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NST over-voltage protection?

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LAN_403

Tesla Fan

Thu Nov 04 2010, 06:28AM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Thanks Dr. Spark

And to all you US residence,

Happy Halloween!

radioman

Thu Nov 04 2010, 01:40PM

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

Tesla Fan,

You raise very good questions! I am in the process of acquiring parts to make my first Terry filter. Herr Zapp has recommended that I leave out the MOVâ€™s because they are â€œredundantâ€. Also, they are not appropriate when using a variac, must be reconfigured if you go to a different voltage transformer, and donâ€™t have an unlimited lifespan. He also says to omit the RF chokes. Does anyone else have ideas on omitting the MOTâ€™s?? Perhaps a Moderator could comment on this.

Thanks,
Radioman

Steve Conner

Thu Nov 04 2010, 01:51PM

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

The important thing to understand is that the RF voltage from transients is unevenly distributed throughout the winding, because of its self-capacitance. Most of the voltage appears across the first few turns.

Therefore, it takes considerably less voltage than a NST's rated voltage to burn it out, if it's RF voltage.

Therefore a plain safety gap, set to a NST's rated voltage, can't be guaranteed to protect it, because the gap always breaks down at the same voltage, no matter whether it's 60Hz or RF. This is the reason for the low-pass part of the Terry Filter.

All of this is (was?) documented in a lengthy paper on Terry Fritz's site, where he explains the purpose of each component in the Terry Filter, with experimental evidence.

He also proved by experiment that the RF chokes were useless if not worse, hence why he replaced them with resistors.

Tesla Fan

Fri Nov 05 2010, 12:50AM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Thank you all for your input, you have been all very helpful and i can say that i have a much better understanding about the RF issue now.

I found an easy R-C low pass filter calculator here

, what do you think the target frequency should be ? If your coil has a resonant frequency of 250KHz for example, would you set the filter to cut frequencies higher than 250KHz, set it to 300KHz for example? Or is this just over-calculating and the usually used circuits (using 12 or 14 3.3nF/1500V caps in line) like the ones below will be just as good?

Any other opinions about the MOVs? Sounds like MOVs and Safety Gap do the same thing, so if you make a S.Gap you can leave the MOVs out?

AleSeg

Fri Nov 05 2010, 11:37PM

Registered Member #2727 Joined: Tue Mar 09 2010, 02:39PM
Location: Montevideo - Uruguay
Posts: 33

MOVÂ´s can disipate energy because they are resistors. Spark gaps are more like a quasy zero resistor activated by voltage. This is the big difference. And take in consideration that the heat accumulated can destroy the MOV !
The action of the MOV is less abrupt than the spark gap discharge in reference to the applied voltage between its terminals. Then we expect a progressive turning on for a MOV very close to the limits, and nothing for a spark gap.
I think spark gaps are better for the amateur, because its robustness, but they are not the best technical solution.

Sorry for my bad english

See you

Luca

Sat Nov 06 2010, 10:21AM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Tesla Fan wrote ...

I found an easy R-C low pass filter calculator here

No, that's a low-pass filter, it means that frequencies below cutoff-freq. can pass the filter, while freq. above cutoff are stopped (almost...). So, if you want to stop frequencies around your coil resonant frequency, hou have to set the cutoff of the R-C filter at a frequency well below your resonant freq. I would say something in the tens of kHz...

For the MOVs, I would leave them...

Regards,

Luca

Tesla Fan

Thu Nov 11 2010, 03:23PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Luca,

I agree with the definition of the low-pass filter, i made a mistake with the â€œhigher than 250KHzâ€ statement, make that â€œlower than 200KHzâ€. I assume that you have to set the cut-off F ( i mean the high-end of the â€œcutâ€ band, eg if you cut 0-100KHz, 100KHz would be the cut-off F) higher that the resonant F (eg resonant F is 250KHz so you set a filter to cut 0-249KHz, so resonant F is not â€œin the cut zoneâ€). Is this not true? Wouldnâ€™t it â€œdisruptâ€ the coil's function somehow a filter the would â€œcutâ€ the resonant? Please correct me if i am wrong.
You mention â€œtens of kHzâ€, isnâ€™t that a bit low? If your resonant F is 250KHz, wouldnâ€™t 150KHz be a good cut-off with plenty of â€œsafety marginâ€ (for harmonics and such)?

According to my calculations ( again here

) the Terry Filter of the first of 2 pics i showed on my previous post, is a 2x 289KHz low pass filters (1Kohm resistor plus 6x3.3nF=550pF cap) and the second pic is 2x337KHz low pass (1K plus 471pF). So the Tesla Coils that these filters where used on had resonant Fs higher than 290KHz for the first and 340KHz for the second? I think these are the filters that most people recomend for any (?) Tesla Coil (kind of â€œuniversalâ€ circuit)?.

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