# 100% homemade railgun - capacitor question first!

Hello all! I just registered on physics forums and already have many questions!

First of all (lets start with just one) my friend and i are attempting to build a homemade railgun. We are tying to make it completely by ourselves - nothing bought besides the materials needed. Since massive ammounts of power was the first problem we ran into - almost instantly - i was hoping i could find some help here.

We are stuck trying to store very large amounts of power in a capicitor that will release it all almost instantaneously. We understand the basic concept of a capacitor and have been playing around with aluminum foil and ceran wrap in the kitchen and have gotten a whopping .4 volts and 35 some amps. Does anybody have any suggestions on what materials to use (thicker metal, denser, or just greater area?) or on the optimal power source (such as DC vs AC and what to charge capacitor with)? Any help would be greatly appreciated!

~diabloizzle

p.s. we are tying to make a railgun that has some legitimate range...not just a couple of feet - so when i say power, i mean UNLIMETED...POWAAA! (star wars reference...sorry)

## Answers and Replies

berkeman
Mentor
Hello all! I just registered on physics forums and already have many questions!

First of all (lets start with just one) my friend and i are attempting to build a homemade railgun. We are tying to make it completely by ourselves - nothing bought besides the materials needed. Since massive ammounts of power was the first problem we ran into - almost instantly - i was hoping i could find some help here.

We are stuck trying to store very large amounts of power in a capicitor that will release it all almost instantaneously. We understand the basic concept of a capacitor and have been playing around with aluminum foil and ceran wrap in the kitchen and have gotten a whopping .4 volts and 35 some amps. Does anybody have any suggestions on what materials to use (thicker metal, denser, or just greater area?) or on the optimal power source (such as DC vs AC and what to charge capacitor with)? Any help would be greatly appreciated!

~diabloizzle

p.s. we are tying to make a railgun that has some legitimate range...not just a couple of feet - so when i say power, i mean UNLIMETED...POWAAA! (star wars reference...sorry)

Welcome to the PF. Can you tell us a bit about your backgrounds? What grade/year in school are you guys? What experience do you have so far in building circuits and apparatus? What voltages have you worked safely with so far, and what types of projects have you built so far?

Okay- well my friend and I are juniors in high school. We have not actually built anything this big with circuits. We are the kind of kids who usually get half way through a project (go-kart, M&M counter, lazer show, etc...) and then drop it. But not this time! We have always been pretty into building gadets and toys - lego mindstorms if anyone's heard of that - but this is a jump. We have safely worked with voltages like...5 or 10. pretty weaksauce. We also have done a lot of work with computers. We have two junk computers assembled that we built from spare parts and have started working with linux.

I know our resume doesnt seem that impressive: but we do have a lot of ideas and really hope to get into a bunch of fun projects this summer. For our current project...we are tying to up the volts a bit!!
thanks!

