# .50 cal Coil Gun

After building a small 6mm coil gun I decided it was time to upscale... To a .50cal...

Youtube video of it in action.

This would mean a dramaticaly larger capacitor bank and equally large solid state switches. Whilst under construction I was approached by a Professor in the Engineering Department asking whether I could adapt it for use as a demonstration piece on electromagnetic acceleration. It only involved a few safety measures so I agreed and they gave me funding for components. The big buy was going to be a high current and voltage solid state switch. Luckily a Mitsubishi CM400HU-24F IGBT was supplied by a member of staff in the department who works on the power systems in trains.

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## Step 1: The Theory

A coil gun works on the principle of electromagnetic attraction. The coil gun itself is akin to an air cored solenoid. A ferromagnetic armature is placed at the breach of the coil and is electromagnetically attracted towards the centre of the coil when a current is passed though the coil. When the armature reaches the centre of the coil it will start actively decelerating if the current is still passing through the coil as it is magnetically attracted to the midpoint of the coil. This means that the current pulse length needs to be of a finite length in order for the armature to be accelerated up to the centre of the coil and not ‘sucked back’ as it tries to leave the coil via the other end. This is more specifically a ‘reluctance coil gun’, a diagram is shown below.

This is the biggest disadvantage to coil guns, the current pulse length needs to be controlled in order to cut the power when the armature reaches the middle of the coil.

The second largest challenge to overcome with coil guns is delivering the electricity as quickly as possible. It stands to reason that the more energy you can put onto the coil gun, the more energy will be transferred into kinetic energy in the armature.

The main losses in this system are the eddy currents in the flyway tube and the projectile/armature, these can be minimised by slotting the flyway tube or using a non-conductive material. The projectile must be ferromagnetic thus this means that limiting eddy currents cannot be achieved by using a non-conductive material. The best solution to this is to use a powdered Iron in epoxy resin matrix. Although this improves initial efficiency it also removes the ability for the gun to be operated as an ‘inductance coil gun’.

MainAvel6 months ago

And what do I do, if I don't want a lightbulb?

absolute zero1 year ago
Sorry if you mentioned this and I didn't see it, you have a very good article here. Where should your projectile be at its still position? I've only ever used round projectiles which were placed just outside the edge of the coil.
kschmidt21 year ago
so if I had a way to get 20kv to peak for 50 microseconds, what do you think the result would be going through a coil? too quick is what I'm thinking, I'm also thinking that the coil would burn out unless it was about 3 mm thick. What are your thoughts on this?

This is purely theoretical, I'm just really interested in knowing what would happen.
ausieking1 year ago
what is the charge time to fire a projectile
LetsBuildOne (author)  ausieking1 year ago
Wizzup2 years ago
Is that simulator available to use somewhere for everyone? I would like to use it to design my coilguns coil better. If not, any chance you could do the sim with my current coils parameters?

Cap bank:
3300uF 370V = ~226J
Switching is made with a big SCR

Coil:
1mm thick magnet wire
It is wrapped on a 16mm outer diameter plastic pipe
3 layers, the whole coils outer diameter is 23mm
41mm long

Projectile:
Material is iron, it is 28mm long, 10mm diameter
Mass 17 grams

This is my first coil for this so i would like to know how it performs, i think i would get much more speed with more coil layers and shortening the coil a bit but i would like to know how the current one does before making another. Also all suggestions for the coil design are very welcome.
LetsBuildOne (author)  Wizzup1 year ago
Yes, I would be happy to e-mail you the simulator. Just drop me an e-mail at letsbuildone@live.co.uk asking for it and I'll send you the .zip!

As for optimizing your coil gun:
As a guide my coil gun is 217J and KE is about 10J @ just over 20m/s. I optimized for maximum kinetic energy at the limit of 800 Amps peak that my switch could handle

Your design currently puts out a predicted 2.2J at 16.7m/s

First you tube is unacceptably thick walled. The magnetic field strength reduces with the square of the distance to reducing your 16mm tube to a 12mm tube results in:
2.85J at 18.7m/s, If you managed a 10.1mm coil by forming the coil in epoxy you could improve that to 3.12J at 19.59m/s. I'll stick with 12mm for subsequent optimization though.

