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Coilgun or Railgun? Answered

               I want to build either a coilgun or a railgun, using a bank of camera capacitors as the power source.  Experimenting with small coilguns, I've found that the efficiency is very lacking.  I haven't done the math, but five camera capacitors in parallel with a medium-sized coil will barely move a half-inch screw.  So which should I go for for better use of available capacitors--coilgun or railgun?

P.S.  I've found places on the internet that give tutorials on making capacitors, which I would also consider doing.  If I'm able to make large-capacity capacitors myself and use those, should I build something other than what I should make if I only have access to camera flash capacitors?

P.P.S.  I heard the following rule somewhere here on this site:  "The more amps in a coil, the more powerful its magnetic field, the more voltage in a coil, the larger the magnetic field."  Is this true?



Best Answer 8 years ago

Regarding your PPS, I'm not sure what the difference is between "powerful" and "larger".  The statement you quoted is either nonsense or redundant, depending. 

If you are using normal conducting wire, then the voltage you use determines the current.  The two are not independent, they are simply proportional (homework problem:  why is that statement true, and who said so first?).

A magnetic field falls off as a power of distance (1/r3 for a dipole), so a field which is "more powerful" at a given distance will also appear "larger", in the sense of having the same strength as the weaker field but at a farther distance from the source.

If you want to use a single capacitor with a very high storage capacity, you would be better off purchasing or scrounging one (from a car audio system, an old CRT, or something) than trying to make it yourself.  The problem is defects in the materials you use (small holes in your dielectric, for example) will either reduce the final capacitance, or make the device nonfunctional.

        I should have explained that better.  I think what was meant was that the higher the voltage of the capacitors, the larger the field, the higher the capacitance of the capacitors, the stronger the field.  for example, If there are two capacitors linked in series that are powering the coil, the field will be larger but not as strong as if the same coil was powered by the same capacitors connected in parallel.

I still don't understand what you mean by "larger" vs. "stronger." 

The shape of the field -- how far it extends in any direction (for a fixed fall-off in stretch), the angular dependence of the field -- depends only on the geometry of the system.  By "geometry," I mean way the coils are wound, their shape and their spacing, and the core you have them wound around.  The electrical configuration does not (cannot!) affect the field shape.

The strength of the field -- how much force it exerts at a particular location -- does depend on the electrical configuration, but that dependence is simple.  The more current you run through the wire, the stronger the field.  That's it.  You can express that same dependence in terms of the voltage applied (there's that homework problem again), but you won't get a different answer.

The capacitors supply current (or voltage, if you want), and how much current they supply depends on how you configure them.  Parallel vs. series will make a difference (more current faster, or less current for a longer time), and that difference in current will affect the field strength, as I've outlined above.

        Okay.  You actually answered my question (well, the P.P.S. one, anyway):  more current  through the coil, stronger field.  So to maximize power of the coilgun, I need a a balance between coil and projectile size, as well as the length of time that the magnetic field needs to be present to pull a particular size of projectile through the coil and shut off.  Makes sense.  Thanks!

Yes!  Sorry it took so long, but I think you've got it.  You'll also want to adjust the geometry to go along with your projectile.  There is software out there that can compute magnetic fields (in their full 3D vector glory) given a coil design and current flow.

An ampere is defined as this: A unit of electric current equal to a flow of one coulomb per second.

a volt is defined as this: The SI unit of electromotive force, the difference of potential that would drive one ampere of current against one ohm resistance.

they will each have different effects on the power of the magnetic field. visit this site for help. http://physics.stackexchange.com/questions/129538/why-does-magnetic-field-increase-when-the-number-of-coil-turns-increase

A capacitor does not produce the electricity, it stores the energy produced from your power source then releases it very fast. Think of storm clouds as giant capacitors, when enough energy is created it releases it in the form of lighting. And for a railgun, even a row of camera capacitors would not provide much power. For a small coilgun it would be OK. So I think you should go down the route of a small coilgun, but be careful for there is still a large amount of electricity involved and you can get severely injured of even killed from it.

Start with coil guns and move up to rail guns, you will learn much about capacitors, energy, and inductors. Rail guns are more advanced, it is better to learn cheap. The best results I have had with smaller caps (120uf @330) is to put them ins series, and charge at high voltage(ie 5 caps*330v=1,650v) you can buy a bug zapping fly swatter for about $7. at walmart this charges to about 2600v(i think + it is rectified). It will be slow and will eat batteries, but it is easy to convert. I have an old charger I made from a plug in bug zapper($35) that allows me to get as high as 4,000v(I rectified it) The best coil I have found that is premade for this small amount of energy is a solenoid from old Polaroid cameras(the ones that spit out a picture instantly). You can find those old cameras at goodwill for under $4. an added bonus is it comes with an armature, (3/16 I think), and a bigger capacitor. The main problem is that you are using too little energy, you need more capacitors, or larger capacitors(more energy higher uf, or voltage). 5 caps at 120uf and 330v = capacitance/2 (voltage squared) .00006/108900 6.53(5 caps)= 32.65joules.... and you will only see a fraction of that converted to mechanical energy electronics assistant: (will help you calculate energy in a capacitor) http://www.electronics2000.co.uk/download.php You can still make a good working model from about 5-10 120uf 330v caps. Good luck.

Go for a rail gun, on the grounds that they are currently under-represented on the site.

    That they are.  However, I'm trying to make something semi-portable and with a quick reload capability, and I don't think I want to have to deal with a pneumatic injector.  I think I'll go for coilgun.  Thanks to everyone for your advice!