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ramses

Thu Jan 31 2008, 09:32PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

I am building an induction CG:
12 2400uf 450v 525v surge 'lyics 4x3 configuration
open air mechanically closed switch

i was wondering how much current the clamping diodes would have to handle. (can't find the datasheet, 1.5a rms), or if i would even need one at all. the cap's can't hold more than 12v reverse voltage, and would be allowed to ring down immediately.

also, on the projectile, copper or aluminum? i know copper conducts 60% more than aluminum, but aluminum is so much lighter, i just can't stop arguing with myself. i want to use a 1cm diameter rod, at least that long. weight is a big concern, since i want velocity, not just energy.

Thanks in advance,
Ramses

FastMHz

Thu Jan 31 2008, 10:15PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I think the copper vs aluminum experiment would be something interesting to perform....I've always fired aluminum on my ILs...if I managed to obtain copper plate I would certainly perform this test myself....there are a few variables that might make copper better in some instances, and aluminum better in others...namely, projectile thickness. I'm thinking thicker projectiles might be better in AL and thinner ones better in CU. I've always been curious about this!!

ramses

Thu Jan 31 2008, 10:44PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

i was actually going to shoot "slugs" out of it. so like a coilgun, but with the center of the projectile right in front of the midpoint of the coil. lots of duct tape to hold the coil together!. i will experiment with materials if i get the chance.

Shaun

Fri Feb 01 2008, 05:18AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hey did you get those capacitors from that guy on Ebay who was selling like 1200 of them? Cause I bought a pack of those too...

4 of mine I felt something shaking around inside them, but I charged them to 500V and they held up...thats good enough for me!

ramses

Fri Feb 01 2008, 03:30PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

yep. BCD parts... screw terminals a little small, and quite dirty when shipped. anyway, can we stay a little more on topic please. i know i am a n00b and not a moderator, but it would be nice. i am not flaming you or trolling, or anything, but... ummm... yeah.... Anyway, Shaun, how do they hold up to the surge? i mean an induction CG is basically a short, and the ESL of the caps might be more than the L of the whole circuit. that doesn't seem too good. I hooked 2 in parallel with 12 AWG wire, and put them on my reluctance CG. somehow, 2, 3 amp diodes from radio shack in parallel are taking all the back-emf without a problem!

Shaun

Fri Feb 01 2008, 08:10PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Somehow I doubt their internal series inductance will outweigh the inductance of any wire coil. As for the diodes, I could see a pair of 3 amp diodes having enough of a pulse rating to handle 2 of theses, especially if they are good quality.

They discharge as fast as any other capacitors of same type, but it still varies a lot based on load and also varies a bit from one cap to another. I have been using a homemade o-scope current probe to take measurements, which is a very valuable tool to have when working with these types of devices. It's a little tricky sometimes getting an analog scope to catch the pulse properly, but it works. I would highly recommend making one if you are concerned about this.

ramses

Fri Feb 01 2008, 08:30PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

one problem: I lack an O-scope, and i am too cheap to buy one, but i will check ebay.

if 6a worth of diodes = 2 caps does 36a worth of diodes = 12 caps? and do i even need back EMF protection? on Fastmhz's 24Kj thread someone implied that he didn't.

and it is going to be really low inductance like <10uh

Shaun

Sat Feb 02 2008, 03:55AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Its true that as long as your using a spark gap switch or any type of switch that conducts both ways, the discharge forms an LC circuit that will ring down at a resonant frequency. So your caps will be reverse charged, then normally charged, then back to reverse, and so on with amplitude slowly decreasing.

I don't claim to be an expert on the behavior of electrolytic caps under stress, but I can't even imagine that letting them ring down is safe. I mean, not even big energy discharge capacitors like too much voltage reversal during ringing, and they are non-polar! This is also why TC capacitors must be so durable; because they are ringing constantly during normal operation.

For the diodes themselves; I really couldn't say. On my bank (exact same as yours, remember) the back EMF diode is one leg of a very large 60A 1200V bridge rectifier. I've been told that bridge rectifier diodes have excellent surge capability, and as far as I can tell they do since mine still works. I would look for one of these since they can be had on the cheap on Ebay (usually).

Oh and if you're looking on ebay for a scope, make sure the picture of it shows a baseline trace and preferably a waveform. I learned the hard way...

ramses

Sat Feb 02 2008, 06:46PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Edit: found the datasheet on the diodes i have. 1.5A and ONLY 20a for a 8.3ms pulse!!! that is CRAP. The ones i use now are from radioshack (slits wrists) are 400v, 3a, 200A surge.

oh and i have determined with Barry's LCR simulator thatt if the ESR is around 125mO, it will be critically dampened, which will be awesome and stuff,making it not need an antiparrelel diode. YAY!! Anyway, do you know the ~ESR of the caps? Thanks, and i wish i had gotten 50 of those caps.

additionally, do you think that the 52watt light bulb i am using on my variac controlled MOT is keeping the voltage down. I can only get ~850v and i should be getting ~2kv. can i just use the variac for current limiting?

Thanks,
Ramses

FastMHz

Sun Feb 03 2008, 07:55PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I use a straight variac for my systems...I fused it though so that I don't overload it if I get too excited and crank it up too quick.

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