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Ant59

Sun Nov 21 2010, 10:16PM

Registered Member #3431 Joined: Sun Nov 21 2010, 10:07PM
Location: Great Britain
Posts: 5

I really want to build a coilgun for A2 Physics, however I have absolutely no idea where to start. I've done loads of research a lot of different websites and got together some components and stuff, but I'm unsure what to do first. If I can get started, I can work everything else out (I think lol). Can someone point me in the right direction please? Thanks in advance! :)

Alex Smith

Sun Nov 21 2010, 10:45PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Haha I just did this for my A2 coursework on Thursday!
Firstly, find out your capacitors, as this will probably end up defining all the parameters for you coil and projectile etc. You may need to check with you teachers btw as its probably restricted to 50Volts or so (if you want ive got loads of 9000 microFarad 50V caps). If not then use photoflash capacitors (lots of info on the everywhere) which are 330V. Youll also need a way to charge them - hopefully your department will have a HV power supply.
Then try and find a projectile, probably nails? and a firing tube (preferably not metal) after which you try and find a coil size using barrys coilgun sim.
You'll also need a way to switch the current. I used a SCR, which worked great but is a bit of an outlay, other guys here will hopefully have an alternative.
Thats basically the whole of it - don't over complicate if you dont need to!

Oh and what's your budget btw?

- HTH Alex

Ant59

Mon Nov 22 2010, 09:44AM

Registered Member #3431 Joined: Sun Nov 21 2010, 10:07PM
Location: Great Britain
Posts: 5

Thanks for the advice. I don't think that budget should be a huge issue, as I should be able to get most of the stuff off the science department here. I have capacitors, but I haven't seen them yet, so I don't know their ratings. I'll start with them then, and work around that.

Artikbot

Mon Nov 22 2010, 12:55PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Edit: oh crap, someone delete this. Wrong topic.

Ant59

Mon Nov 22 2010, 02:37PM

Registered Member #3431 Joined: Sun Nov 21 2010, 10:07PM
Location: Great Britain
Posts: 5

Yay :) My school has given me 2 41000uF 50V capacitors (HUGE! lol). Where do I go from here? I've set up a simple circuit, and the capacitors work, and I get a magnetic pulse through a simple coil. What's next from here? What sort of equations should I be looking at. (Sorry if I'm just being dumb with asking all this).

Alex Smith

Mon Nov 22 2010, 03:00PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

WOW. They are mental. Dont mess around with these, if they blow up... *shudder*

You need to aim for a pulse length of around 4ms (see Barrys site)
Anyway, you will probably only be able to use 1 cap as at 50V that much capacitance will take too long to discharge even with 2mm wire. Alternatively use them in series to get 100V at 20500uF (this is what I would do), but this will evidently need a 100V power supply (DC remember or youll blow them up).

Are they electrolytic? and/or do you have model numbers of them so we can find some sort of datasheet?

Heck looking at barrys sim you may still have too much capacitance at 100V. Using a quick estimate of 2.0mm wire ), the discharge time is way over 4ms (around 13ms atm) and there is a massive spike of negative voltage which would probably annihilate those capacitors. Critically dampening it would reduce the negative spike to almost zero and the zero voltage time would be circa 15ms, but you would need a really really heavy projectile and the circuit is going to be quite inefficient (well in terms of coilguns anyway).

I'm not particularly well versed in this stuff though so hopefully somebody else will have some suggestions or find a error in my reasoning which means it'll all be ok ;)

Neuuubeh

Mon Nov 22 2010, 08:06PM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

Try going with one for the first try. 2 might work I believe, but you will need really really thick conductors, both for your coil and for the rest of the curcuit. Thick short ones :D. Id rather look at something like that rather than putting them in series. Then again, maybe this kind of supply is more use for a small railgun or something..

Do you know what you will be switching with? Will you be able to get some measuring equipment from your school as well? An LCR meter to measure inductance and resistance at even low levels? If you manage to get your hands on this, it should be really helpful to optimize your design

I would imagine a diode to rectify the negative swing and possibly to feed back to the caps would be nice as well?

