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EvilTesla-RG

Wed Nov 12 2008, 10:48PM

Registered Member #1523 Joined: Sat Jun 07 2008, 02:05PM
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Posts: 97

I am currently designing my seconed Tesla Coil. And I always have trouble desiging the capacitors.

So I have two basic questions. and I realy hope that I get at least one intelligent responce.....

Is the plastic sheating (found in round rolls, measured in mills (millionths of an inch I thingk). Such as the most common is 6 mill) Is it Polyethaline, or Polyehtaline Teraphtalate. Becouse these two materials are vastly differnt in design considerations.

And my more important question, how many layers of 6 mill plastic will I need for a capacitor in a SGTC powerd by a 15 kv NST?

I would realy like a difent answer, or a referal to a definite answer, and not a list of equations that might or might not match actully experience.

rp181

Wed Nov 12 2008, 11:33PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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isnt mills thousands? (millimeter)

Firefox

Thu Nov 13 2008, 12:18AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I've used 4mil (mil is indeed thousanth of an inch) PET overhead slides in 10kV tesla coil operation. A moderate estimate of DC breakdown voltage is around 3kV/mil, or about 120kV/mm.

rp181

Thu Nov 13 2008, 02:56AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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how can we say what plastic it is with no details? Can you provide a link? If you insist on making you own capacitors, teflon film from mcmaster-carr will work great. For each mill, it can stand atleast 500v (DC).

Arcstarter

Thu Nov 13 2008, 04:01AM

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

You can just go to a hardware store and buy the polyethylene sheets that you use for covering a floor while painting. Look here on tesladownunder

and you can make them with that. I tried it and it did stand up to a 12kv nst but a square that was about 1 square foot only gave me something like .5nf.

EvilTesla-RG

Fri Nov 14 2008, 08:25PM

Registered Member #1523 Joined: Sat Jun 07 2008, 02:05PM
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Posts: 97

Thanks for all the replies.

you can just go to a hardware store and buy the polyethylene sheets that you use for covering a floor while painting

That was the stuff I was talking about.

Is it Polyethaline? or Polyethaline Teraphtalate?

I like the idea of PET overhead slides, but I can't find them anywhere.

Mills being thousans, not millionths makes alot more sence!!

I do insest on makeing my own caps, I have neither the money, nor the soldering skill required for an MMC, or anthing simaler.

Alright, please tell me if my calculations here are correct, that would help me alot.

Assume: NST of 15 Kv. Material of PET, breakdown voltage of 3 KV a mill, 4 mill a sheet (thanks firefox)

since it is AC, and breakdown voltage is DC, half to break down voltage from 3 kv/mill, to 1.5 kV/mill.

so 15/1.5= 10.

So ten mill per layer. 10/4= 2.5

Rounding up, I'd use three sheets per layer, is this correct?

Hazmatt_(The Underdog)

Fri Nov 14 2008, 08:58PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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Posts: 1735

I think you're forgetting a few important factors for your cap.

1. TC ringdown is > ~ 2xVpk because as the capacitor exchanges its energy with the inductor the voltage swings from + to - in its oscillation. Example, if you are charging to peak, your peak value is 15KV. That means in ringdown your cap will fully reverse its charge from +15KV to -15KV, or 30KV peak swings. So 15KV rating does not work here.

2. Safety margin! You may want a 10KV safety margin built into your cap to make it last longer. Now your rating is for a 40KV cap.

3. You will have to do some soldering with the cap, and possibly close to the sensitive dielectric, so you might want to practice a bit.
The cap will require a fairly sizable bus to handle several hundered amps during ringdown, so you may have to solder the buses onto the end foil of your cap as your lead outs.

4. Aside from all of that junk, you may also want to pump down your cap, although I had a great deal of trouble with this one. At the very least let it sit for a few days under oil before testing.

EvilTesla-RG

Sat Nov 15 2008, 12:23AM

Registered Member #1523 Joined: Sat Jun 07 2008, 02:05PM
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Posts: 97

Ah! thanks for all the info Hazmatt.

1. TC ringdown is > ~ 2xVpk because as the capacitor exchanges its energy with the inductor the voltage swings from + to - in its oscillation. Example, if you are charging to peak, your peak value is 15KV. That means in ringdown your cap will fully reverse its charge from +15KV to -15KV, or 30KV peak swings. So 15KV rating does not work here.

That is why I cut the breakdown in half.

. Safety margin! You may want a 10KV safety margin built into your cap to make it last longer. Now your rating is for a 40KV cap.

AH! thanks! this is why I put those calculations here

3. You will have to do some soldering with the cap, and possibly close to the sensitive dielectric, so you might want to practice a bit.
The cap will require a fairly sizable bus to handle several hundered amps during ringdown, so you may have to solder the buses onto the end foil of your cap as your lead outs.

4. Aside from all of that junk, you may also want to pump down your cap, although I had a great deal of trouble with this one. At the very least let it sit for a few days under oil before testing.

I was thinking I would build my cap as vertical plates. they would be compressed and placed in oil (verticle so they don't need a pump-down, just some tapping to get out the bubbles)

long Tinfoil tabs would lead to to bolts, which lead to the outside of my capacitor, where my wire will be connected.

Thanks very much, this is what I needed to hear.

last thing I need to know is where can I get these magical PET 3 mill transparency sheets. Are these the normal transparency sheets bought at any office supply store (If so, then what type?)

Also, what is the dialectric constant of this stuff? (Is the material P.E.T.?)

EvilTesla-RG

Sun Nov 16 2008, 06:27PM

Registered Member #1523 Joined: Sat Jun 07 2008, 02:05PM
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Posts: 97

Does anyone have any info on PET transparencies?

I did some reaserch, if I am right, most transparencies are Cellulose Acetate

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