The coil isn't big enough either and it is discharging too quickly. Increasing the number of layers from 3-7 increases the velocity to 7.37J KE at 30.1m/s.

Further than this I will need to know what the peak current your switch can handle is and also if you are optimizing for velocity or kinetic energy.

LBO

Tristan232 years ago
i want to know are you using a multi meter
LetsBuildOne (author)  Tristan232 years ago
Yes
ohforgodssake2 years ago
You could get much better performance from a slingshot. How can one increase the performance?
LetsBuildOne (author)  ohforgodssake2 years ago
If your aim is to do damage at range then a slingshot would be a lot cheaper, easier, and effective. A slingshot can't accelerate a projectile using electricity and magnetism without physically touching the projectile and the acceleration will never be any where near as fast, coil guns also have no upper limit to the speed to which you can accelerate the projectile and hence how powerful it can be. Maybe the speed of light... Tacheons though... and that particle they found at cern maybe...

You can make a coilgun more powerful by putting more energy in. That isn't quite as easy as it sounds though. You could increase the length of your projectile, the coil, and increase the size of your Cap bank in accordance with this. You can also place a secondary coil after the first to further accelerate the projectile when you reach the limit of what can be done with one coil. You can then keep stacking coils but each coil adds much less power than the one preceding it so it's diminishing returns.

The US military are researching them for use in long range weapons for ship but the big problem is that when they try and fire it, the recoil would sink the ship! They were also considered in WW2 for shooting down aircraft as the projectiles are cheaper and easier to produce. All it needs is electricity to fire them and it's rate of fire is almost unlimited, storage of projectiles is less dangerous because there are no explosives involved.
Speedmite2 years ago
Would this work for a solid state switch if I had 120 of 330V 100uF capacitors? It would be like 3 of your bank in parallel.

http://search.digikey.com/us/en/products/FGA180N33ATDTU/FGA180N33ATDTU-ND/1916864

I would have to buy 15 IGBTs or so to put in parallel to handle the amps, and that would cost around 60 bucks, but I was wanting to build another coilgun/railgun, and though as fun as manually triggering it is, it looses a lot of bang. I built a railgun before, now Im thinking coilguns are more efficient. Eh, only difference is rails or coil, if I change my mind Its only gonna cost me like 5 bucks.

So about 15 in parallel wold work?
H203 years ago
My science fair project is based around coil guns, but I don't know how to measure the velocity. I've looked at Youtube videos and have seen the phototransistor method, but I'm not sure if the projectile will go fast enough. Is it OK to place to PT's 1 cm apart?
LetsBuildOne (author)  H203 years ago
1cm apart wouldn't give you an incredibly accurate measurement. The further apart the better. Also the PT's will probably involve some fancy circuits to work out the velocity or the use of an oscilloscope preferably with computer input.

The best method I have found is to stretch out two pieces of paper one meter part and put a microphone in the middle of them (out of the projectile flight path of course). Record the sound on your computer as you fire the coil gun and then use audio editing software to measure the time between the two bangs as the projectile hits each sheet.

Alternatively you can put it 1m off the ground and shoot it horizontally into sand. You can see then measure to the imprint where it landed and use SUVAT equations to calculate the initial velocity.

Finally you could video it shooting in front of a background of equally spaced black and white strips and measure how many it goes over per unit time or how much time it take for it to travel over say ten of 1cm width.

Does this help?
3 years ago
if I were to use phototransistors I would find the velocity using Audacity. I have thought about using slow-motion footage but don't have a slow-mo camera or knowledge of any software that could slow the footage down. Do know of any software?
LetsBuildOne (author)  H203 years ago
How would you get the phototransistor signal into the computer in audio format for audacity to analyse? I have some phototransistors for just this purpose, if you could tell me how I'd be very great-full!

I have done the camera approach and a standard camera films at around 25frames per second so over a one meter length it could measure up to 25m/s. I found that at 21m/s it was inaccurate as the projectile was blurred from the lengthy exposure time of my camera. You don't need to slow it down. You need to see it frame by frame with the frame time stamp to 100ths of a second. I used a free trial of Adobe's software available from their website. These others are also available:
http://www.desktop-video-guide.com/top-5-free-video-editing-software-review.html

As long as you can guarantee that the projectile will pass through your two light gates and you can get that signal into the computer then that will be your most accurate and easy option. I would posticulate that the PT signal would be easier to analyse than the microphone method as it will have less noise and a clearer step change as the beam is broken.

