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R.F. Choke?

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radioman

Thu Oct 14 2010, 01:48PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

My first SGTC is nearing completion with 24 inch sparks. Now, I want to protect my hard-to-find surplus NST. I already have a safety gap. I know about the Terry Filter, but have no more money to buy a lot of parts; my wife says the project is already over budget! Does anyone have a â€œrecipeâ€ RF coil I can easily wind? Also, will putting a .001 cap directly across the NST output do any good? Thanks, all.

dingo27

Thu Oct 14 2010, 04:33PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

radioman wrote ...
my wife says the project is already over budget!

dont worry, all women do that, dont listen them!

Herr Zapp

Thu Oct 14 2010, 08:08PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

radioman -

Regardless of what you may have heard, you probably don't want to just add simple RF chokes to your system in an effort to "protect" your NST. RF chokes, used alone, may cause resonances that apply more stress the NST than no chokes at all.

Terry Fritz was probably the first to systematically study the stresses seen by NSTs used in spark-gap Tesla coils. Some of his papers are archived at:

With a little shopping on eBay, the parts for a Terry Filter can be found for less than $30. The actual construction of the R-C filter array is pretty flexible: if you can't find two 100W 1K resistors, use two 500 Ohm resistors in series in each leg, or any combination of resistors that yield ~1K ohms and can handle the power. The bypass caps can be common 500pF, 20kV ceramic doorknob caps. I typically don't include the MOVs, as I may may use the filter with different NST voltages, or use a variac to boost NST output. Actually the MOVs are a bit redundant, if you carefully set your safety gaps.

Depanding on how much you paid for your NST, especially if it's a hard-to-find 60ma unit (or a very rare 120ma unit), the cost of a Terry Filter is insignificant.

Herr Zapp

dmg

Thu Oct 14 2010, 10:58PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

A filter in general is a nice investment for your system, NSTs are quite fragile things.
I wouldnt recomend using just 2 coils and a saftey gap for an NST, but rather building the filter zapster mentioned.
(although this worked for me in the past, I mostly use MOTs for coils)

as for the wife, if she complains about the price of your coil, then you should have every right to complain about the price of her shoes

is there any pictures of your coil so far anywhere?

radioman

Fri Oct 15 2010, 01:49PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Herr Zapp

The Fritz link you gave me shows a very simple arrangement of just one capacitor across the NST output followed by a 1,000 ohm, 250 watt resister in each leg. Will something this simple do the job for me? Also, the more complicated Fritz diagram I have seen on line shows the NST case along with the safety gap both using the dedicated RF ground -- not the house ground. Is this O.K.?

Thanks Herr Zapp,
Radioman

radioman

Fri Oct 15 2010, 02:08PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Gatedbreakdown

Here is my first coil. It is completely mobil so I can easily hide it when my wife has company over. I only operate it outdoors, so I have ground stakes all over the yard! Thanks for your input.

Radioman

radioman

Fri Oct 15 2010, 02:10PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Herr Zapp

Fri Oct 15 2010, 10:59PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

radioman -

Possibly Terry's "final" NST protection circuit did not get archived.

Here is the schematic, at the bottom of the page:

Again, I do not use the MOVs, but do use 3-ball safety gaps across both the "input" and "output" sides of the filter.

A typical implementation of the filter can be seen at:

My preference is to use actual brass balls 1/2" to 3/4" in diameter rather than the acorn nuts, which tend to spray corona from the sharp edges when used with 12 or 15kV NSTs.

Herr Zapp

radioman

Sat Oct 16 2010, 02:41AM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Thanks again, Herr Zapp. I will check it out

Radioman

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