~diabloizzle

The first capacitors were Leyden jars, invented in 1745 by someone living in Leiden. See http://en.wikipedia.org/wiki/Leyden_jar. See also http://www.alaska.net/~natnkell/leyden.htm
You can also use a similar jar to make an electrocscope. see http://en.wikipedia.org/wiki/Electroscope
A better capacitor can be made by wrapping two large strips of aluminum foil with two sandwiched layers of wax paper (which I used as a kid) (or with thin mylar, kapton, or Saran wrap as dielectric). Make sure the wax paper or Saran wrap sticks out about 1/2 inch in every direction. Two layers of wax paper are NOT better than one; it only reduces the capacitance (see below). I have seen, but used, 1 Farad capacitors (about $400 ea.). The capacitance C (Farads) is roughly C= 1 to 10 x 10-11 A/d, where A is the foil area in square meters, and d is the foil separation in meters. Last edited by a moderator: vk6kro Science Advisor A capacitor doesn't deliver any more power (or voltage) than you put into it. And it doesn't necessarily deliver if very fast either. That depends on the internal connections. So, if you are going to charge it with a car battery, for example, you don't need the capacitor. You can just use the car battery, which is capable of amazing output, to power your rail gun. You might like to have a look at this link from a few weeks ago (April 26th on): https://www.physicsforums.com/showthread.php?t=309838 I know it isn't what you want to hear, but maybe you could choose something a little less ambitious as a project. You need some projects that actually have a chance of working for you. If you like Star Wars analogies " DO ..... OR ..... DO NOT.......NO TRY"....Yoda. I believe Amateur Radio has a lot to offer, even in these days of Internet (and zero sunspots!), as a practical way of learning about some amazing areas of electronics. Last edited: Thanks for all the help and the links guys! We have made two capacitors so far-both out of seran and aluminum foil. We tried wrapping them up in a tight roll (very sloppy job might i add) and did not get very good results. We also tried using 4 and 6 layers of aluminum that were connectected (alternated each sheet). That formula will help - Thanks! (also the idea about a car battery is nice!) vk6kro...i know it sounds like we are in over our heads - thanks for the advice but dont worry...we have a bunch of projects going at once...none of them really anywhere near completion. Im pretty sure the railgun idea has been put on the backburner since we discovered the power of IR sensors, touch screens, a wii, and a projector!! Still fiddling though...always fiddling, never producing! one more question: If we have serveral capacitors hooked together, will that increase the amount of power that the railgun can use all at once or will it just all 'leak out' slower? thanks! ~diabloizzle vk6kro Science Advisor Maybe you can get someone to measure your capacitors for you? Many multimeters have capacitance measurement on them so someone at your local Ham Radio club or even a small repair shop might be able to do this for you. Ask politely. That will also tell you if your capacitors are working or not. You can have a fairly small capacitor that works on high voltage or a very big capacitor that works at lower voltages. So, you need to know what voltage you intend to put on your coils and this will tell you how big the coils have to be. If you go for high voltage, you need a source of high voltage and good medical insurance. Your question about capacitors in parallel. If you do this, you just add up the capacitances. So, the capacitor pair will hold more charge and should give it out at a faster rate. They shouldn't leak at all. If yours do, they may be faulty If you charge up two capacitors and then put them in series, you get the sum of the voltages on the capacitors, just like batteries. sweet! thanks! Medical insurance sounds good. I think maybe we will assemble the capacitors/railgun in a house next door to the ER! thanks a bunch! ~diabloizzle Not to try to stop your creativity or drive... but make sure you stay in school and do good. You have a ton of projects because you are still learning. You will learn a lot in college if you study engineering/physics or whatever. At that point these projects will be easier, cheaper, safer, and more bad***. Have fun though. This is the friend mentioned in the first post. In response to bassplayer - Both my friend and I do exceptionally well in school. We are currently enrolled in physics and other advanced placement courses and don't have any intention of dropping out of school. We both do a lot of learning and experimenting outside of class, and so far, we have probably learned more by ourselves than we have in school. Hey, I am also a junior in high school. You might want to look into getting some photoflash capacitors to power your railgun. I built a nice coilgun last week using 18 330V 120 uF capacitors that I got for free from a local photodeveloper (like at Wlamert or a drugstore). All you have to do is learn how to *safely* dissect the circuit and you can make huge electromagnetic pulses that can drive any electromagnetic projectile launcher. I built mine by reusing the flash circuit and sticking the caps in parallel, discharging through a 3 cm 22 gauge wire coil. Works very nicely. I am not sure that you can get the current you need from a homemade Leyden Jar. Scott jenesen I know that post is pretty old, but it contains many mistakes. First of all, capacitors are not like resistances or batteries, if you want to sum up their capacities, you must place them in parallel. Secondly, the point of using capacitors, as many know, is to provide instantly a huge amount of energy, which a battery can't do. Here some useful formulas to calculate the energy provided by your capacitors : Ue (potential electrical energy) = 1/2*C*deltaV^2 (this is the easiest one to use) (c=capacity) Some other formulas uses the charge on the capacitor : Ue = Q^2/(2C) (Q = the charge) or Ue = 1/2*Q*deltaV The link between the charge, the deltaV and the capacity is : C = Q/deltaV were the charge is measured in coulombs Finally, there is also a way to measure the capacity of a capacitor using a formula (i don't recommend it since some of the measures are hard to get precisely) C = epsilon zero*A/d (epsilon zero is a physic constant, A is the area of a sheet used to build the capacitor and d is the distance between the two sheet) I live in Québec, so we got what we call CEGEP. I have learn all that stuff in a general formation given for people who deserve to go at university in science programs. That was from my second physic class called : electricity and magnetism. We also learned the principle rear the concept of rail gun. It's a question of current in you projectile and of magnetic field created by the apparition of current in the rails of u rail gun. The formula to calculate the force applied on the projectile is : F = I LxB where L and B are vectors and where the x represent a vector multiplication. B is the magnetic field and L the length of the projectile in which the current pass. Finally I is the current in amp. You can also calculate the energy given to the projectile by this force : Ek = F * D (D is the distance on which we apply the force) Then you can calculate the speed of the projectile by isolating the speed in the formula : Ek = 1/2 m*V^2 (Where v is the speed and m the weight of the projectile) However it is important to consider that a huge amount of energy is lost with friction. Also you can find Ek by simply consider that it is equal to the energy of the capacitors. Once again only a fraction of this energy is transferred to the projectile. In fact, for a rail gun made by student with a small budget the fraction can be of 1-3 %. Hope this will help anyone that read that topic Jean-Yves Breton ...F = I LxB where L and B are vectors and where the x represent a vector multiplication. B is the magnetic field and L the length of the projectile in which the current pass. Finally I is the current in amp. You can also calculate the energy given to the projectile by this force : Ek = F * D (D is the distance on which we apply the force) Then you can calculate the speed of the projectile by isolating the speed in the formula : Ek = 1/2 m*V^2 (Where v is the speed and m the weight of the projectile) However it is important to consider that a huge amount of energy is lost with friction. Also you can find Ek by simply consider that it is equal to the energy of the capacitors. Once again only a fraction of this energy is transferred to the projectile. In fact, for a rail gun made by student with a small budget the fraction can be of 1-3 %. ... Hey guys! I wish you the best of luck on this project, as I am currently working on the same exact thing! I'm building a railgun as my senior project, and I am a senior Aerospace engineering undergrad at Cal Poly (come to Cal Poly, and do Aero if you wanna do AWESOME CRAP). Anyways. Ditch the idea of making your own capacitors. Sorry guys. Keep going to/get a summer job and save up around$500. You build one of these guys that works, posts a youtube video and make a website talking about your efforts (you may get some donations!!), and you can do whatever you like with your life. You will officially be awesome.

