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Ash Small

Sat Oct 26 2013, 03:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

And what is the gap, in millimetres? (or gaps, in your case)

MrFlatox

Sat Oct 26 2013, 04:02PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

The gap is set to about a millimeter on each side.

Ash Small

Sat Oct 26 2013, 04:23PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

The gap is set to about a millimeter on each side.

I've run static gaps at more than twice that gap with hand rolled PE capacitors with no oil or vacuum.

Mine was powered from a DC flyback, though, and only fired once the voltage across the capacitor was sufficient, but they withstood higher voltages with no visible corona.

Can you post some photo's of your capacitors?

redruM69

Sat Oct 26 2013, 05:24PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

Careful!
I used an acrylic rotor in my spark gap initially, but after about 30 seconds of running the electrodes got hot and melted the acrylic. Luckily I caught it in time. Spinning @ 8k+rpm, those electrodes will fly out with considerable force!
Phenolic plastic sheet is the material of choice.

MrFlatox

Sat Oct 26 2013, 05:35PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

The disc is not made out of acrylic, it's lexan, a rather tough plastic. The disc can withstand the full rpm of the angle grinder (even if I will never go up to 5krpm in use). However, I am not sure of how this would come up with the heating of the electrode.

What do you mean by picture of a cap ? How they are made ? a burnt capacitor cracked open for analysis ?

Ash Small

Sat Oct 26 2013, 07:39PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

What do you mean by picture of a cap ? How they are made ? a burnt capacitor cracked open for analysis ?

I was wondering just how you roll them, wrap them?

It's just I never had the same issues with corona with mine, even at flyback voltages, which are generally higher than the voltage you've stated above.

MrFlatox

Sat Oct 26 2013, 09:48PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Indeed, i also found it weird to have corona issue only at 4200vac. On the PP cap, the corona flashing was occuring when the capacitor was discharged through the gap. Corona is definitely occuring because the caps smell ozone after being used.

As for the construction, i used a 4cm clearance on the side of the plates, and roll as tight as i could around a pvc tube. I then secure the rolled cap with duct tape.

I already made some rolled capacitor for a 40kv dc flyback a few years ago, and they still work fine with no failed cap, and no such ozone problem. Would this be caused by the fact that this is DC whereas my tesla coil caps are seeing AC ? It's strange...

In a desperate try, I made 4 beer bottle caps just to see how many of these would be needed to achieve 130nF, and the number i found was a joke : 174 bottles.
It definitly does not worth the time and effort to make those to eventually get a poor efficiency and a 100+kg capacitor bank.

twist2b

Sun Oct 27 2013, 08:23PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

For anyone that is interested, I've found a couple useful sites, but I really like this one for a good explination:

I'm also trying to make my own capacitor. I bought Polycarbonate:

and if you read the electrical details on wiki:

Good permittivity and dielectric strength of 15â€“67 kV/mm.
With What I ordered with one sheet I could have a minimum of 30kV (cross fingers) and dial in the capacitance with parallel plates. :D
(I am building with a 15KV neon transformer probably...)

I guess I still have to worry about carona though so what kind of oil should I use? I guess some googling will tell meh.

Ash Small

Mon Oct 28 2013, 11:00AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Pharmaceutical grade mineral oil seems to be what most people use, and is apparently quite cheap.

Cheap cooking oil can be used, but should be boiled first to drive off any moisture (probably applies to mineral oil too)

I read recently that coconut oil is supposedly better than the 'professional' transformer oils .

MrFlatox

Mon Oct 28 2013, 02:44PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi,

I am now trying with plastic bottles capacitor instead of glass bottles and it turns out to be much more reasonable, because I would only need approx 25 bottles to achieve 130nF.

With glass bottle capacitors, it's said that their main drawback when it comes to efficiency is that the glass is lossy. But with PET bottles, would it be better ?

Because there is no sharp edges due to water, corona might not be such a problem anymore. I plan to go with pressure rated bottles like soda bottles because the plastic is thicker. The lossiest part of the cap would be the "electrolyte" ? Conductivity wise, is it better to use sodium hydroxide instead of salt to make the water conduct more, and therefore reduce the liquid conductivity losses ?

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