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New Coilgunner, need advice and tips!

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Rokko8652

Fri Jun 25 2010, 06:45AM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Hello, I am new to the forums and to Coil guns. I wanted to build one for a summer project, so I set out on plans. I planned to use Photoflash capacitors, 330v 80uF each. Would ~20 of these give an average power for a coil rifle? If not, any suggestions? I don't have much of a budget for parts, just about $20 for switches and things. If about 20 of those capacitors are enough for an average amount of power, how could I charge it in ~10 seconds? Also, I plan to use an 11 layer 50mm long coil using 16awg wire. Any suggestions for a long, skinny barrel?

Any tips, tricks, advice, or anything to further my project is GREATLY appreciated!
EDIT: Also, does a longer barrel increase accuracy? I would think it would...

UltraMagnus

Fri Jun 25 2010, 01:16PM

Registered Member #2875 Joined: Mon May 24 2010, 08:28AM
Location: England
Posts: 42

those caps should be enough, in fact they could be too much and end up causing suck back. this is where the coil is still energised once the projectile has passed through it.

how you charge your capacitors will depend on if you want to be portable. if you want it to be portable you will need to make a boost converter or flyback style transformer. if you want it mains powered a transformer followed by a rectifier would do.

a longer barrel will have no effect on accuracy, a coilgun pulls the projectile rather than pushing it and is more like a magnetic slingshot than a gun.

Rokko8652

Fri Jun 25 2010, 06:42PM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Thanks for the help, Magnus. So if it does cause suck back, just take one capacitor off at a time until it shoots?
EDIT: Also, what is a flyback transformer and a boost converter? I've heard of people linking the circuit board chargers from the cameras in parallel to charge.

UltraMagnus

Fri Jun 25 2010, 11:44PM

Registered Member #2875 Joined: Mon May 24 2010, 08:28AM
Location: England
Posts: 42

The is calculations you can do on suckback, hopefully someone better at maths than me will explain it. Or you can use light gates a microcontroller or similar, and an IGBT.

camera chargers in parallel will work, but be inefficient and slow.

GhostNull

Sat Jun 26 2010, 12:00AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

There is lots of information in previous threads on this board. Barry's site has great information for newbies http://www.coilgun.info you'll find design process, easy to use simuators and plenty of simple easy to understand information. I also recommend you look here:

I recommend you look through them, there is a wealth of information. Coilguns can be complex presion engineered machines to something you whack together in an after-noon. If this is your summer project I'm assuming you're going to make something relatively good.

In answers to your questions:
1. You have 20*330v 80uF photoflash caps, this gives you a power 87.12 Joules. Given by E=(C*(V^2)/2)
This is should be enough to build a coilgun that can launch a projectile. Just 1 photoflash cap is enough to lauch a nail, not far though. More advanced people build coilguns that go into the kJ using computer grade capacitors, then depending on their efficiency they can shot through a tin can. If you look on youtube you can get an idea of kind of power you get for what kind of capacitance. You power should be able to make a decent coilgun. What is your goal in terms for power?

2. If you're asking what to use for switching then for the lowest price people use relays, then SCRs then IGBTs. Each has its advantage and disadvantage. Relays are very cheap, below $5, but arcing on the contacts causes reduced efficiency and limited lifespan. you must also remeber the inductive kick back of the relay. SCR's are solid state so you don't get arcing and it's associated problems. SCRs are also faster to turn on but they are more expensive, you can get a suitable 1 for around $20 on ebay. Then there are IGBTs these are solid states too but unlike the SCR you can control the pulse, turn it on and off, but they are even more expensive in the 30+ zone. Another thing with solid state switches is that they must be protected from the inductive kickback of the coil. All have their up and downs you must weigh them up and decide which is best for you.

3. As for charging there as many methods. The cheapest and probaly slowest and least efficient is to use the photoflash's capacitor charger. Then there are people's boost, buck and buck bost converters. These are much more efficient and charge up your caps fast. Here is a good guide on how to make one:

Another method is too use current limited rectified mains, I don't know too much about it but I think it is relatively cheap. Then you can get specialised IC capacitor chargers, I making one and It has ended up costing me more than $40 XD.

I don't have much have much time left so I'll point you here:

here:

and here

play around with the slider until you find something that works.

hope that helps!
-Ken

Rokko8652

Sat Jun 26 2010, 12:14AM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Thanks GhostNull! I really appreciate the organized response with links. I do plan to make something relatively good, possibly something to shoot through, maybe, a potato.

Rokko8652

Sat Jun 26 2010, 04:43AM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Ok, after my first day of having the electronic parts, I got 12 330v 80-180uF capacitors. Would the varying uF amount matter? Also, I got the charging unit wired up to a switch for the safety and charging toggle. I just have to make the barrel and the capacitor bank, then wire it all together then put it into a case, in this case a modified airsoft gun body. The barrel is making me nervous, since most guides focus mainly on the other parts, like the capacitors. I also don't have something tight enough to use as a barrel. Would having the barrel fit the ammo fairly tight ( As in not having 1-2 cm room to move around) help, as I think it would?

GhostNull

Sat Jun 26 2010, 08:40AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

1. If you are planing to put them in series you will have to balance them using balancing resistors

but since u are using 330v caps you will probably not need to series them. Other wise you should just take the normal precautions with any hv capacitors.

2. Your projectile should fit snugly in the barrel, as tight as possible while still being able to easily slide up and down. The closer the projectile is to the coil the better it will be able to interact with the magnetic flux of the coil. Therefore the barrel hould also be as thin as possible, the barrel will only need to direct the projectile so it does not need to be extremely tough. The barrel should also be non-conductive as to avoid eddy currents. Also a longer barrel will mean better accuracy but there will be greater fiction and therefore your projectile will have less force when it comes out. I would be more concerned about how straight the barrel is and how well the projectile fit when looking at accuracy.

The answers to all your questions so far are answered in the links I have given you so please read through them, however boring it maybe because it will save to the pain of problems later on in your construction.

I am also concerned that you haven't put much thought into designed of your coilgun.
What will you use for switching?
What shall be your charger?
What is the total capacitance of your bank?
What that will your coil be?
What will you use to trigger your coilgun?
What you you going to do to control the coil pulse timing?

Cheers
-Ken

Rokko8652

Sat Jun 26 2010, 02:53PM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

I'm going to be honest:
I don't have much thought into it, that is because I am unsure what to do, so I came here.
So to answer your questions,
1) How do you mean?
2) The camera charging circuit (I know it will be painfully slow)
3) I have to measure this out, as I had to stop for the night because it was getting late, but I will do this today.
4) My coil will be 1in long 7 layer of 20awg enamel wire.
5) A momentary switch
6) Aw geez, another thing to take care of? Oh well, I'd rather put in alot more effort and get something to be proud of :).
(Sorry if I messed up the questions, I just woke up)

GhostNull

Sun Jun 27 2010, 12:03AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

1. What are you going to use to control your pulse?
2. What you can do is put a couple in parallel to increase output power
3. The total capacitance will aproximately be the sum of the value listed on the side of the caps
4. The coil must be properly made so that the RLC time is short enough to not cause suck back whilst still being able to pull the projectile, here is a calaculator for it

. You will have to take into account the ID, OD, resistance of the wire, and must importantly the inductance of the coil.
6. As mentioned with the in 4. with the coil, it must be properly made to so that the pulse is short enough to not cause suck back whilst still being able to pull the projectile.

You should really at least read the 4hv wiki on coilguns

and their basic theorys

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