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Homemade plate stack capacitor, surface tracking problem

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johnf

Sat Apr 01 2006, 09:14AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Noted in your pictures are the sharp edges and by other obsevant replies used insulator sheets and even worse some with toner on them. In air corona discharge is your enemy at voltages of over 10kV (depending on cleanliness and humidity). Toner (black ) contains carbon and usually a wax binder and will measure as an insultaor at low voltages but as the electric field builds up islands will appear in the field produced by the isolated conductors, as these islands are very small the field irregularity will get to a point where at 1-20 volt per micron emmission will occur to other lesser charged sections. The patterns on your insulators look like lichtenstein patterns as the discharge has traversed across and through the surface layers of the insulators,
If you do not want to try oil to help limit the dischargs try vacuum potting the cap in resin or putting the cap in a pressurised environment of 80% nitrogen and 20% carbon dioxide at 200 PSI ( gas must be dry of course.

Recap
no sharp edges
no air gaps
no used insulators
encapsulated
= good cap

Steve Conner

Sat Apr 01 2006, 10:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The sharp edges of the aluminium foil plates are where corona usually starts. The corona leads to surface tracking and eventually breakdown. That seems to agree with the diagram you drew showing where it arcs.

Kurt Schraner has a good article about this, that he wrote after trying to make capacitors for his tesla coil out of copper-clad PCB material.

http://home.datacomm.ch/k.schraner/pcb_cap.htm seems to be in German only, bizarre, I was sure there was an English version.

You can try folding the edges of each plate over to make them double thickness and slightly rounded. Schraner says rounding the corners is pointless, but it wouldn't hurt to make sure they're not razor sharp points. Vacuum impregnating the whole thing in oil might help too, but that's a much bigger hassle.

Dr. Shark

Sun Apr 02 2006, 09:34AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I finally had a chance to power up my OHP capacitor today. Here is what the setup looked like:

the big white thing is the original cardboard box the OHP sheets came in, I used a brand-new pack.

Conclusion: no arcover. See for yourself

The funny thing is that it did not just puncture one layer, but about 10 OHP sheets had a hole in them! Looks like it is time for a major rebuild.

I have no idea at what voltage this occured, but since the bang was not very loud and it happened a fraction of a second after I plugged in the mains, I suspect it was not very high. Maybe the OHP sheets I was using are of poor quality after all?

EDIT:
I did get it to flash over! I just removed the burnt layers and plugged it in again. It sounded very much like pop-corn and left a series of tracks that unfortunately dont show up well on photographs. Most of them happened along the edges where the connections were made, so they crossed a single path of 4.5cm. Again this presumably happened at fairly low voltage, since no damage happened, and the cap just went on popping.

I guess now it's time to get a vacuum pump and try harder!

The Wumpus

Sun Apr 02 2006, 11:49AM

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

What kind of OHP transparencies are you using, the plain ones for just writing on, or the ones that can handle laser printing/xeroxing? Also what thickness? I used the printer/copier type in my capapacitor, they are different materials since the sheets have to stand higher temperature when toner is fused. Do you know which material your sheets are made of? I suspect mine are mylar or something similar, since they stand very high voltages without punch-through (mylar is about 7kV/mil, or 28kV/0.1mm, which is the thickness of a sheet).

Dr. Shark

Sun Apr 02 2006, 05:06PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

The sheets are for laser copiers, and they are 0.1mm thick. The brand is GB plast. I don't know about the material, but since they melt when they get hot, it must be some kind of polymer.

What is really weird is the following: I was testing for surface tracking distance and placed two strips of Al foil at a variable distance beween two of the films. It only flashed over when they were about 13mm apart. Inside the capacitor, I had them flash over distances of up to 50mm! I have no explanation for this.
My flyback transformer seems to put out less than 20kV peak, since I can just get it to jump around 1cm between wire electrodes.

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