PS. 50v 41mf caps, they are electrolytic and I dont really see what will happen to them? Just put them in a box or so and all will be good, 50v is not really dangerous, and you will probably be charging them to 42-45 volts tops anyhow

PS2. For us non-british people, when do you guys have those Physics a2 classes?

Ant59

Mon Nov 22 2010, 08:11PM

Registered Member #3431 Joined: Sun Nov 21 2010, 10:07PM
Location: Great Britain
Posts: 5

Alex Smith wrote ...

WOW. They are mental. Dont mess around with these, if they blow up... *shudder*

Ok, so those are pretty huge. After some more searching in the store cupboards, I've found a huge stash of 25V 2200uF caps instead.

Atm, I have made a coil with 22 SWG wire. It's 60mm long and has an inner diameter of 12mm. I've used a plastic pipe inside, and used the lathe to make a 10mm wide, 60mm long bullet out of mild steel. I have a solid state relay to fire the pulse through the circuit. I only have 1 coil layer, so I guess I'm going to need to do some more winding for more layers? According to Barry's inductance sim, my coil has 0.05mH :S Surely this is too low. I'm hoping to achieve 4ms of timing using the relay circuit, saving me having to critically damp the circuit and loose power. Am I going the right way about this? TIA :)

Ant59

Tue Nov 23 2010, 12:24AM

Registered Member #3431 Joined: Sun Nov 21 2010, 10:07PM
Location: Great Britain
Posts: 5

I've stopped using the huge caps, since they arced earlier, and I'm not going near them again haha.

Neuuubeh wrote ...

Do you know what you will be switching with?

At the moment, I'm using a solid state relay.

Neuuubeh wrote ...

Will you be able to get some measuring equipment from your school as well? An LCR meter to measure inductance and resistance at even low levels? If you manage to get your hands on this, it should be really helpful to optimize your design

Not sure what an LCR meter is. The best I have is a simple multimeter.

Neuuubeh wrote ...

I would imagine a diode to rectify the negative swing and possibly to feed back to the caps would be nice as well?

I've seen this before, but not greatly sure how to go about it. I have 6 diodes, I'm just not sure where to place them. I've heard they're to be put in "anti-parallel" to the coil, but I'm lost on that :S lol.

Neuuubeh wrote ...

PS. 50v 41mf caps, they are electrolytic and I dont really see what will happen to them? Just put them in a box or so and all will be good, 50v is not really dangerous, and you will probably be charging them to 42-45 volts tops anyhow

As said in my previous post, I'm using much smaller caps now, and I have ample supply of them :)

Neuuubeh wrote ...

PS2. For us non-british people, when do you guys have those Physics a2 classes?

A2 Physics is the second year of an A-Level Physics course. I have college every weekday, and atm I'm spending a little time everyday on this project.

Alex Smith

Tue Nov 23 2010, 05:03PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Ant59 wrote ...

Not sure what an LCR meter is. The best I have is a simple multimeter.

Neuuubeh wrote ...

I would imagine a diode to rectify the negative swing and possibly to feed back to the caps would be nice as well?

Neuuubeh wrote ...

PS2. For us non-british people, when do you guys have those Physics a2 classes?

A2 Physics is the second year of an A-Level Physics course. I have college every weekday, and atm I'm spending a little time everyday on this project.

1) Google is your friend, heck I didn't know what an LCR meter is, but i do (vaguely) now.

2) Look at the max negative (below the x axis) amperage in barrys sim and try to find a single diode which has a surge current rating somewhere above that and also has a breakdown voltage alot above your capacitors max voltage rating.

Anti parallel is when the diode blocks the forward flow of current from the caps - so it all flows through the coil, but if you get that negative spike it should allow the current to continuously drain itself with the resistance from the coil.. Clicky

3) A2 is the year before uni and our results affect where we go, so its important ;)

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