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Questions regarding variacs & protection

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infinitum

Sun Mar 12 2006, 06:51AM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

I plan on building a simple spark-gap tesla coil for my highschool graduation project. I haven't really found any plans yet, but I'm pretty set on using a NST and building some salt water caps (the project has build time requirement, so I'll just "waste" time by building caps

). I'm not really sure of the size yet, but I want something that's capable of atleast 12" arcs.

Anyways, I've read about tesla coils causing RF interference and possibly destryoing all electronics connected to the same power source...which I obviously don't want (bye bye computers, televisions, etc).

How do I prevent this from happening? What type of grounding do I need? What type of protection do I need to keep the tesla from killing the NST (or the NST AND everything else in my house)?

Also, I've read about variacs, which from my understanding go like this: 120V source --> variac --> NST --> tesla coil. I take it that they control the voltage to the NST. Do I need one? Do they take any load off the coil, as in starting at a low voltage and slowly going up would put less strain on the coil?

Thank-you!

vasil

Sun Mar 12 2006, 07:37AM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Use separate groundings for HV part and for RF base current:
-the mains ground for the NST case and the middle electrode of the safety spark gap
- a separate grounding (water pipe, dedicated ground in your yard, counterpoise) for the secondary base
If you use the same ground (possible for low power SGTCs), it is possible to inject RF noise in your home electric frame.
The variac can be used to tune your coil at low power input. If you can tune other way (ex: tuning with a scope), you can manage without the variac.
For example, try first with a quick made primary (15 turns of high insulated HV wire) and take off one turn each time until you get in tune. Run with a small distance between the gap electrodes (the spark gap, set in parallel with the NST output) will act as a voltage regulator.
Build then your primary, with the number of turns you found +2 more turns to have more space to tune. When runing the new primary, you will open more the gap to the get the best of its potential, until the safety gap across the NST windings start to fire. Then just decrease a bit the gap, to work properly.
Do not allow sparks between inner primary turn and the base of the secondary, or between toroid and the primary turns. They can kill your NST.

infinitum

Sun Mar 12 2006, 07:32PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Should I use an RF filter when I built my tesla? Also, what is a an RC filter, and what do they do?

vasil

Sun Mar 12 2006, 07:53PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

If you are getting much RF noise in your mains frame it is better to use a filter (in reverse, i think).
The RC filter is some stuff you put accross the NST terminal, to protect the windings from the RF kickback from the tank circuit. It is composed by caps, in parallel with the NSTs and high watage resistors on both HV arms.

See here:

http://www.hot-streamer.com/TeslaCoils/MyPapers/rcfilter/rcfilter.html

infinitum

Sun Mar 12 2006, 11:27PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

I was just reading about rotary spark gaps, and from my understanding synchronous motors rotate at an EXACT speed, which allows the capacitors to charge to the max capacity. Asynchronus motors rotate at however fast they want to.

I want to be able to play with the tesla's arcing through the use of a variac. I know asynchronous motors can be speed controlled, but can synchronous be speed controlled? Also, should the spark-gap motors be run off the same variac that controls my NST, or a different one?

Thanks for helping, vasil.

Hazmatt_(The Underdog)

Mon Mar 13 2006, 12:22AM

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

For NST I find that 1.6x or 1.7x Cres works well even in rotary spark gap. You can go 1.8x Cres, but I'm getting less performance that way. Some places you see 2.6x Cres, I wouldn't go that high. I've been performance testing off and on with my systems for about 2 months and the best performances were ~1.6xCres.

I tried to control a sync. motor a few times but at best you get a little deviation back and forth. The largest problem is that if you get too far out of phase it stalls or it tries to start itself over and over. Your best bet is to find an 1800 RPM motor and put 4 electrodes on it for 200BPS. Now this will not give you 120BPS all the time. It leads and lags somewhat because there isn't really a load on the shaft unless your rotary weighs as much as a grinding wheel.

For an async. you will need to use a variac different from your power variac because you will want to find a particular spot to stay at. It's hard to know where exactly to adjust, but for now mine is at about 45-60VAC. I want to build a tach and control to keep it at 1800.
My rotary works okay, its not perfect, but its getting better.

I think for a rotary I would either say go for a sync motor because you know its at the right speed (which is a major headache for me! I can't tell you if its at the right speed or not)
Or if you want to go this way, a DC motor that you can pulse control and really adjust/control with a feedback system.

infinitum

Mon Mar 13 2006, 12:27AM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Ok, where should I be able to find plans? Most of the plans I've seen on eBay are xerox copied crap. I want a synchronous rotary gap machine that uses a 9-15 kvolt NST that can do 1-3 foot arcs. I want to use salt-water caps, but I would be ok with using a small bank of caps.

I would like to find plans that have text AND step by step pictures that show how something should look and how the grounds, variac, NST, etc. should be connected.

So, where would I be able to find such plans?

edit: Plans for the RC filter that show how they are built and where they are connected would be great, too.

Hazmatt_(The Underdog)

Mon Mar 13 2006, 02:59AM

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

you will need to do a lot of homework

Part Scavenger

Mon Mar 13 2006, 04:00AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

If you're willing to spend a whole lot more than you have to, "Electronic Gadgets for the Evil Genuis" has two sets of plans. It does give a good idea of construction though. Check it out at most B&N stores, etc. It has some other neat stuff in there too.

Oh, and deepfriedneon

is a good place to start. I'm working on my site, I have just added two tools I wrote to help lay out your primary. They are pretty self-explanitory, there is a circle divider calculator and a slit-post calculator(this helps you determine where to cut the slits for the primary on each post.) There is a tutorial on secondary design, and bottle caps. The others are in progress.

Here are some other helpful sites to get you started:
Loneoceans.com had a good blog on his TC.
richieburnett.co.uk has some good theory stuff, especially on SSTC's. Go there if you want to know about coupling coefficients.
tesladownunder.com has GREAT pictures.
Steve Ward's high voltage has good info on SSTC's and DRSSTC's
Teslina's high voltage? I think that's the name; great info and lots of it. Confusing site though, you may never find something you know you've seen before there.

Steve Connor's site, scopeboy.com is great.
Greg's Garage

Jochten's high voltage

Check out the old fourm and look at "questions/comments about first tesla coil". And look at some of the stuff "madmaxx" did. Only don't do it that way, look at the help other people tried to give him. And best of all, Tesladownunder's thread on the old fourm "Add your tesla coil here" is pretty cool. It gives you a good idea of what all kinds of variables do, plus some pretty amazing pictures

Hope that helps

infinitum

Mon Mar 13 2006, 09:32PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Before I start buying equipment, I have a question. I know that the larger voltage rating a NST transformer the more powerful it is. However, what bearing does the milliamp rating have on my tesla coil? Will a high ma rating mean a more powerful spark?

Specifically, I'm looking at this .

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