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My first 'real 'coilgun

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MrFlatox

Fri Jan 25 2013, 06:38PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

That's right, these capacitors are the ones I use for my ETG. They work fine, of course these are not "top notch" capacitor for pulsing application, but they do the job and are quite cheap. In fact, I think this is the best Joule/Euro ratio on ebay.

Hanzie

Fri Jan 25 2013, 10:27PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Okay thanks, I'll just buy a couple then.
It's mainly for a proof of concept, but it's always nice to have some extra power. We'll see when they arrive.

Hanzie

Mon Feb 18 2013, 02:49PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Caps are in!
Now I can start working on the coil and stuff. From Barry's sim (

I found 50uH to work pretty well at 450V, 3000uF and a resistance of around 200mOhm. Something tells me 50uH is a bit low though, any thoughts on that? I feel like I'm missing something...
As I'll be shooting 6mm projectiles, the inner diameter of the coil will be about 8mm. I seen the rule of thumb that the coil should be about the same length as the projectile being used quite a lot, which would put it at +/- 3,5 cm. Probably gonna use 2mm copper wire. I've seen arguments for shorter coils too, but efficiency isn't my main concern right now so I'll just go for the easy way out. ^^
Peak current seems to be around 1400Amps in a 2msec pulse, so I'll also have to take a look for a usable thyristor (Or multiple smaller ones in parallel. I read that could work too.). Any recommendations? Ebay seems to be a good source for those as well.

PSCG

Mon Feb 18 2013, 05:31PM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136

Since it is your first coilgun, keep it simple and try to use only one thyristor.

If you find one that you can acquire, check it's datasheet and see if you can identify the Itsm (*) value. This will show you how much current it can conduct over a specified period of time (for your system, how much current for 2 ms). If it is lower than the 1400 A figure that you have now, you either have to look for a different thyristor or increase the time figure by changing the coil inductance. Else, you are good to go.

------------------------------

(*) Itsm is usually given for time periods of 10 and 8.3 ms (half cycles of 50 and 60 Hz). You can calculate the approximate current for other time periods by the surge overload current graph. If you have problems identifying it, post a link for the datasheet.

Hanzie

Mon Feb 18 2013, 07:11PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Thanks!
Would that go linearly? As in, say 200Amps for 5ms gives 1000A for 1ms?

Yandersen

Mon Feb 18 2013, 09:51PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

No, probably 200A for 5ms will give only 400A for 1.25ms.

Hanzie

Tue Feb 19 2013, 12:50AM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Alright, so it's exponentially. I'll see what I can find then. Thanks for the advise guys.

Hanzie

Thu Feb 21 2013, 11:04AM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

K, I found a suitable thyristor: the BTW69-800 seems to do the trick. (Datasheet:

I can get it on ebay and I've also found it in some other sources, so that should work.
As for the coil, I'm not sure if I can even get 2mm copper wire around here, highest I could find was 1.5 so I might have to go for that. Ebay only sells it per 20 meters or so of which I probably won't even use half. I'll look around some more.
Also need some diodes to protect everything from the reverse voltage spike from the coil. I'd think you only need to put one anti parallel with the coil and one in series with the capacitor bank, right? I'm not entirely sure about the latter, as the thyristor itself already works as a diode, or would it blow out? I've read things about placing multiple diodes in series or using zeners around the coil to increase efficiency, am I correct? Again, efficiency isn't the biggest concern on this particular project as it's mainly a proof of concept. (I might do another one later though) So what kind of ratings should I expect? Can I just use diodes rated for >450 volts that can handle a surge above my expected numbers?
Thanks again!

Yandersen

Thu Feb 21 2013, 11:37AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

The wire thicker than 1mm is incredibly hard to wind by hand anyway - consider few smaller diameters in parallel.
Protection diode must be placed antiparallel to cap to prevent it's repolarization. One power diode, nothing else. If you were using non-polar cap or transistor switching you could play with zeners, TVS or multiple diodes in series, but not in your current simple setup.

Hanzie

Thu Feb 21 2013, 11:47AM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Hm, I've seen that technique being used in larger electric motors for my rc planes/car. Never thought about using it in a coilgun. Wouldn't you essentially have two coils then, and doesn't that have any weird effects on the magnetic field or anything?
Do you mean I don't need a diode at the coil then? I have some diodes laying around here from an old pc monitor, what would be considered a power diode?
(Man, I'm asking a lot of questions... xP)

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