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Suitable caps for pulse application (railgun)?

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LAN_403

The Wumpus

Fri May 19 2006, 03:39PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

This summer I plan to build a medium-sized electromagnetic projectile launcher, probably a railgun since I've built a number of coilguns in my halycon days of youth and am looking for the challenge. Anyway I just picked up four cheap electrolytic caps rated 12000uF at 350 V:

The caps are marked

S+M: B43465-S4129-M1

I don't find any info about these being pulse rated. They do have sturdy aluminum terminals with M6 threaded holes.

Any idea if they are suitable for pulse applications, and also, is there a simple way to measure ESR or at least get a rough estimate using multimeter/scope and common components? Thanks in advance.

Desmogod

Fri May 19 2006, 03:45PM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

I have used non pulse rated caps for pulse applications before with no worries.
I think problems start to occur if you cycle them too quickly, like in a Tesla coil.
Others will know better, but I would say it should be fine.

The Wumpus

Fri May 19 2006, 04:30PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

Yes, I used to parallell lots of power supply filter caps (400V 330uF) for coilguns and never had any catastrophic failures. However this was at a few hundred J max.

Cesiumsponge

Sat May 20 2006, 12:25AM

Registered Member #397 Joined: Wed Apr 19 2006, 12:56AM
Location: Western Washington
Posts: 125

Those should work in a pinch and I think basically everyone that has ventured into pulsed power has used electrolytics on one scale or another due to availability and pricing. Real pulse-rated, rectangular cased capacitors are usually very expensive, or you have to get very lucky (and heavy too).

If you're going big, even those terminals tend to spark (in my experience) so I'd personally suggest using some conductive electrical joint compound to cut down or eliminate that. Perhaps I didn't crank down on my terminals hard enough, but it's very easy to strip aluminum threads and they aren't rated at high torque to begin with (plus aluminum oxide is always pesky).

pulslaser

Sat May 20 2006, 11:59AM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

I used smaller caps (30 x 5700Âµ) of similar construction and they work very well in a railgun. You see the caps behind the ignitron. Like Cesiumsponge I got sparking at some of the contacts as long I didn't use soft tin foil between the studs and the busbars

The Wumpus

Sat May 20 2006, 07:30PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

pulsslaser, that's a very neat setup indeed. I was contemplating sanding away the oxide from the studs and wrapping a few leafs of sheet gold on them just before mounting to the busbars. Might be a bit overkill, but at least that should ensure good contact.

McFluffin

Wed Jun 28 2006, 05:59AM

Registered Member #119 Joined: Fri Feb 10 2006, 06:26AM
Location: USA
Posts: 114

I used lock nuts on the bus bar along with washers on top. I have yet to have any problems sparking on the capacitors. HOWEVER, I have had several bad exerpiences with sparking between contacts. Even when I was using some silver plated copper contacts, it still would eventually get dirty or something and start sparking. I found that having things connected in at least two places, even if weakly in the second, seemed to help a lot.
It also seems to me that running a 10kV railgun like I see some do is not very well suited even for several kJ. Were you getting near complete discharges on your caps? I usually only had less than a hundred joules left even after a several kJ shot on my mini railgun. You can see in the attached pic where I took damage from firing.

Self Defenestrate

Tue Jul 04 2006, 05:42AM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

Look fine to me. As for oxide build on terminals, I always polish my contacts with a buffing head on my dremel. Crank it tight, and you should have no problems whatsoever with contact.

Sulaiman

Tue Jul 04 2006, 09:18AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For pulse discharging the currents are very high (hopefuly)

The busbars on the capacitor bank receive large mechanical impulse forces
due to the magnetic field caused by the current pulses.

Check the tightness of the connections frequently to avoid sparking.

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