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Coil Gun

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daviddoria

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I have found several instructions on how to build these online.... however my question is why does everyone use all these capacitors and transistors and stuff? would it not work just as well plugging a big coil wrapped around a tube into a 120v AC?

for all of you who read my "burned my carpet" thread (haha), i am still looking for a big enough coil... looking back through my physics book... i found some stuff about the resistivity of copper and i remember it being the smaller the wire diameter the more resistive it is... so would I be better off with like 4 100ft. coils of 32 AWG or like 1 100ft. coil of 14AWG? Also, trying to figure out what kind of resistance i need for the coil... i didn't know what i wanted the current to be... how much current should there be lol?

David
 

Gene

New Member
David - I haven't seen the circuit you mentioned but I'll bet it is similar to the ones model railroad people build to operate the solonoids that move a section of track (turn-out?). Rather than simply apply voltage to the coil, they build a little circuit that causes the solonoid to react very quickly to a charge. Sort of 'super charged.' I'm sure there are some circuits on the web if you do a search in that area. More info? Let me know and I'll ask a model RR friend.
 

Dean Huster

Well-Known Member
Rail gun?

This sounds like what is more commonly known as a rail gun, an idea that was first fostered in the early sci-fi days for getting payloads off the moon and other low-gravity planetoids, where a long electromagnetic rail is used to propel the cargo similar to the way a catapult works, except that it's electrically driven vs. steam driven. (That last sentence was 'way too long.) In essence, a rail gun is a SERIES of coils that are energized in progression, pulling the cargo along the rail at progressively higher speeds. The caps in the circuit you mention probably hold a big charge while the transistors progressively switch these caps into each coil to insure a big bolt of energy to beef up the magnetic field. As the "cargo" goes farther down the rail, it's moving faster and each coil doesn't have to be energized for as long a period.

Techically, a device like this can be very dangerous, as a "muzzle velocity" of a large ball bearing could get high enough that the rail gun could be classified as a dangerous weapon capable of destroying property or killing. Be really careful with your experiments. It's always been a fascinating type of a project to me, but I've never considered putting one together in the classroom environment because of the weirdness of teens and their normally unstable mentality. Our auto mechanics programs have always had that problem as at least once a year, a student will put a ball bearing into a length of pipe and hit the other end with 125 psi from an air nozzle and do serious damage at the other end of the shop.

Oh, by the way ... mag lev trains can be propelled using similar techniques.

Dean
 

daviddoria

New Member
haha dean, the idea here is to shoot BB's, not large balls to do damage! hehe. i think it has to be much bigger to be called a rail gun.
 

daviddoria

New Member
ok.. so i went and got a rectifier (AC to DC) and it seems to be converting the voltage just fine...

then i got a 10W 51ohm resistor

i put a 100ft. coil in series with the resistor and the resistor started smoking.

then i put a 100 ft coil and a 400 ft. coil in series and nothing was smoking... however now when i measure the resistance of the resistor it is 110k ohm... is this huge increase due to the fact that it burnt? i didn't measure it in the first place but it says 51 ohm right on it....

hm... also, do they make bigger resistors (power wise) for applications like this? because as i see it, P=IV, so <>5A * 120V = ALOT MORE THAN 10!!! lol

i think i am giving up on the huge coil idea, but i'd still like to know about large circuits like this.

also if anyone knows anything about the small, more pheasable circuits that'd be great too.

thanks
david
 

bogdanfirst

New Member
the idea is
1) you put more resistors with smaller resistance in series with 10W or whatever...
2) more resistors with a highter value in paralel....
either way you choose....by putting them in paralel you decrease the current trough each one, so i see this solution better....
 
Dean, I'm not quite sure what you were describing, but it certainly wasn't a rail gun =). Its definately an intersting idea though, sort of like a multi-stage coil gun without coils being wrapped around a tube. Its worth looking into.

A rail gun doesn't even use a classical magnetic force. The force responsible for the motion of the slug is called the "Lorenz Force" (or something). What it all comes down to is that turns of an inductor want to push away from eachother. Now, a rail gun simply consists of two rails (solid bars or metal, preferrably silver for conductivity) and you just hook them up to some very big capacitors. Anything over 4kJ worth of capacitors will probably be pretty impressive. Next, you get a non-ferromagnetic slug (brass is good, for thermal and conductive properties) and inject it between the rails at some velocity greater than 0, otherwise it will spot-weld. The slug closes the circuit, and if the capacitors are even half decent, you'll get 100,000+A running through the whole setup for a few dozen microseconds. Thanks to our buddy Lorenz, the whole thing acts as a single-turn inductor, and the slug goes high into the sunset. The efficiency of a rail gun can be improved greatly by the presence of a vertical magnetic field parallel to the rails (above or under the firing track) but the field must be so strong that often the performance gains aren't worth the losses of powering hefty electromagnets above and below the rails (similar to the coil design you were speaking of).

Coil guns work simply by creating a magnetic field that draws the projectile toward the center of the coil. Optimally, the capacitors discharge before the projectile goes beyond the center, so it is not pulled in the oppisite direction. Coil guns generally achieve an efficiency of 2-4%, so you need big capacitors. Efficiency goes up tremendously in multi-stage setups, which is what Dean described, but using coils around the projectile instead of electromagnets below it. 20%-30% is achievable by a hobbiest with this setup.

For more info on railguns:
http://www.railgun.org/
http://www.powerlabs.org/

Coil Guns:
http://www.powerlabs.org/
http://www.oz.net/~coilgun/mark2/hints.htm
 
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