About the quote above...the Forcing is wrong on the projectile. The force is calculated as

F = I*IvXB

I = the MAGNITUDE of the current running through the rails/projectile
Iv = the vector DIRECTION of the current running through the projectile
B = the vector DIRECTION of the magnetic field caused by the current running through each parallel rail. Its important that you have pretty near-perfect parallel rails, or you will get side forces that can do some serious damage. [CAREFUL ON THIS PROJECT...]

Thats the basic way that you want to do it, but not the easy way to calculate your forces. Your next question should be...how do we find B?? NOT EASILY. B is brought about by the inductance of the rails per meter (permeability).

After all that reading you do...here's a nice formula for the estimation of the force on the projectile:

F = 0.5 * L' * I^2

L' = being the permeability (or inductance per meter) of the rails TOGETHER!!!
I = current through the rails

A pretty good forum talking about his very topic: https://www.physicsforums.com/showthread.php?t=323052"

For more on how to find L', google inductance per meter of railguns. There are a lot of professional papers out there on it, and it is not a simple topic AT ALL. (were currently working on that as well)

Here's a good place to start and see how hard it really is: http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=559966"

Capacitors that you want to look into: electrolytic, oil-filled (maybe), or flash.

Ratings on capacitors you should worry about:

- Voltage rating - the voltage at which that single capacitor can be charged
- Capacitance - the bigger the better. You will want something around 10-1000 uF for a low budget gun. I'm going slightly larger budget and will most likely be paying well over \$500 just for the capacitors.
- Current....is hard to find a rating for overall current through capacitors. Ripple current is a rating on a capacitor, but most of them are less than 35A, and your homemade capacitor did better than that. So when you get capacitors you can either try and analyze the heat generated by the internal resistance of the capacitors and see if it reaches the melting point of the capacitor materials...or just build it and hope they don't break...

If you want to read up on someone else that 'fiddled around' and made a rail gun, look at Sam Barros' site(s) and read everything. Extremely informative, and he may help you with questions you may have if you email him. Don't take my word for it, and if you're reading this Sam, don't hate me.

http://www.powerlabs.org/railgun2.htm" [Broken]

Guys, this is a big project and should be well-documented and NOT taken lightly. To get any kind of force in the projectile you must have upwards of or more than 100,000Amps of current and at least between 450V to 1000V of voltage. That is plenty of power to kill you where you stand, and anyone else that happens to be touching you at the same time (up to 10-15 people in series :tongue: ). I can't reiterate this enough BE CAREFUL and GET AN ADULT to help. Find your quirky neighborhood physics teacher or washed-up engineer doing software and get him on board!! This is not safe for you to do on your own, you will kill yourself. At these voltages, you can be thrown across a room merely by voltage alone, if it doesn't just kill you. BE CAREFUL

If you have any questions I will be very happy to answer them. You can contact me through this forum, or you can email me at jeffmaniglia@gmail.com

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