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Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi,
ive started to build a coilgun, and the coil i have wound is 1 ohm, and i am using 400v, so obviously a maximum current of 400A will flow, do you know which scr or transistor would be suitable? because i cant find an scr which has a surge current rating of 400A, that isnt a massive thing with a big bolt and a wire an inch thick
and also, without controlling the scr with a pulse of specific duration, and instead just to discharge the capacitors directly into the coil, what would be the maximum capacitance at 400v with the 1 ohm coil, that would increase the power of the gun, and at what point would the capacitance pull back the projectile? cuz at the moment i have a miserable 700UF (57J) of capacitors, and i know that if i stay with 400v, and wanna inrease the capacitance, eventually i will have to reduce the resistance of the coil, or the pulse will be too long and the bullet will be pulled back to the coil.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Well, 400 Amps isn't actually all too much. ST samples some TO-3 Isolated SCR's that have a pulse rating of something like 586 amps, if i remember right.... Same with Vishay.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Yeah, 400A is easy for an SCR; the big pucks can do 2 kA continuous and 10's of kA pulsed. Even IGBTs will switch that kind of current continuously (thermally limited, of course).
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
You're not going to get anywhere near 400A. With a 1 ohm coil, you must have thousands of turns which means a very large inductance. Even if your cap is 400V, it can't supply that continuously.
What is more likely is that your meter that you measured the resistance witch isn't made for low resistances. How many layers does your coil have and what are its dimensions? Also, what is your wire gage?
Figure out your coil and then simulate it with Barry's sims to get a good idea about the actual current peak.
Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi turkey9,
i would about 200 turns of wire which is about 1mm thick, round a ballpoint pen tube thing, and i measured it with a kane DM393C multimeter, and its resistance was 1R, so there will be a current peak of 400A right? and obviously itll drop till the scr cuts off.
and have you any examples of a lil scr that will handle in excess of 400A repetitive peak current? cuz it would be handy if you knew of a cheap one so i dont have to look through pages of specs.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
I'm no coilgunner, but I can tell you that 200t around a tiny ballpoint pen is not going to give you very good performance. As for switching devices, I would personally use a BIG SCR. They're cheap on Ebay, indestructible (at your current energies) and allow for expandability.
Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi,
lol i know it wouldnt be very good, do you mean the efficiency wont be good, or just that it wont be very powerful? what should i do to make it better?
i have a big scr,, it looks almost identical to this one, except that it doesnt have those gate trigger wires, it has a pin sticking out of the ceramic which must be the trigger connection-
the only problem is that i dont have the datasheet, have you any ideas what its rated values would be? and how would i wire it? and could i trigger it with a AA battery? and finally what would be its cut off current?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
how long is the coil? The wire diameter is 1mm, right? About what diameter is the entire coil? Going strictly off what you said and guessing the geometry an amateur coil gunner would use I'd estimate you have a 87uH coil. This with a 700uF cap bank at 400V would give you a current peak of 912A. This is just a wild guess due to lack of info. If you could provide the following I could give you a good guess on peak current: Wire gage or thickness Coil inside diameter Coil outside diameter coil length
Also, your meter is not reading right. Normal DMM are not meant to read such low resistance. Your coil is more likely to be around .1 ohm.
Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi,
i will post those measurements later :) so does the inductance make much of an impact on the current peak? or is it just as simple as V=IR? and also i am now just connecting the 2 wires together manually and there is a big spark etc, could this be wasting a lot of energy? cuz my coil gun is 57J, and it doesnt even make a dent in a cardboard box, whereas this guys 85J one, shoots a hole through a cardboard box -
my coilgun as you know is just a ballpoint pen tube and some wire, what would be a better way to build one, that is still small, but will work better than this one?
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Check out the coilgun chapter of "15 Dangerously Mad Projects for the Evil Genius" by Simon Monk and see if it fits your goal. Excellent instructions. The book is new and just went into production last month.
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a few questions about coilguns.
