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Ruyck

Thu Dec 27 2007, 02:50AM

Registered Member #1195 Joined: Thu Dec 27 2007, 02:31AM
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Posts: 6

Hi all, this is my first post here so go easy =)

I just bought a lot of 12, .47uF 2KV capacitors, and I'm looking to build my first coil. The ebay lot said they would work for tesla coils but the more research I do... the less I believe the seller. I have a 10KV 30mA neon transformer, and I've tried some calculators online, and i dont think I can get the desired capacitance for resonance. Hopefully I'm wrong. Please tell me I'm wrong

Thanks in advance.

Shaun

Thu Dec 27 2007, 04:38AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Welcome to 4HV!

We need some more info about the caps to say if they really will hold up. Are they film/foil (good)? Or maybe ceramic (bad)? How about some pics?

Now, assuming you are on 120V, 60Hz mains, TeslaMap tells me that for a 10kV 30ma NST you need an 8nF resonant cap, or 11.1nF LTR. All your capacitors in series will only come down to about 39nF, too much for resonance. If you were to rectify your NST (requiring HV diodes), you wouldn't need to worry about the 60Hz resonant value and could simply compensate by tuning your primary. Until then, although these caps may be capable of TC use, you may not have enough to get to a resonant value.

MOT_man

Thu Dec 27 2007, 07:05AM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
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Posts: 139

Yep. Shaun is right on - we need more info on the caps before we can say - yey or neh - who makes these caps? What are they made of?

Ruyck

Thu Dec 27 2007, 02:52PM

Registered Member #1195 Joined: Thu Dec 27 2007, 02:31AM
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Posts: 6

Thanks guys. They are CBB81 Metalized POLYPROPYLENE FILM CAPACITORS from elecsound. And yes I am on 120V 60Hz mains. Another question: If I was to put my 12 caps in series and build a coil disregarding the resonance, what's the worst that could happen?
1227070941small

MOT_man

Fri Dec 28 2007, 07:05PM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
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Posts: 139

....I would be careful ! These capacitors are not designed for pulse discharge - however - poly film is usually pretty durable. I'd run lower current first and then stiffer current second... You will see a performance difference!

Tetrafluoroethane

Sat Dec 29 2007, 07:12PM

Registered Member #127 Joined: Fri Feb 10 2006, 03:36PM
Location: Cincinnati, OH - USA
Posts: 44

Resonance is the entire operating principle of the tesla coil. So the "worst" would be that it wouldn't work at all. Really though, you just have to tune the primary + cap to the secondary + cap (self and topload). So if you can't get any smaller a set of capacitors, you just need a primary with fewer turns. Most of tesla coil design (when you are on a budget anyway) is fiddling with what you can afford to change and just dealing with the rest.

There are several great calculators out there that you could use to play with the numbers. Plug in what you know and fiddle with the rest. Check out WinTesla or JavaTC. Or just search for the formulas for calculating inductor capacitor resonant frequencies. It will be very enlightening either way.

Just one note: While you can probably tune it, that doesn't mean you have made a good TC. Normally you need to calculate the tank capacitor size based on what you are going to use to power the TC. After all, a huge cap will only charge to only a small % of it's total capacity with only a small transformer. Since the smallest value you seem to be able to get with your 12 caps is 39nF (quite large) and you have a 10-30 NST, you fall into this situation.

Personally, I would ditch the caps for this coil. But save them, they might be useful later.

cbfull

Sat Dec 29 2007, 09:19PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

If those capacitors any anything like the maroon colored dipped caps I have, you might be surprised at what they can do. I ran my 15kv60ma tesla coil with of string and I don't recall any heating or signs of breaking down. On the other hand, those might be nothing like the caps I was using.

Jeff Sadowski

Sun Dec 30 2007, 01:42AM

Registered Member #1185 Joined: Thu Dec 20 2007, 04:40PM
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Posts: 17

I'd use a plastic rain gutter as a shield worked good for me when my cap blew.

Ruyck

Sun Dec 30 2007, 03:07AM

Registered Member #1195 Joined: Thu Dec 27 2007, 02:31AM
Location:
Posts: 6

Well I just blew my NST...the arc on it is very weak now, and theres a black pinhole on the NST secondary. I had a problem with the transformer just shutting off the moment i turned the coil on. I think it was overcurrent protection or something. So I started turning an adjustment screw on the NST and something it just blew... quite spectacularly actually, an arc jumped right through the HV insulated wires...oh well. This gives me a chance to do things right. I'm going to buy some of these

and a new transformer. Is there a recommended NST without current protection, maybe something 15Kv 30mA (EBay Link?)? Anything wrong with the caps I chose? And as for those 2Kv .47 uF caps, are their any other good projects I could put them towards?

Some Pictures of what I have so far.
1229072143a

J. Aaron Holmes

Sun Dec 30 2007, 05:25AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

The CDE 942 series of caps is very popular for MMCs, but the 2kV .15uF ones are the most popular by far. That's 942C20P15K, I believe. The ones you chose will work, you'll just need more of them.

As for eBay: I'm sure you can figure out the Search feature yourself!

There are always LOADS of NSTs on eBay. Generally, the older units tend to lack the GFCI, and that's good. Newer GFCI-equipped and/or solid-state NSTs are relatively uninteresting as coiling goes. If you can't tell from the listing what you're getting yourself into, just ask the seller. And if they don't know, just walk away. There will be others, I guarantee it!

Cheers,
Aaron, N7OE

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