Most sound cards run at 48kHz so over 1cm you could measure up to 480m/s though accuracy would be much better at velocities closer to half that.

You should also make your measurements as large as possible as if you can only place your PT's accurate to +/-1mm then you have 4mm of uncertainty in the PT's position which is 40% of the measure so your velocity is effectively an estimate. If it were 100mm then it would be 4%, 1000mm 0.4%uncertainty, you get the idea. Also it means that the time would be longer between the gates so you could measure that more accurately in the software.

st5k3 years ago
ok.... i have been working on building a coilgun for a while now and i wanted to pick your brain a little. first off what im working with are 10 450v 1000uf caps, and i was using a homemade static tranfer switch. essentially 3 bolts in a small box with a 20k neg ion gen, worked pretty good too. anyway, should i hook these caps up in series or parallel? i had them in parallel but i wasnt getting the oomph i was looking for. that could have been because of my coil too im not sure, it was pretty crappy not gonna lie. i cant remember the gauge of the wire, i wanna say it was 16. the coil was approx 4 inches long with 3 layers, and i was using a plastic tube with a 5/16 bolt as a projectile. also what should be used as a charging system? i was using a flyback transformer, but was a bit slow. i would love any sort of input u can provide.
LetsBuildOne (author)  st5k3 years ago
You want to hook your caps up in parallel. I doubt you have a 4.5kv supply hanging around to charge the series bank.

At 450v you could use full wave rectified mains maybe from a 12v-240v inverter passed in series through a neon tube on the AC side of the bridge. This would only charge to around 340v though. a little short of your 450v capability. You could use two in series to get 680v then use a charge regulator to limit it to 450v. Or simply watch the voltage across the caps and cut it before it goes over 450v.

Your coil and your projectiles are undoubtedly what are costing you performance. Firstly your 5/16 bolt has a thread which will mess up your flux linkage from the coil to the projectile and effectively introduces an air gap. It will also have more friction and air resistance. You want a smooth non-electrically conductive, ferro-magnetic projectile with flat ends. you can buy ferrite cores off ebay in short lengths.

Your coil will also be stunting its performance. I don't have my simulator on this computer so I can't tell you what coil you want right now but the dimensions of yours seems wrong to me. Firstly the L/D ratio is too high. Your coil should be the same length as your projectile and your projectile should be roughly 5-9 times as long as it is diameter. 4"=100mm is too long. 5/16"=7.9mm so 40-72mm (2"-3") is about right.

At that much power you also want thicker wire, 14AWG or thicker. I can't be sure without checking the maths but three layers seems to few to me. My experience suggests that somewhere around double that would be better.

Are you going for raw power, as in projectile kinetic energy, Piercing power, as in pressure per unit area, or projectile velocity? Each is a different coil and projectile. High velocity wants a light weight projectile and a longer coil with fewer layers. Raw power means you want a large diameter heavy projectile with a short fat coil. Your talking over a 0.50cal. For piercing power there is a compromise to be drawn between diameter which you want thinner, and weight which you want higher but you can't increase the length too much...

Finally it sounds like your coil is badly formed. your plastic tube may be too thick, your projectile may not be a snug enough fit in the barrel or too tight and stick. Your coil should have end plates too that the coil is formed between so it is nice and square in cross section. You should coil it nice and tight and glue each layer in place, one at a time. Try to minimise the gap between the coil and the projectile. The easiest way to do this is to use your projectile as the coil form. just wrap a piece of paper or plastic around it first, one thin layer.

To be honest I know nothing about Static Transfer Switches. This could be inefficient and waste energy compared to solid state alternatives. I appreciate that as these voltage and currents they can be rather expensive though. I'm using a pneumatically actuated spark gap switch with two ali blocks and a copper peg with a morse taper submerged in mineral oil. No current and voltage limits in my range and loses are minimal.

