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REALLY Strange capacitor issue.....

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Hon1nbo

Sun Jul 15 2007, 08:27PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

Ok, I am using a 6Kv, 30 mA Neon transformer, and EVERY CAP I buy, or make, ends up with a problem..... But the latest is weird, I was testing the dielectric strength of some plexi, and it wouldn't hold until four sheets, THEN it would "pop", and not do anything, and that was with a sheet of aluminum foil underneath, and a wire tip on top. But then, as I was adding foil on top, I noticed that more foil meant longer "pops" and longer duration. Now I understand this, but even when I used an Air Gap on the Tesla Coil, SHORTER than the plexi thickness, it still goes through the plexi! Any ideas why?!?!?!?......The first cap i bought shorted, then the replacement shorted (and these are rated 10 Kv), then the wine bottles were not enough, and now this : - P

GreySoul

Sun Jul 15 2007, 09:31PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

Once your plexi has failed once it will have a small (you might not be able to see it) trail of fractures and carbon build up... basically a short.

plexiglass really isn't a great HV dielectric in my experience. I used some as an insulator for Kirilian photographs once and got bit pretty bad when it failed - now I use 4 sheets of 8 mil polypropylene film from a plastics supplier, no problem at all.

On your other caps... do they fail when you over voltage them? Unless they're marked for an AC voltage you can usually only put about 50% of the DC voltage across them before they'll fail.

-Doug

Sulaiman

Sun Jul 15 2007, 09:46PM

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

Obtaining a good primary capacitor is not trivial;
ALL of my DIY attempts have failed, some immediately,all eventually.
NONE of my MMC using foil/film polypropylene capacitors has failed.
I have only used Arcotronics A.72 range (RS in uk)
and Philips/BC components 376-series
but I see other types being used successfully.
A good MMC is a good investment.

I doubt that your commercial capacitors were 'killed' by over-voltage
The types that I mentioned can be charged to full rated voltage in typical SGTC use,
and are tested to 1.5x to 2.5x rated in the factory.

More likely the high frequency/current caused localised heating.

Avoid PET/Mylar/plastic caps, most metalized caps, and ceramic capacitors other than 'transmitter-type'

Hon1nbo

Mon Jul 16 2007, 04:24AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

thanks

Hon1nbo

Sat Jun 14 2008, 01:29AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

yes, again this is from the dead, but I thought I might post this little update: when I did this I used a homemade inverter since I wasn't allowed to use the school mains: but I recently dug it up, and as I was testing, it turns out it was no longer at 60Hz, and after doing some testing, the change had induced some resonance in the NST (weird I know, but it might also be the modified sine wave that I used) and it was outputting more than 6Kv (after discussing it with my teacher, it was estimated to be around 15Kv)
dont know if that theory holds, or it was mistranslated (the thing was french and the ratings were in words and not numbers for some reason)... I am moving to a mini-7Kv transformer from amazing1.com... battery powered.

Arcstarter

Sat Jun 14 2008, 04:03PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

DaJJHman wrote ...

I am moving to a mini-7Kv transformer from amazing1.com... battery powered.

That will not work for tesla coils. They run at too high frequency for a capacitor to charge unless it was a really high current transformer or small capacitor. And the spark gap would not be able to quench fast enough to trigger at a few khz.

flannelhead

Sat Jun 14 2008, 04:07PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Is it MINIMAX7? I think it will be able to run a Tesla Coil. Just rectify the output with some high frequency high voltage diodes. People have used DC flybacks for powering TCs, so why couldn't that power supply work, too?

Hon1nbo

Sat Jun 14 2008, 04:11PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

NOT minimax7, it is the universal driver on this page: http://amazing1.com/hv-hf-power-supplies.htm

it's part number is gradriv1, but I am going to try that full circuit, and the transformer from it on a 555 timer (which I have done, and it works for a coil... but I never improved efficiency, and I scrapped it... I really cant keep more than one coil around at a time, so I cant experiment much... school space etc etc)

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