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Rebuilt Cap Bank - Now with a 200 Pound Pulse Discharge Cap

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rp181

Sun Dec 28 2008, 02:59AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

A bit late, but there are some 50kJ capacitors (Maxwell) here for 2200$

El_Roberto

Sun Dec 28 2008, 03:13AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Posts: 135

Yea but theres no way of knowing how old and abused those are, as it is a surplus store. I got caught out by that when I was buying capacitors for my coilgun, the electrolyte had dried up - They still worked but I dont know how long till they explode.

lhl_henrylui

Sat Jan 03 2009, 03:50AM

Registered Member #1498 Joined: Thu May 22 2008, 07:08AM
Location: Hong Kong
Posts: 72

Wow!That'a the biggest capacitor I've ever seen!
11kV!

Neil

Sun Jan 25 2009, 09:16AM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
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Posts: 51

My Rogowski coil has finally arrived from England, it came in a nice box. I tested it out last weekend and it seems to work great, im very happy with it.

1232873662 1458 FT58983 Rogowski Coil Box

This is the mount for it on one of the output leads, bolted together with plastic bolts

This is the wave it made when I fired the capacitor to blow up a peice of wire at a charge voltage of 5,000 volts. What I find interesting is that it actually has a chance to reverse several times before the wire has blown up. The y axis is 20,000 amps/div and the x axis is 100 us/div. It reached about 55,000 amps in about 60 microseconds, I was fairly happy with that.

This is a piece of copper tube that I crushed, it actually reached a bit over 60,000 amps but I never got a picture of that one.

Backyard Skunkworks

Sun Jan 25 2009, 04:00PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Neil wrote ...

What I find interesting is that it actually has a chance to reverse several times before the wire has blown up.

You mean the circuit is ringing down, right? I donno where you'd ever find a large enough clamping diode for this magnitude of current though...

aonomus

Sun Jan 25 2009, 05:04PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Nice current probe, how much did it cost?? I'm also surprised that there is ringing in the circuit for a non-inductive load... Could the wire layout be simply enough inductance to cause it?

rp181

Sun Jan 25 2009, 05:05PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

The 4 diodes i posted in the for sale section =)
Each of those can take 5400A for 10ms, all 4 can quite alot of power for only one ms. Keep in mind rating goes up exponetially. so 20kA for 10ms could easily take it.

EDIT: where did you get that voltage readout next to the variac? i remeber seeing it on ebay, but i can't find it now.

Neil

Mon Jan 26 2009, 12:38PM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
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Posts: 51

aonomus wrote ...

Nice current probe, how much did it cost?? I'm also surprised that there is ringing in the circuit for a non-inductive load... Could the wire layout be simply enough inductance to cause it?

The probe ended up costing $2,660 AUD. I guess with any wire with electricity passing through it, there is a surrounding magnetic field, and is just concentrated with a coil because of so much wire being in the one place, but to only have 2 short wires exiting the capacitor as the output leads and basically a resistive load being between them, im suprised it can store power in such quantity and efficiently enough to pump 40,000 amps back the other way. I wonder how much the capacitor itself contributes to this, with fields built up around each conductive path inside?

rp181 wrote ...

The 4 diodes i posted in the for sale section =)
Each of those can take 5400A for 10ms, all 4 can quite alot of power for only one ms. Keep in mind rating goes up exponetially. so 20kA for 10ms could easily take it.

EDIT: where did you get that voltage readout next to the variac? i remeber seeing it on ebay, but i can't find it now.

I had thought about diodes also, although with the charge voltage there would need to be several in series. Given the time frame of the discharge, a few in parallel could possibly handle the currents involved, however the turnoff time of going from conducting to blocking then comes into it. Are there any diodes out there that can do this under 30 microseconds? Or under 10 even, the quicker the better.

I had built a small cap bank a while ago using 4 x 6100 uf x 350 volt electrolytics. On this I had placed 4 heavy duty diodes in parallel on the output, Semikron SKN 130 stud type diodes. A while ago with a quickly constructed passive rogowski coil, it looked like it was actually reversing before the diodes could catch it, and I would imagine these to be in the millisecond range for their discharges. The diodes are quite old however, technology has most probably advanced a lot since they were made. Now I have a good Rogowski coil I will have to check it out to see if in fact it is reversing. But this I guess makes me a little dubious about diodes being fast enough.

1232972035 1458 FT58983 Capacitor Bank Modifications

I was just hopefull that blowing up the wire would do the trick, no wire, no conduction, but it doesnt seem to work that way, maybe its too slow or maybe there is plasma left that allows it to conduct.

The voltage readout you are after is like one of these items currently listed on ebay Here

badastronaut

Mon Jan 26 2009, 02:42PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96

If you know what the initial voltage is and you have the graph of current versus time, you can estimate what the inductance of your circuit is. Based on the initial condition of the ideal RLC natural response, the inductance is equal to voltage divided by di/dt at time zero.

You can also use the log decrement method to estimate the resistance based on how fast the pulse decays. Better yet is to use parameter estimation to curve fit your current waveform.

Knowing what R, L, and C are can help you to formulate a mathematical model to better predict the performance of your setup. It could also help you avoid blowing up things you don't want to.

...

Mon Jan 26 2009, 05:17PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

As to the diodes, yes you can get diodes with switching time down in the ns range, any 'fast' diode will work for you (not to mention the superfast, ultrafast, hyperfast, etc diodes that you find from all the big manufactures these days

)

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