If you want any further help just ask :)
newwarhammer23 years ago
could i use 1 x120,000 Microfarad capacitor at 16 volts?
LetsBuildOne (author)  newwarhammer23 years ago
Yes, you could. Don't be expecting amazing results though as the energy stored in a capacitor is 1/2cv^2 so my 4000uF bank at 330v would give 217Joules, I charge it to closer to 400v, that's 320J. Your bank would be 0.96J Therefore less than 0.5% as powerful as this one. This is 13J KE so yours would be 0.06J KE roughly speaking. To put this in perspective, a powerful airsoft gun is 1J. Personally I'd go to your local camera shot and get soldering!
spark master3 years ago
how hot does the projectile get?
LetsBuildOne (author)  spark master3 years ago
Not noticeably warmer. Once, after several shots of the same projectile in rapid succession I noticed it was slightly over ambient .
3 years ago
could use thin walled capped iron pipe and add a tail and fins like olde tyme grenadiers did, accuracy would be improved. This gizmo is quite a nice lawsuit waiting to happen I must say , I want one! Then again my land is too small, and the rabbits run as it is, stttiiiillll ....... Wosted wabbits fweshly wacked in the yard.... Bacon wrapped of course....
LetsBuildOne (author)  spark master3 years ago
As I explain in my instructable on projectiles, You don't want circular conductive paths in the projectile as they allow eddy currents to form and waste energy. The volume/mass of ferro-magnetic material in the projectile is also important as you want enough so that it fully saturates in the magnetic field but not so much that it doesn't fully saturate and you waste energy accelerating dead mass. Essentially, the size and density of the projectile need to be balanced with the inductance to get optimal performance. If you read this instructable and the one on projectiles you should understand what affects coil gun performance. If that still leaves you with questions or you want clarification I will be more than happy to fill in the blanks :)
3 years ago
Shucks, I was hoping for a hollow projectile. Since you like high voltage check out

http://205.243.100.155/frames/shrinkergallery.html

Bert is a very cool maker of strange things , (like the magnetic gun), when he makes his items he will erase your cred cards magnetic strips and the end result is a hoot.

thanks fer th efeed back

spark master3 years ago
awesome, how heavy are the projectiles, are they ball bearings, pieces of iron pipe?
LetsBuildOne (author)  spark master3 years ago
I've writen an instructable on this subject: http://www.instructables.com/id/Coil-Gun-Projectiles/

Getting to the point: They are variations on a 55mm long, 13mm diameter solid soft iron bar. They weigh from a little over 40g.
3 years ago
Almost a 2 ounce weight I will check out your other instructable
XOIIO3 years ago
Can you give me any help making a faster charger for my coil gun? I have 14 330v capacitors rigged up for testing, but I have 145 cameras to take apart and scrap from. Can you give step-by-step instructions on how you ade the charger for your capacitor bank?
LetsBuildOne (author)  XOIIO3 years ago
Yes I can, I am however currently in Southport celebrating my birthday and as such have none of my stuff with me. Having said that I'll see what I can do now.

Right, you have a couple of options.

First you could use full or half rectified mains. The simplest way to do this is to use a diode from the live followed by a mains rated switch and then a light bulb fitting with a 20W light bulb in it. In the UK you'll get just under 340v DC out. In the US if the mains is 120v then you'll get 170V which isn't a lot of use to you. If you use this method you need to use a voltmeter to monitor the charge level and make sure you don't overcharge them. Also put a couple of mega ohm resistor across the capacitor bank to make sure it can't be left charged and will naturally discharge itself.

You could use 12v DC to 240v AC inverter to source your mains. they're available on the net for about £20 and will run off any 12v source though a decent size lead acid or gel cell would be recommended.

If either of the above options are used then a diode bridge could be used to double the charge rate with full wave instead of half wave rectification..

The other way of doing it is to use a step up buck boost converter but they are a lot more complex to make and explain here.

You could also just link up several camera flash chargers in parallel or add a second 1.5v battery. This is a good way to blow up circuits though so keep a few spares. Camera flash chargers aren't designed for that kind of punishment and don't usually last that long.

Hope this helps, If you want more help then let me know and I'll draw you some circuit diagrams when I get home.
3 years ago
Well I do have 137 circuits from all the cameras I took apart, my current charger is two of the worlds smallest shocker circuits because they have more oomph. but they do take a few minutes to charge it, and I'm looking for something much faster. I could use a car inverter with a power supply, and that seems to be the easiest option aside from trying to replicate the camera circuit with a larger transformer, but I would need someones help sizing up the resistors, and the transistor might fry.

tazerboy3 years ago