160J Coilgun

A multiple stage coil gun with an adjustable output goal of up to 160J.

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A multiple stage coil gun with an adjustable output goal of up to 160J.
Most examples I've seen on youtube are in the 5~20J range. I'm out to up the ante.

For reference,
Air BB's = 20~40J
.22ga round = 159J
9mm round = 520J

This project is still in the planning phase, and will probably take a year or 4 to complete. So don't hold your breath. I also need to figure out what legal requirements there are in OH about building a firearm, among the myriad of technical equations I have to collect.

The power system I have planned is a project in itself, and will knock your socks off. Perhaps literally if you touch the wrong part. ;)

So stay tuned.

There are a few distinct phases that this project will follow:

First, the power system. This will consist of boost converter driving purely capacitive load, so current limit has to be downslope smooth as butter down to short circuit. Behind that a dynamic power bus fed by both an onboard battery and an AC rectifier, both of which are also current and power limited. And the AC will charge the battery.. So there is quite a bit going on here. Luckily high power is my jam, so I think this will be fun. Tentatively the output charge V will be adjustable up to 900V. 

Second, some coil testing and assembly. Despite endless calculations, I've worked with magnetics enough to know that some trial and error is unavoidable. I will likely be testing with a single coil for some time and fine tune it before doing all of the stages. Some things I'm concerned with are saturating the ammo, energy transfer efficiency, inductive spike magnitude, energy recovery schemes, etc. It's going to get real.

Third, physically putting the thing together. Dealing with ammo loading, heat management, rigidity, accuracy, all that. I expect this to be a few years away.

  • A thought about inductive spikes

    dave.m.mcdonough05/03/2014 at 19:50 1 comment

    So, the typical solution to dealing with the massive voltage spike which occurs when the coil turns off is to put a freewheel diode across the coil. This shunts all of that reverse energy through the diode, and whatever resistances are in the circuit. Series R can be used to share that energy, it can be a lot for the diode to handle. In the case of a dry fire, plan on 100% of the charged capacitor bank energy bouncing right back out of that coil. Wouldn't it be a better idea to steer that energy into capacitors instead of shorting it through a diode?

    The current plan involves multiple stages, and each stage will have it's own capacitor. They are separated by diodes, so that firing a coil does not discharge the cap for the next stage. What I realized is that all I need to do is simply move the diode location, and that coil will act as a boost converter, pumping current into all of the capacitors. In this arrangement I'm considering that additional voltage protection in the form of a hard clamp (drawn as zener diode) may be necessary. I may also decide to add protection diode across the switch (redundant to the FET body diode, also somewhat redundant to zener) and perhaps even one across the cap to make sure it does not receive negative charge from the inductor and that all of the juice makes it out as a positive charge. Sounds ridiculous at first but the load is a lot stiffer than the source here and the cap will be discharged. 

    I'm also adding film caps across the charge diodes. This should be enough to pass the high current spike through to the cap banks and not have to use large diodes here, but I'm on the fence on if this is a good idea. This will also cause a very low ESR rush of current when the switch first turns on, squaring up the turn-on waveform of the coil somewhat. It sounds like a good thing but it also means that the current spikes and noise won't be compartmentalized to each coil anymore. A extra filter on the output of my charge converter might make it all good? I'm probably over-analyzing what amounts to slapping an RC snubber in there.

    What I'm more concerned with is that the entire bank of caps will pump-charge itself and then I have an overvoltage condition on the entire system, particularly in the case of dry-fire. How realistic of a problem is this? Well, as long as the charge converter is not running then voltage should get settled back down into the discharged caps and things average out. 

    This led me to a complete brainstorm idea. Can I actually use this as the boost converter to charge itself from battery voltage?? Why add more coils and switches if I already have these? I'm not seeing how to pull it off yet but it's on the burner..


    There is a concern that squashing current in the coil as fast as possible is a good idea as letting it linger in there can retard the projectile speed. If this is the case then I would be better off putting a huge freewheel diode or perhaps even an active clamp in there. Once I have a coil made it will be easy enough to experiment. 

    I'm hoping to negate the issue by calculating the speed the projectile will be at for each stage, and setting up the inductance for each coil so that the pulse duration is timed accordingly. Because the coils will be physically short, the projectile should be gone for the freewheel ringing garbage phase of the pulse. This of course goes out the window for lower-power shots or using heavier ammo. 

  • Stage 1: Power

    dave.m.mcdonough04/29/2014 at 02:29 0 comments

    Well, you have to start somewhere. In this case I had to decide on a power level, and the ammo.. and just started calculating from there.

