# Any Tesla Coil fans out there?

Discussion in 'General Electronics Chat' started by DerStrom8, Feb 16, 2011.

1. ### DerStrom8Super ModeratorMost Helpful Member

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Hello, everyone!
I'm working on building a medium-sized Tesla coil that uses a 9kV 60mA NST for a transformer. It is to run on 120 volts, 60hz U.S. mains. I found an equation that determines the size of the capacitor I would need for that transformer, which is:

C=10^6/(6.2832 x Z x Fl)

My question is this: I am thinking of changing the entire coil over to DC (rectify the output from the NST) because the capacitor I hope to use is rated for DC and not AC. I believe, though, that this equation is made for a sine-wave AC power source, so I am wondering how the values would change if I were to use DC. Does anyone have a formula for a DC tesla coil to match a capacitor to the transformer?
Der Strom

P.S. I have also found some websites that say to never use a DC capacitor for a tesla coil, as the charging process in the tank circuit requires the polarity to constantly reverse itself. Is there any validity to this claim?

Last edited: Feb 16, 2011
2. ### BrownOutBanned

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Where does the eqn come from? What is "Z"? What is "Fl"? Tesla coils don't work in DC, so the cap must be able to operate in an AC environment. You cannot use polarized capacitors.

3. ### DerStrom8Super ModeratorMost Helpful Member

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Thanks for the reply, BrownOut. Check my edit (the P.S.) of my first post.
The equation comes from DeepFriedNeon - Tesla Coils .
I apologize for forgetting to specify the variables. The "Z" is the voltage divided by the current (E/I) and the "Fl" is the line frequency.

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5. ### BrownOutBanned

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Not sure why they use that equation. Looks like they want to make the reactance of the capacitor a tiny fraction of the E/I figure of the transformer. Guess that keeps the power dissapation low. Anyway, a polarized capacitor cannot be used for the tank circuit. Although there are quote/unquote "DC" tesla coils, that only means the power comes from a DC source. However, the primary circuit is still AC, and so a non-polarized capacitor must be used. Good thing you check these things out

Last edited: Feb 16, 2011
6. ### crutschowWell-Known MemberMost Helpful Member

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The output of a Tesla coil is a high frequency, high voltage waveform. Typically this is done by resonating the primary inductance of the output high voltage transformer with a capacitor. This capacitor obviously must handle AC voltages.

The power source for the oscillator is DC, which can be rectified from the AC power line, but the oscillator output is high frequency AC. An NST transformer operating at the line frequency can generate high voltage but it will not be a Tesla coil, which requires high frequency as well as high voltage.

7. ### DerStrom8Super ModeratorMost Helpful Member

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Right. That's what I had originally assumed, but I wanted to be sure. The search for a TC capacitor continues....

I'm not quite sure what you are saying here. I know that the NST is not a Tesla coil. I was just asking about rectifying the output so that the power source is DC, and what effect that would have on the capacitor-to-transformer match for the tank circuit. Let me know if I'm not making any sense here
Der Strom

Last edited: Feb 16, 2011
8. ### tcmtechWell-Known MemberMost Helpful Member

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I don't see why you are having such a hard time finding a HV non polar capacitor. eBay has pages of listings for them and many are surprisingly cheap.

Its where I have bought all of mine.

9. ### DerStrom8Super ModeratorMost Helpful Member

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That is really strange.... I have always had trouble finding cheap, decent high voltage capacitors that would be suitable for Tesla coil applications.
I guess I'll have to take another look!
Thanks a lot!
Der Strom

Last edited: Feb 16, 2011

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For what it may or may not be worth back in '64 Popular Electronics did a few Tesla Coil articles called Big TC and Lil' TC. I built the big tc and with some tweaking finally got it working. I used a 15 KV 30mA neon sign transformer. The capacitors were hand made and I remember using copper foil snanwiched between plates of glass all mounted in wood. Each cap I used 3 plates of glass, I remember that much. I found this old refrence to the article on Big TC. If you google enough you might find more on the article(s). However, the caps were home brew with trips to the hardware store.

The Lil' TC was sort of cool and useds a vacuum tube (valve) as an oscillator driving an old B&W TV flyback transformer.

Ron.

11. ### crutschowWell-Known MemberMost Helpful Member

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Yes, I got confused about what you were saying about AC and DC. If that cap is for smoothing the rectified DC from the NST transformer than you can use a DC rated cap. If the cap is part of the high frequency tank circuit, then it must be AC rated.

12. ### DerStrom8Super ModeratorMost Helpful Member

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Ah, sorry. Yes, the cap I was talking about would be part of the tank circuit. I think I might just keep my eyes open for a high voltage AC capacitor. It'll be a lot simpler if I can just keep everything AC (In other words, forget about rectifying the output from the NST).

Last edited: Feb 16, 2011
13. ### nsaspookWell-Known Member

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I made a tube drive coil once out of junk transmitter parts very similar to this one.
Ross-O's Tube Coil
Not a project for the timid when you have 2 kV DC on the plate of a 4CX1000.
Eimac 4CX1000 Ceramic Tetrode Tube Data Sheet

14. ### gary350Well-Known Member

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You can not use DC on a Tesla Coil unless your building a tube type TC.

Tube type tesla coils do not work as well as AC type tesla coils.

You need about .005 uf per 15K 30ma neon sign transformer.

