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finding the polarity of a capacitor

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Shaun

Thu May 29 2008, 07:27PM

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

Teslacoolguy, that capacitor is not pulse rated. I have 2 of the same brand and series (except .5uf, 10kV), and I sent an email to Plastic Capacitors awhile back asking them that same question. They said no, it is a dc blocking capacitor only. I've used it for some large strobe lights but i dont think it can take a whole lot of current, and especially not in TC use.

Oh and solidacid, defib capacitors are non-polar, usually have a rating of around 2-4kV. Capacitance could be in the hundreds of microfarads. Usually theyre around 400J devices. They are in no way pulse rated either.

J. Aaron Holmes

Thu May 29 2008, 07:41PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

quicksilver wrote ...

I am wondering (rhetorically) what would be the physics of the explosion.....If something of the nature of a capacitor popped it would be different than material such as a deflagrent, which burns, produces gases and the gases build with increasing rapidity against the case wall until they could no longer be contained (i.e. black powder, etc), would it not?

Yeah, I'm sure it would be quite different. With a cap like this, I would definitely be more worried about ceramic bushings, bolts, etc., flying off than about the explosion itself. Castor oil is not fuel or gunpowder, after all. In the case of the blue cap above, I believe that arcing vaporized and ignited some oil, but the fire ultimately starved itself for oxygen. After this happened, the capacitor was very, very hot. Oil that had gotten on the outside of the can continued to vaporize, and the paint continued cracking and flaking off, not just from the initial deformation of the can, but from heat.

Cheers,
Aaron, N7OE

teslacoolguy

Thu May 29 2008, 10:27PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Well i wasent going to dream to use it in tc use but i was just going to lash up a potato exploder

but nothing really high power just something fun to show some non ingeneering friends. Also were do people find defibs? because i have been wanting one for months and i just cant find one.

Weston

Fri May 30 2008, 12:38AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

The old type defibrillator can be found on the surplus market now that new safer ones a being made. Try Surplus sales of Nebraska.

Shaun

Fri May 30 2008, 01:34AM

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

Will 50J of energy be enough to explode a potato? Ive never tried but I'd like to know because you and I have the same stored energy in those caps, and it something id also like to see.

Firefox

Fri May 30 2008, 03:29AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I think it depends on the voltage Shaun. I can tell you my 60J 300V photoflash bank isn't making any potatoes go pop any time soon. However, I am going to take a guess and say that that dangerous looking box is up to such a task.

teslacoolguy

Fri May 30 2008, 03:43AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Well lets put it this way if my hand was the potato, after discharging 1ka at 5kv through it i dont think i would have a hand left

Solidacid

Fri May 30 2008, 06:58AM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i got my caps from a trashed semi-broken defibrillator Laying next to a dumpster :D

Hhhhhhhhh

Fri May 30 2008, 12:46PM

Registered Member #1340 Joined: Thu Feb 21 2008, 12:47AM
Location:
Posts: 23

Ive had big cap discharge some of its cap.. through my hand and it made my fingers try and touch my ellbow. be careful

never seen a metal capacitor go pop just leak.

Well I had a metal capacitor pop today

I was trying to crush a can. the lowest voltage capacitor pop out the side, got on video here it is slowed down.

Quantum Singularity

Fri May 30 2008, 02:30PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

HajiWatts wrote ...

Ive had big cap discharge some of its cap.. through my hand and it made my fingers try and touch my ellbow. be careful

never seen a metal capacitor go pop just leak.

I'd be really careful if i were you, I've always told myself if I ever 'accidentally' get shocked by any of my big caps/transformers I'll get out of the hobby, because I'd be lucky to survive and probably wouldn't be so lucky twice!

The worse failure I've seen was on a Maxwell bigger than mine, it was a 25kJ welded metal case discharge cap by 'Bill the Arcstarter' and Paul whom I met in the late 90's. Looks like his links no longer work on his old website, he had pics of the failure and they even dissected the cap with an air chisel.

the link is a little ways down 'Alien Capacitor Autopsy' but doesn't appear to work anymore. As I recall there was an outward bulge about the size of a softball in the case. But if memory serves I don't think it even ruptured at all but I can't remember for 100% certainty. But still, just to cause such a bulge had to been several kJ of energy probably >10kJ... I don't remember what kind of charge they had when it failed. But I am sure the casing on the 'energy discharge' rated Maxwells are going to be a bit stronger than your typical non-pulse rated cap. The failure Bill & Paul had was ultimately an internal short which would have done its damage very quickly in one big internal blast, not heat up over a period of time like in TC usage. I think they were quarter shrinking at the time. But I agree that it is always a good idea to have extra safety precautions for flying fragments, boiling oil spray, burning oil, etc.

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