    1/2" ball bearing ammo


    Some caps scavenged from dumpster: 560uF 450V

    Internet sourced efficiency expectation: 6%

    24 stages. 33" long barrel. 6" tall. It will be bulky but it's shaping up to be possible.. I'm not ready to share my spreadsheet of massive calculations but at this point I've got it spilling out inductance goals for each coil stage.

    Where it gets interesting is the power system. Higher voltage will pack more punch, so I'm shooting for 900V. A custom boost converter is in order to step up the battery voltage to and adjustable output of up to 900V, and it's going to be spending the whole time in current limit pumping up a purely capacitive load. 

    The battery pack I have lined up are some LiPO4 cells strung up to 48V. 10A continuous, 100A for less than 30s. So right there I'm looking at 480W continuous 4800W peak. So, obviously we won't be using a flyback out of a disposable camera. This will have some meat to it.

    I plan on handling those batteries with kid gloves, it will have it's own separate power handler for current limit, undervoltage limit/shutdown, thermal limit/shutdown, and charging. 

    Charging will come from the AC when it's plugged in. The AC will be limited to 1500W/15A, so again another current limiting. Where they all tie in will be a sort of soft buss where each system is just putting in what it can while watching it's own back.

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dave.m.mcdonough wrote 12/22/2018 at 18:19 point

As evidenced from the complete lack of updates, I've moved on. I have no need for this, and it wasn't long before I had realized that ball bearings for ammo is _extremely unsafe_ because of how well they bounce. Same with most available ferrous metal shapes, they are too hard to wad up into the target and instead can bounce. I do still plan on dynamic power supply for other things.

  Are you sure? yes | no

lantramnamcodon wrote 09/26/2016 at 12:02 point

there is many of capacitor and it's just 160j?if i have 2 450v 470uf capacitors,how many jules i would have?thank. 

  Are you sure? yes | no

dave.m.mcdonough wrote 05/03/2014 at 17:21 point
I've decided to share a few links I'm reviewing today. They are 1/10 the power of my goal but same theories apply.

VERY good site explaining the fundamentals:

Anyone know Russian? Translate only does so much. :)
I like what I'm seeing here.

This project turned out nicely:
The equations he used are not totally translating for me because the power system is different but I appreciate sharing the details.

this craziness off wiki??

Interesting concept using multiple switches to increase effective coil voltage.
I probably will not be doing this because of the size/cost of my switch devices, the current losses in them, and the additional high-side gate drive circuitry. I'm also not going to be hurting for charge voltage.. But I likely will be getting complicated with management of the inductive kick when the coil shuts off. To sack it all in a freewheel diode seems like a waste.

  Are you sure? yes | no

dave.m.mcdonough wrote 05/03/2014 at 15:41 point
There's another coil gun project on here:

Fun to find similar projects, I hope to share findings and collaborate.

  Are you sure? yes | no

Eric Evenchick wrote 04/30/2014 at 05:01 point
Where did you get your hands on that much capacitance? I have a feeling this thing is going to be terrifying, so I don't blame you at all for not providing build instructions.

  Are you sure? yes | no

dave.m.mcdonough wrote 04/30/2014 at 12:18 point
I know where there's a good dumpster to check for power supply scrap. I will probably try to source as much as I can there because I don't have any real budget set aside for this yet. Between batteries and other parts I'm sure I'll eventually spend $600+ on this.. but yes I will save where I can. To buy an equivalent cap would probably be this:
So that's $580 right there.

And yes, it will be terrifying. I should probably run a check to see if this much stored energy is an official arc-flash hazard, even if not it's surely enough potential to be concerned with. I'm concerned with setting up a safe way to test. I'm concerned that the ammo (ball bearings) might be really BOUNCY and just ricochet off everything even at a shooting range. Normal ammo is very soft metal which will deform and absorb/transfer most of it's energy.

That said, when things start to come together I'll take plenty of pictures and things. I don't mind sharing or even selling certain parts such as the power supplies, battery management.. I think they will come in handy for a variety of projects. But the final stages of turning it from coils into a gun I will be thinking twice and probably seeking legal advice first. I certainly will not be selling a complete kit for that, ever.
On that note I did mention the project to a lawyer friend of mine, she's going to check out laws regarding building your own firearm for me. :)

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Eric Evenchick wrote 04/30/2014 at 20:30 point
That's definitely a good dumpster! Lawyer friends can be very handy.

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