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4hv.org :: Forums :: High Voltage
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finding the polarity of a capacitor

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teslacoolguy
Thu May 29 2008, 03:27AM Print
teslacoolguy Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
I have this high voltage discharge capacitor sitting around dooing nothing and i looked at it today and noticed that there was no indication of the plus and minus terminals. I am pretty sure (correct me if i am wrong) that it is dc bipolar but if it is not how can i find what the polarity is?
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rp181
Thu May 29 2008, 03:31AM
rp181 Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
its probably not polarized, but if it is, Diode function on a multimeter might work.
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Weston
Thu May 29 2008, 03:37AM
Weston Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
If it is high voltage and pulse rated it is probably not a electrolytic cap(only they are polarized). Also shape might help determine, electrolytics are generally cylinder shaped.
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teslacoolguy
Thu May 29 2008, 03:44AM
teslacoolguy Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
pictures can say 100 words
1212032648 1107 FT46130 Cap
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...
Thu May 29 2008, 05:15AM
... Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
That monster is not polarised.

It should even be good for a fair bit of average power running through it smile
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J. Aaron Holmes
Thu May 29 2008, 06:04AM
J. Aaron Holmes Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Doesn't look like it's meant for pulse discharge, however. The terminals would be much fatter. And as I mentioned in another capacitor thread last week (or maybe the week prior): I'm very uneasy about pulse power experiments using these sort of metal- or glass-encased caps. If you don't know what you're doing, or "push" things, they're as good as grenades. Certainly I would never dream of using this cap in a Tesla coil (well, unless it was a nightmare wink)

Lest anybody assume that high voltages are the only immediate danger in our hobby, below is a picture of what could have been a fatal accident had it not been noticed pretty quickly. This was the primary cap in a friend's coil a couple of years ago. I was standing very near when it started spouting boiling oil and we both ran for our lives suprised Notice how the sides and ends are bulged out. It was even more of a balloon before it cooled down and finished letting out most of its oil. This was a pulse-rated cap, and we suspect the failure was internal arcing caused by resonant overvoltages in the primary, which may in turn have been the result of the RSG misfiring during tuning.

Cap Balloon Thumb

...which is why I like stuff in plastics these days! smile

Play safe!!

Cheers,
Aaron, N7OE
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Solidacid
Thu May 29 2008, 07:16AM
Solidacid Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102
i got 6 capacitors from a defibrillator i took apart a few months ago, i havent tried charging them since they have no ratings or polarity markings and because they are scary cry
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Quantum Singularity
Thu May 29 2008, 01:22PM
Quantum Singularity Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
J. Aaron Holmes wrote ...

Doesn't look like it's meant for pulse discharge, however. The terminals would be much fatter. And as I mentioned in another capacitor thread last week (or maybe the week prior): I'm very uneasy about pulse power experiments using these sort of metal- or glass-encased caps. If you don't know what you're doing, or "push" things, they're as good as grenades. Certainly I would never dream of using this cap in a Tesla coil (well, unless it was a nightmare wink)

Lest anybody assume that high voltages are the only immediate danger in our hobby, below is a picture of what could have been a fatal accident had it not been noticed pretty quickly. This was the primary cap in a friend's coil a couple of years ago. I was standing very near when it started spouting boiling oil and we both ran for our lives suprised Notice how the sides and ends are bulged out. It was even more of a balloon before it cooled down and finished letting out most of its oil. This was a pulse-rated cap, and we suspect the failure was internal arcing caused by resonant overvoltages in the primary, which may in turn have been the result of the RSG misfiring during tuning.

Cap Balloon Thumb

...which is why I like stuff in plastics these days! smile

Play safe!!

Cheers,
Aaron, N7OE


Aaron, just curious (I know you have said about the metal cans in the past) but have you actually seen a metal cap grenade? You show pics of a rupture, and I have heard of many instances and also seen some huge caps fail like this, but only bulging or slight rupture, which IMO is a credit to the metal casing. I've never heard of a cap failure on a thick metal cased cap actually fragmenting violently before. I ask because I have some of the big welded metal maxwells, and I have seen some even bigger maxwells blow before but the case has always contained the failure.
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J. Aaron Holmes
Thu May 29 2008, 04:13PM
J. Aaron Holmes Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
I have not had a metal-cased cap blow up on me, fortunately. At the same time, I can't believe that a capacitor whose geometry is significantly altered by a failure was designed with much thought given to how that particular failure mode would be handled *by the cap itself*. These things are usually mounted in large metal cabinets and/or away from humans when operated "in the wild". So while I must concede that I have not witnessed a "capacitor grenade", I also do not believe in the safety of these units much, high voltages aside. Certainly not in close proximity to humans, much less amateurs. To contrast, the plastic Maxwells just pop with a dull thud long before the 100's or 1000's of PSI develop that would send debris flying. I have witnessed those failures too.

Cheers,
Aaron, N7OE
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quicksilver
Thu May 29 2008, 06:21PM
quicksilver Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
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?

Superficially, it seems that the oil, etc would heat and expand. This may also produce a very dangerous situation, no doubt. However it seems that this may not have the same expansion speed as a contained explosive....

But perhaps I'm missing something....Perhaps the oil (or whatever) would not respond in that way but even more violently in a manner unique from common gas producing compositions.....(?)
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