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First tesla coil thread - Dual Mot

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Hydron

Tue Oct 22 2013, 06:28PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

600A is the peak current, not RMS. You'd normally use RMS to specify the current over a long period, not just one burst, so it will be much lower (and related to the break rate).
They both matter, but stress different parts of the capacitor. If your connection to the foil is good it should handle the current fine. Note that if 160 microns is the skindepth then your foil is likely thinner and not limited by it, though you can't just use a simple skindepth number for anything other than an isolated round wire.
I'm not sure how the litz wire suggestion relates to capacitors, but while it would be useful for connections in the tank circuit I don't think anyone uses it once they find out the price! I just have some 16 mm^2 welding cable for my DRSSTC primary connections (nice and flexible, good silicone insulation) - it gets a bit warm with long runs but not enough to matter. The primary also gets warm - skin effect certainly hurts here!

MrFlatox

Fri Oct 25 2013, 08:03PM

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

Hi !

I did some new tests with different dielectric, and it's a fail...

This time I used 2 layer of PP foil which gives 140 Âµm insulation thickness. The capacitor lasted about 30 seconds across the spark gap without coil before it burnt. As a sidenote, the PP foil I used was not "cristal clear" it was a slightly textured plastic. I do not know the influence of this texture in the capacitor other than adding a small amount of air. When the rotary spark gap was firing, the whole capacitor was glowing purple due to corona. After a 15 seconds run, this capacitor was as warm to the touch as the ones made with OHP sheets.

I read somewhere that it is bad to test a capacitor just across the spark gap without the primary coil, because it puts a lot of stress in the capacitor. I do this because I think that if the capacitor can withstand this kind of treatment, it will survive wired in a tesla coil right ?

I can not get a design that at least survive more than 30 seconds... what a shame... What is the problem with all the designs I tried ? Too little insulation thickness ? Wrong dielectric choice ? Lack of oil ?

I looked at MMC, and I do not want to throw atleast 150 euros to get a capacitor for the coil...

Kizmo

Fri Oct 25 2013, 08:52PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

There are two issues:

1.) Plastic sheets used in real capacitors are different grade and go through intensive quality control to make sure that there are no defects. Normal PP foil or OHP sheet may seem like water/air proof but it will more than likely have impurities and microscopic defects that will cause it to fail quickly as capacitor dielectric.

2.) As you guessed: air. Any air space will work as place for ozone and heat to be generated by corona discharge.

Many coilers have tried home made capacitors and in the end it turns out that MMC would have been the cheapest and by far most reliable option after all.

EDIT:

I have pile of used but still perfectly good 942C 150nF 2kV caps laying around doing nothing. I could sell them for reasonable price and since we both are in EU region, postage wont be too bad and there are no import fees ;)

Ash Small

Fri Oct 25 2013, 09:35PM

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

Oil should stop the corona, especially if you can get it under some sort of vacuum, it doesn't need to be much.

Personally, I'd use a dielectric with a smooth, even finish.

teravolt

Sat Oct 26 2013, 01:59AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

One of the first home made caps I made was out of sheets of 1/4" glass and aluminum foil. it may be pausible to use 1/16"th glass. I used a mix of motor oil and paraffin wax mixed together. I worked great to prevent tracking around the glass. At the time this sounded like a good idea but it was a little messy but it worked. if you could fit the glass in a glued acrylic box or plastic container and fill with mineral oil to prevent tracking I think it would work well. it will be more capacitance than the beer bottle caps because of the increased surface area. You could try a comical cap like

MrFlatox

Sat Oct 26 2013, 08:39AM

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

That makes sense that corona is causing those heating and failing problem.

As for the oil, It should solve the corona problem, but I will still have a non-professional plastic grade that is not evenly thick, have some weakness, inclusions, impurities...

Ash Small

Sat Oct 26 2013, 09:54AM

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

I noticed a couple of weeks ago that my local hardware store (well, Homebase) is selling sheets of ~1-1.5mm thick polystyrene sheeting.

I've not tried it yet, but polystyrene is one of the best dielectrics you can use. Obvoiusly this isn't 'capacitor grade' polystyrene, but it does look to be of reasonable quality.

This approach could end up being expensive (read 'waste of money') if it doesn't work, but I'd try using oil with your existing design, and see what happens.

I'll be making some more soon, so I'm interested in any progress you make.

All you really need to do is stop the corona, and then ensure that the dielectric is thick enough to withstand the voltage, although imperfections in the dielectric could lead to 'some' failures.

EDIT: using extra layers of dielectric will 'neutralise' some of the imperfections, but will lead to a larger capacitor.

MrFlatox

Sat Oct 26 2013, 10:10AM

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

I actually don't have any oil that would be appropriate to make an oil capacitor, neither I have no vaccum pump. I do not want to buy all these things if it's a dead end.

1mm to 1.5mm is really thick, I imagine the capacity obtained with this would be very low, and you would need an enormous amount of plastic and conductiong foil to achieve decent capacity, it might not worth the time and effort involved.

Using multiple layer of insulator "may" cancel out imperfections, but they will still be there and poses a threat to the reliability capacitor reliability.

Ash Small

Sat Oct 26 2013, 02:45PM

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

cheap cooking oil is ok to use if you boil it first, to drive off any moisture.

I think an old fridge pump can be adapted for vacuum use. Plastic tubing and silicon will seal it. You don't need much vacuum.

Even without a vacuum some oil should reduce, if not eliminate the corona.

What gap are you using?

MrFlatox

Sat Oct 26 2013, 03:12PM

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

What gap ? Spark gap ? If so, I am using the rotary spark gap that would normally drive the coil.

Here are pictures of the actual setup :

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