI machines do have a very powerful magnetic field, theres some interesting pictures on the internet of what happens if you have a metal chair/gas cylinder/anything metal at all, in the same room as one.

Even so, they're not really suitable for coilguns and at the price they are theres certainly better options

Big caps, a jumbo SCR and some good coils are pretty much the starting point of any powerful gauss gun. Do you have any plans to go on at the moment?

But, then again, it is very hard to get much energy out of a coilgun, full stop.
Short of using a diagonal halfbridge configuration, IGBTs, digital timing, at least 1kV, multiple stages and external iron to increase efficency above a pitiful percent point or two, power will be lacking.
Not to mention you'll need a Cockroft-walton voltage multipler running from the mains, ZVS driver running a suitable transformer, or a boost convertor just to charge the thing moderately fast.

Creating a powerful coilgun is not easy. However, if you get it right, you can create accelerative forces unknown to conventional spudgunners. However, the low efficency will always come back to stab you in the back.
And modern capacitor banks have fairly low energy density - About 0.5 J/cm^3.

Combine that with an average efficency of no more than about 5%, and to get a muzzle energy the same as a good spudgun, you need a capacitor bank the same size as a typical battle tank, and you'll need to throw a lot of cash at it.

Amazing. Rag's finally started to actually post here

Despite all the low efficiency worries, I recall you mentioning you were thinking of knocking up a decent one yourself sometime. Is it going anywhere off the thought-board yet?

Hotwired wrote:Despite all the low efficiency worries, I recall you mentioning you were thinking of knocking up a decent one yourself sometime. Is it going anywhere off the thought-board yet?

It's still on the thought board. My electronics are a bit shabby, and given the price of some key components in the design, I can't afford to spend the money until I'm mostly sure it will work.

It would be helpful if the circuit sim program I used actually wanted to accept one of my circuit diagrams as being a working design.

At the right here and now, the design that's top of the drawing board is a combustion BBMG.

umm...
MRImachine = sarcasm, anyway, i got about 1125 volts worth of caps and i have a camera circuit which charges them in under the hour it takes with a nine volt...
i was wondering as to how i hsould wire the capacitor
s for the best results...
i have a basic plan of one coil, a camera charging circuit and all my capacitors, i dont like to draw plans out

First, figure out how much energy you are likely to get into a projectile. A well designed coil gun might be 1% efficient.

The total energy in your cap(s) is E=(1/2)cv². Where energy (E) is in Joules, capacitance (c) is in Farads and voltage (v) is in volts.

If you put the caps in series then the overall voltage adds but the capacitance goes as
(1/C_total) = (1/C₁) + (1/C₂) + (1/C₃) ...
I doubt that a camera flash circuit will charge anything to more than about 350 volts.

If you put the caps in parallel then the maximum voltage is the minimum of the power supplies output voltage and the smallest voltage rating of the caps. The capacitance is the sum of the capacitance of all the caps.
C_total = C₁ + C₂ + C₃ ...

Once you've figured out the total energy in your caps then multiply that by 0.01 (1% efficience) to get a rough estimate of how much energy you might get into a projectile.

If you know the mass of your projectiles then the velocity (V) will be about;
V = sqrt(E/mass)

or, doing it all together
Velocity = sqrt((1/2)cv²/mass)
Velocity = v*sqrt((1/2)c/mass)
where velocity (V) is in m/s.

Ask yourself if it is worth the effort. There is a good chance that you can throw the projecitle with more energy than your coil gun will.


Usually, unmatched caps do not work well in a coil gun. You need caps of the same rating and type, perferably from the same manufacturer.

i see with teh caps, well ill just jack a few more 200v 330uf ones from my friend, lol he has a bagfull
and ummm.... coilgun = silent

vov35 wrote:i see with teh caps, well ill just jack a few more 200v 330uf ones from my friend, lol he has a bagfull
and ummm.... coilgun = silent

So is throwing the projectile with your hand. And you can throw it harder with your hand the the coilgun can fire it.

But go for it. Coilguns are fun and you'll learn alot.

Couldn't you use an inductor coil with a very high turn ratio in the secondary coil? That should be able to induce massive voltages, and therefore, a strong magnetic field. Simply charge a good capacitor, and you'd have more voltage than you'd know what to do with. Just make sure your recent diet wasn't high in iron, lol.

paaiyan wrote:Couldn't you use an inductor coil with a very high turn ratio in the secondary coil? That should be able to induce massive voltages, and therefore, a strong magnetic field. Simply charge a good capacitor, and you'd have more voltage than you'd know what to do with. Just make sure your recent diet wasn't high in iron, lol.

Nope. The magentic field generated by a coil (the launch coil) is proportional to the time rate of change of current (dI/dt). To get maximum magnetic field you want a large change in current in the shortest amount of time. Using a step-up transformer increases the voltage but decreases the current (the total power stays the same when using a transformer if you neglect parasitic losses).

The reason people use fairly high voltage sources on coilguns is because the current is related to the voltage and the coils resistance and inductance. The higher the voltage you put into the coil the more current will flow through the coil.

But a step-up transformer swaps current for voltage. So you can step 100V up to 10KV but you'll only have 1/100 the current. If you put the stepped up voltage into the coil there is too little current available to get the job done.

The nice thing about high capacitance / high voltage cap's is that they will deliver both high current and voltage at the same time. Even a pretty basic coil gun setup can deliver 300 volts at several hundred amps, but only for a fraction of a second.

If the caps could put that out continously you would be talking about 100,000 watts or so. 100,000 watts is roughly the maximum amount of power that four houses are allowed to draw from the electrical grid.

So then just wire up a couple dozen 100V caps in parallel maybe?

EDIT: Hey, I wonder if you could make a capacitor bank, try to make it compact, then put it in some sort of case with two leads. You could maybe modify a cell phone charger or something to charge it up. You could make a bunch of them and just hook up a new one for each shot.

In a perfect world, yes, but caps tend to leak over time. A single photoflash cap might only retain a good voltage for a few minutes, depending on the quality.
At least, the ones I'm using leak fairly fast.