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Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
hey guys So i'm whipping up a 1kJ cap bank to use for various testing purposes/fun. I've got the charging and switching figured out I think (variac->extension cord->tripler+bank and a massive relay that i'll use one of the tripler capacitors to fire shut.) I'd like to use this test bed for many types of coils from lots of turns (coilgun) to very few (induction launcher and wire detonation) Since I'm using lytics in the bank (4x 450v 2400uF) I'm worried about huge back emf's ruining them.
How do you guy's usually handle the back emf in induction launchers? could the approximate reverse charging voltage be found from C*V^2=L*I^2 -> V=I*sqrt(L/C)? In my coilgun designs I always just use freewheeling diodes antiparallel to the coils, but for this case, I don't have a diode that could handle >20kA peak.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
I used a 800V 350A stud diode on my 450V 1KJ bank. I was still able to launch a platter at least 70+ft (referencing off the fact the building was 50ft) and at school, I made a crappy coilgun with maybe 40 turns of 16awg, and was able to shoot a custom made metal bullet 30ft across the room from a bench, and after 10 tries i finally got one to stick in one of the 2x4's we had angled against the wall.
So, I didnt see much a drop for me. But, my bank, being 4X 450V 2400uF as well, Probably the same capacitors, worked fine with a 800V 350A stud.
Id hear what others have to say first, but if your interested, a seller on ebay use to have a ton, i think he just has to relist them. I'll keep a lookout for you, i think they were $18+ship. I did no math. I just went with the "Buy one heck of a beefy diode and your safe" I used this diode on a 16 bank of these for 4KJ 900V, worked fine, i just stopped charge at 800V
You can find my bank, which had the stud diode, here.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
ah well i'm flat broke (except rolls of change) until my university jobs start paying me in a few weeks, so I can't order anything. Do you think about 10-20 firings will flat out ruin the caps without protection? I'd just like to do a few demos for my friends before I go back to school. Did you get your caps off bgmicro when they were $5 a piece? They look the same as mine.
Registered Member #4034
Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138
I happen to have a massive diode lying around (1n3738) stud type. It's of no use to me so I'de be willing to trade for some things (MOSFETS if at all possible). I can't remember the rating on the diode so ill put up the data sheet later.
Registered Member #4034
Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138
Ok I've tried t find the data sheet but my browser keeps crashing when I try to open pdf's. But from the looks of it its 400V 250A continuos so I think the pulse rating will be adequate.
Registered Member #4034
Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138
sorry should clarify that. Some IRFP250's or some IRFP450's preferably some IRFP450's. (Tell me if I'm being unreasonable here I have no idea what it's worth + I'm happy to pass it on to someone that will use it, instead of it lying around not being used.)
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cap bank reverse emf
