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Registered Member #1106
Joined: Thu Nov 08 2007, 01:15AM
Location: Seattle, Washington
Posts: 25
I am planning on making a single stage coilgun. This is going to be my first coilgun but i do have a deepbackground in "potato guns"
So here are some questions i need some answers to:
1. which set of caps should i use?? 2x 400v 3900uf....or.....4x 200v 7600uf ...(parellel config)?? 2. Will this SCR work for triggering? http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=230028760447&ssPageName=MERC_VI_RSCC_Pr4_PcY_BIN_Stores_IT&refitem=230048673233&itemcount=4&refwidgetloc=active_view_item&usedrule1=StoreCatToStoreCat&usedrule2=CrossSell_LogicX&refwidgettype=cross_promot_widget and just confirming this, the npt thread side goes to the coil, the other side, the larger ring goes to the (+) end of the caps and also goes to my triggering device while the small ring goes to the (-) end of my triggering device? 3. i plan on using a 1/4" pvc thin wall barrel(sdr21 type) and launch a 1"x1/4" metal projectile (bolt cut into pieces). how would i calculate the amount of mag wire i should need and also the gauge? 4. this question confuses the hell out of me and is a question that you guys probably wouldnt answer straight away. but how would i charge my caps? i am planning to use dc power but i dont know how?? i know that i have to have a transformer of some type but thats about it. i've searched many places and i cant find anything thats straight foward. a link would be great. "noobs guide to building power supply" or seomthing along those lines would be great.
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
Hi Cwazy and welcome to the forum.
Both the cap banks you have described store over 600J of energy, a 1" by 1/4" projectile will weigh about 5g. This is a lot of energy to transfer to such a small projectile. The pulse length will be about 1ms so the current will be huge, the 25A SCR will not be anywhere near big enough. A rough estimate of the peak current would be about 5000A, you would need a hockey puck type SCR rated for over 200A to switch this sort of power.
Dealing with such high currents will make this project much more challenging than the average first coilgun. If you can make the pulse longer then the current will be lower, there are several ways this can be done.
1) Use a bigger heavier projectile. 2) Use a smaller cap bank. 3) Use a 1" long projectile in a 2" long coil. 4) Any combination of the above.
Try this, work out the energy stored in your cap bank then asume an efficiency of 1% for a high power coilgun or 2% for a low power coilgun. This gives you the kinetic energy of the projectile, If you know the mass then you can calculate the muzzle velocity. Then use equations of motion to work backwards and get numbers for acceleration, time etc.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
a 25A scr will definitely not work, try something along the line of SKKT 132/12E, these are from semikron and have a 4.7kA pulse rating for a 8.3ms half sin. No puck mounting trouble. I have been using one of these to switch my 1kJ bank.
at 600J, assuming low rate of fire, <1 per minute, wire doesn't have to be thick. 14AWG range sounds about right. #turns and dims you have to do some calculations, or try Barry's RLC sim.
To charge the bank from DC, assuming you are talking about batteries, so say around 12 or 24V, boost converter should be the easiest. goto anothercoilgun site for more info.
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