RE: [TCML] Terry filter - does one need the current limiting resistorswith a current limited NST?

["Lau, Gary" <Gary.Lau@xxxxxx>]

One may indeed calculate the cutoff frequency of the R-C filter, but I think the answer would surprise most folks.  If R is 1K and C is 500pF, the -3db frequency is 1/2pi*R*C or 318KHz.  But most medium-large coils run at a much lower frequency than that, so the attenuation would be much less than 3db.  Shouldn't we be shooting for a much lower cutoff frequency to ensure a thorough attenuation?

No.  Contrary to intuition, it's not the coil's resonant frequency that the R-C filter is there to attenuate.  Assuming that the spark gap is across the NST, when the primary is ringing at its resonant frequency Fr, the spark gap is ON, pretty much a short circuit, so the NST sees very little in the way of Fr, even with no filter.

Pasting from my web page on this topic (please view the page if the context of the excerpt below is unclear): http://www.laushaus.com/tesla/protection.htm

"I believe the more significant NST hazard (aside from too-wide gaps) is due to VHF voltage bursts generated during the gap conduction. These bursts are twice the peak tank voltage and in the tens of MHz. As I mentioned, gap conduction is not really continuous, but ceases briefly each time the tank current crosses zero as it rings at its resonant frequency. At the moment that tank current crosses zero and the gap ceases conduction, the voltage across both the tank capacitor and the primary coil are at maximum, or Vtank. The primary coil has some parasitic self-capacitance, C-self, associated and in parallel with it, and it is also charged up to V-tank. As soon as the gap opens, the primary coil and its C-self ring at their own resonant frequency, about 24 MHz in the case of my primary. This ringing is in series with the tank capacitor, sitting at Vtank, so the combination of the tank capacitor and the ringing primary inductor in series now present a voltage of 2Vtank across the non-conducting spark gap and to the NST if no protection network exists! But 24 MHz is child's play for an R-C network to filter out, and is easily attenuated."

Regards, Gary Lau
MA, USA




> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of miles waldron
> Sent: Sunday, April 06, 2008 11:19 PM
> To: 'Tesla Coil Mailing List'
> Subject: RE: [TCML] Terry filter - does one need the current limiting resistorswith a
> current limited NST?
>
> So, do the R's in the terry filter along with the caps, make a low pass
> filter? If this is true, then it should be easy to figure out what the cut
> off noch is, or make a bode plot.
>
> Miles Waldron
> 678-557-7737

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