You can roll your own capacitors or buy them. You can not use just any old capacitor. Buy some of these. 0.15uF 3kV High Voltage Capacitor HV Tesla Coil Ham - eBay (item 150307333292 end time Feb-18-11 18:01:59 PST)

You mount the capacitors in a plexaglass sheet so they are connected in series to get a voltage rating double the power supply voltage. You put a quantity of these capacitor banks in parallel to get the UF rating you need.

Built a Variable Speed Vacuum Fan Spark Gap using a vacuum cleaner motor and a variac for speed control.

Wind a flat plate primay coil using 1/4" refrigation type copper tubing 1/2" spacing center to center with 2.5" clearance between the primary coil and the secondary coil with about 16 to 18 turns on the primary.

Secondary coil is 950 turns of #24 enamel coated copper wire with 2 coats of polyurethane varnish to glue all the wire in place. Leave 2" of PVC pipe at each end with no wire on it. The PVC pipe needs to be 4" pipe it actually measures 4.5" on the outside diameter.

You need 9 spark gaps made from 3/4" diameter by 2" long copper tubing spaced.030" between each piece. The vacuum fan cools the tubing and brakes up the sparks.

http://i67.photobucket.com/albums/h292/mikeweaver/gap1.jpg

http://i67.photobucket.com/albums/h292/mikeweaver/gap2.jpg

http://i67.photobucket.com/albums/h292/mikeweaver/gap3.jpg

http://i67.photobucket.com/albums/h292/mikeweaver/gap4.jpg

http://i67.photobucket.com/albums/h292/mikeweaver/gaps5.jpg

This is my 4" TC

This is an early model 4" and 6" TC. The 6" has 3 neons it makes 5 ft sparks.

This is my 10" TC 950 turns of #18 wire on the secondary. It makes 12 foot sparks.

This is the correct way to connect your capacitors.

http://www.hvtesla.com/mmc.html

Last edited: Feb 16, 2011
15. ### DerStrom8Super ModeratorMost Helpful Member

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Thanks, Gary. The real problem I have at this point is cost. If I were to do everything you suggested, the costs would add up extremely fast!
I found these capacitors on ebay, and I was wondering how they would work:

6 high voltage capacitors .005 uf, 15000 VDC oil filled - eBay (item 110648287300 end time Feb-19-11 08:04:03 PST)

I believe four of them in parallel would give me a value of 15kV and .02uF, which is pretty much what I need. Would this type of capacitor work?

16. ### gary350Well-Known Member

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Those are DC capacitors they will not work.

Most AC capacitors will not work. The Tesla Coil group have been experementing for many years with MMC capacitors. Only certain caps will hold up. I did some experementing too the caps I experemented with lasted about 3 seconds and exploded.

You can buy used neon sign transformers for \$10 each at a sign shop they will not resell use equipment because it has no warrenty. You can some cheap ones on ebay too. Get a 15K either 30 ma or 60 ma.

You can get free plywood from wooden wire reals at any industrial supply company trash dumpster.

Refrigation copper tubing is much cheaper than water line copper tubing.

Get some #24 enamel coated copper wire on ebay or buy some at your local electric motor repair shop.

You can get free PVC pipe and wood from the dumster at construction sites.

You need to be a pack rat and a junk collector to save money.

Last edited: Feb 17, 2011
17. ### killivoltWell-Known Member

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If you use 4 in Parallel the Voltage shouldn't change 4 of them would give .02uf @ 15kv

However, I don't know how it works in a tank circuit.

I hope I'm not steering you in the wrong direction? With lower Voltages I know I can go up a bit in Voltage without any problems; you just can't change uf value.

kv

Edit: Looks like a really good buy though.

Last edited: Feb 17, 2011
18. ### DerStrom8Super ModeratorMost Helpful Member

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I didn't realize those were DC caps. I guess I'd better read a little more carefully
I already have two salvaged NSTs-- the 9kV 60mA that I mentioned before, and a 15kV 30mA that doesn't like to be connected to a capacitor (some sort of automatic shut-off or something keeps, well, automatically shutting it off!)
I'll see if I can get a photo of the tesla coil that I have so far. For the primary, at the moment, I have ~10 turns of #12 AWG wire, which I plan to replace with refrigeration tubing, that I can tap at any point. For the secondary, I have #26 AWG wire (~800 turns) on a 4 inch ID PVC pipe, all mounted on a plywood base (two levels--one for the transformer, spark gap, and fan, the other for the capacitors). The topload is aluminum dryer ducting formed into a toroid shape with an aluminum pie plate in the center. Right now, the only thing I'm missing is the capacitor(s).

Always have been, always will be

Last edited: Feb 17, 2011
19. ### DerStrom8Super ModeratorMost Helpful Member

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By the way, I found this Tesla Coil "tutorial" site and they keep saying (under the MMC heading) that they are using DC capacitors. How can they get away with this?

20. ### BrownOutBanned

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Those are AC capacitors. They just don't know what they have.

EDIT: Those DC ratings are just for dielectric breakdown. But the caps are non-polarized. The breakdown probably needs to be derated for AC operation.

Last edited: Feb 17, 2011
21. ### DerStrom8Super ModeratorMost Helpful Member

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Pictures of my Tesla coil

Okay, so I told you all I would post pictures of my TC, so here they are. This is just the basic setup, minus the transformer, spark gap, fan, and (of course) the capacitor:

And yes, that's my cat lying next to the Tesla coil in the first photo

Last edited: Feb 17, 2011