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2.5kJ Capacitor Bank (Complete, finally!)

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aonomus

Sat Jan 31 2009, 03:39AM

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

So I've decided to build a nice big capacitor bank for pulse power experiments, while conventional wisdom says that electrolytic caps aren't up to the job, I'm determined to try otherwise. My plan is to use 38000uF at 450V in electrolytic caps and use a bunch of diodes and SCR's to minimize/eliminate any ringing that would reduce the lifespan or destroy my caps...

I'm going to use a pair of heavy duty (5.3kA peak stud diodes) placed in-line with the load to eliminate most of the ringing, and on top of that I will use 2 puck SCR's (5.5kA peak), one on the +ve side and one on the -ve side of the cap bank, triggered using a GDT so that I get high-side isolation.

I will use another smaller stud SCR to achieve safe-discharge through 3 or 4 incandescent bulbs incase residual charge remained, or due to a failure in the load. Charging will be done either by a cascade with a variac, or by a big boost converter so I can strap a 12V battery to it and use it away from a source of AC. The charging circuit will be connected/disconnected by a DIY contactor I suppose, because the big spike of current and back EMF would likely destroy a small boost converter, and no doubt cause havoc on a 120VAC line.

Also, I have built a rogowski coil and I'll build an active integrator so I can measure the current waveform, and I'll capture it with my laptop soundcard with an attenuator.

The overall design will look like this, minus a few parts:

Construction photos:

Mockup with unflattened copper pipes, one SCR pictured infront, the other in back. Plywood base with caster wheels, eventually there will be a case with handles for easy transportation.

Top-down view of the mockup

Other side

Big diode (in the back) that I'll place across +ve to -ve to get rid of any reverse bias.

So, any thoughts, comments, suggestions?

Backyard Skunkworks

Sat Jan 31 2009, 03:57AM

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

Nice. So what kind of peak current are you expecting, and will we see you use these for a coilgun?

aonomus

Sat Jan 31 2009, 04:03AM

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

Peak current depends on the inductance of all the connections, and somehow I doubt that the buss-bar layout is optimal, however its still going to be very high, at least 5-10kA. The diodes should ensure that I can safely use inductive loads without back-EMF destroying the caps, which means a coilgun, railgun, can crusher, or even a hdd platter launcher are all possibilities. On the resistive side, I can do wire exploding (or any other mildly conductive object exploding).

MRacerxdl

Sat Jan 31 2009, 04:27AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Very Nice! I cant wait to see the complete setup and the results =)

Very good, I was thinking about building one, but the caps are a little expensive here, so I must think before buying :P

big5824

Sat Jan 31 2009, 11:55AM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

how flat have you managed to get the copper pipe? Mine still has lots of little bumps in, which I think will give a bad connection.

rp181

Sat Jan 31 2009, 03:59PM

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

To make copper flat, just hammer it as flat as you can, turn it around and do that again. I also put a big steel block i found outside on it, and hammer that.

aonomus

Sat Jan 31 2009, 05:07PM

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

I've only managed to get it reasonably flat, acceptable for a buss bar, but unacceptable for connecting to a hockey puck SCR. I may have to resort to lapping the end of a piece of copper to get it super smooth.

rp181

Sat Jan 31 2009, 06:32PM

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

the tension should flatten it enough.

aonomus

Sun Feb 01 2009, 08:33PM

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

Ok so a few updates:

1. I've cleaned the big stud diode I got for free with some alcohol and it turns out that its a 1n4047, which unfortunately is only rated for 200V. Instead of this big stud diode, I plan to use 1 or 2 isotop packaged dual diodes rated at 500A surge each (at 800V). It may be a bit overkill, but I sampled these and I have no use for them.

2. I've finished flattening the buss bars, probably some time this week I'll make a jig so I can drill the buss bars evenly and bolt the entire thing together.

3. I've figured out how to mount the other SCR puck, so I'll work on that too.

Unfortunately another bout of discussion on my cap bank says that because I am using diodes in series to block any back-EMF, the work-coil may undergo an extreme voltage spike, akin to a buck converter of sorts... I may have to redistribute the diodes so I have one anti-parallel with the load itself, and one in series. The result of a massive voltage spike would be blowing the puck SCR's and charging the bank in reverse, so I may have to use a resistive load with the anti-parallel diode to solve this. I have a big (100W 3ohm) power resistor which may fit the job.

aonomus

Wed Feb 04 2009, 03:47AM

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

Ok today I made a good amount of progress on the cap bank, drilled the buss bars, positioned and mounted the caps and linked up the 3 sets of caps. The center set of capacitors are shorter than the taller 3100uf ones, so I needed to add a wooden platform to support those caps.

The whole bank currently, note yellow lead, bank was charged up to power and bleed off to test capacitance at low voltage. Yellow lead to short for safety.

A pair of 1awg welding cables, I could use these to join the bank to the test load when I finish it.

Image of wooden platform underneeth center caps.

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