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Questions about my First Tesla Coil

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J. Aaron Holmes

Tue Dec 18 2007, 05:07PM

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

Bomberguy92: Good luck with your first coil!

They're a lot of fun to build and run. We look forward to the results!

bomberguy92 wrote ...

I have some questions about wiring like do you connect the primary to the secondary? I don't think you do but i have read it both ways.

Connecting the primary to the secondary (i.e., the inner or bottom turn of the primary to the bottom turn of the secondary) is a relatively common practice with larger transformers like pole pigs and potential transformers, particularly the wye-connected variety where the HV side consists of a hot and a neutral. But on a 9kV NST with a midpoint ground, you really don't want to do this, as it puts your NST's case and your coil's secondary base at a 4.5kVAC potential difference, which can bring about all manner of nasty surprises!

Cheers,
Aaron, N7OE

bomberguy92

Tue Dec 18 2007, 11:00PM

Registered Member #1181 Joined: Mon Dec 17 2007, 10:48PM
Location: Iowa USA
Posts: 17

Thanks you guys have been great. I just want to clear some stuff up quick.

I thought I was not supposed to connect the NST Ground to the mains ground, but to connect it to a different ground??

Also Tetraflouroethane:I should use an MMC for the filter cap?? how important is the filter cap if I use a safety gap and the inductors. I am pressed for cash
and don't want to buy more caps. I'll have 4 caps leftover from my Tank MMC but that will only make 1200 V for each of the filter caps. I was told by Shuan that they need to be at least 7kv.

One more thing what kind of fuse should I put on the RFI filter and should I just put it across the inputs. I alread have a 20 amp fuse and the front of my controler. my controler goes wall to fuse (the fuse has a bilt-in switch) to dimmer switch to RFI filter.

Thanks

J. Aaron Holmes

Tue Dec 18 2007, 11:11PM

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

Your NST case (secondary midpoint) would be tied to the same RF ground as the base of your secondary. It would also be the midpoint of your safety gap. A safety gap by itself is better than nothing, provided you adjust it correctly. For a Terry filter cap, I'd probably just use a single 30kV doorknob cap off eBay. ~2000pF or something around there. No need to bother with a bunch of tiny caps, and the Yish-brand Chinese doorknobs on eBay are generally considered to be pretty good. They're cheap (inexpensive), too.

Put the fuse between the wall socket and "everything else". It should be on the hot lead.

EDIT: I would also add that the use of filter *inductors* on the HV has been largely discredited through numeroous studies around protecting NSTs. See the TCML archives and Terry's papers. In some cases, it was suggested that the inductors did more harm than good. Hence the Terry filter and its RC-ness.

Cheers,
Aaron, N7OE

Hazmatt_(The Underdog)

Tue Dec 18 2007, 11:32PM

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

I was doing analysis of the TC system and transient filter as part of my optimization work and Aaron is almost right. For maximum surge clamping you want the smallest capacitence with highest voltage obtainable. For my design I found that the most effective capacitence for arresting the transients was under 50pF. 2000pF did not clamp the surges much in comparison to 50pF in simulation.

My present system is using 560pF doorknobs because that's what I have here. But Surplus Sales has some 10pF doorknobs that are suitable for the RF filter and will handle the surge current. I still have to buy some more.

Tetrafluoroethane

Wed Dec 19 2007, 02:15PM

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

I would not use the same caps for your RC filter that you used for your tank cap. That would be kind of a waste of money. Aaron is right, if you can get a doorknob cap it will work great. Otherwise you can get cheaper polypropylene caps at most electronics suppliers and series them for the voltage you need. They work just fine. The filter caps really won't have to stand up to quite the abuse the tank caps do.

J. Aaron Holmes wrote ...

Your NST case (secondary midpoint) would be tied to the same RF ground as the base of your secondary.

Aaron, are you saying it should *also* be tied to the RF ground, or *only* to the RF ground? The advice I got here when my first coil started toasting network cards on was to *also* tie it to RF ground.

HV Enthusiast

Wed Dec 19 2007, 04:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

For NST powered coils, I tie everything to EARTH ground (same ground) with no problems whatsoever.

If you want to go dedicated grounds, then just ground everything on the primary side to one ground (i.e. 3 prong plug earth ground) and the secondary coil RF ground to its own dedicated ground.

Obviously, the higher power you go, the more reason you would want to use a dedicated RF ground so not to induce large voltage gradients within your mains wiring.

J. Aaron Holmes

Wed Dec 19 2007, 04:19PM

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

Tetrafluoroethane wrote ...

Aaron, are you saying it should *also* be tied to the RF ground, or *only* to the RF ground? The advice I got here when my first coil started toasting network cards on was to *also* tie it to RF ground.

Unless you have no dedicated RF ground, I cannot think of a good reason to *ever* tie your NST's case to the mains ground. I was suggesting RF ground *only*. Using your power ground at all just means you'll be circulating RF currents through the mains ground. This current may be small if your RF ground's RF impedance is a lot lower (and it should be, or else what good is it?), but it's still there, and may still cause harm or interference to other electronics in your house.

In a neon sign application, the NSTs case ground serves to make the case "safe". In a Tesla coil, who cares? You're not touching the case while you run your coil (I hope!).

Cheers,
Aaron, N7OE

Steve Conner

Wed Dec 19 2007, 06:00PM

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

Well, if you have a poor RF ground, you can arc the NST internally between its primary and its core.

J. Aaron Holmes

Wed Dec 19 2007, 06:55PM

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

Steve Conner wrote ...

Well, if you have a poor RF ground, you can arc the NST internally between its primary and its core.

If your RF ground is so bad, ... ?

In the end, if you just unplug your HD TV and computer and ham radio and anything else you're worried about frying, then sure; connect every ground you can find lying around to your NST

HV Enthusiast

Wed Dec 19 2007, 08:35PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...
Using your power ground at all just means you'll be circulating RF currents through the mains ground.

It really makes no difference. Whether you earth ground the case (which earth grounds secondary centertap), RF currents will circulate through your mains power anyways. I've made measurements using RF current transformers with spectrum analyzer and the RF currents are present whether you tie a dedicated RF ground to the secondary midpoint of the NST or directly to the earth ground of your 3rd prong power outlet. There isn't any magic RF isolation between primary and secondary on an NST. Any RF on the secondary will couple right back into the primary.

The issue with tying the secondary RF to earth ground (3rd prong outlet) is that depending on the impedance / length of the earth ground connection, you can develope very large transient voltages which can damage electronics equipment.

So I stand by my original recommedation. Earth ground your NST. At the least, do it for safety reasons. Thats what its there for to begin with.

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