[TCML] Terry filters

Tue Jan 1 07:43:06 MST 2008

Gary,
    Thanks for your well thought out reply to my comparison of low pass 
filters (as used in audio) to transformer protection. Now it is clear to me 
the basic flaw of using chokes on our NST's.

Best regards, Dave Goodfellow

----- Original Message ----- 
From: "Lau, Gary" <Gary.Lau at hp.com>
To: "'Tesla Coil Mailing List'" <tesla at pupman.com>
Sent: Monday, December 31, 2007 9:21 PM
Subject: RE: [TCML] Terry filters

Hi Dave,

Your analogy about using just an inductor for a woofer low pass network made 
me think hard for a moment about my post.  You are exactly right about the 
requirements for 6/12/18 dB/octave filters.  But the woofer situation is 
very different than NST protection networks and I maintain my position. 
Allow me to explain.

In the woofer circuit, the load, the speaker, is an 8 Ohm resistor, and 
forms an integral part of a 1-pole R-L network.  There is some inductance in 
the woofer, but it's predominantly resistive; otherwise it wouldn't consume 
power.  The choke in series with a *resistive* load does indeed form a 6 
dB/octave low-pass filter.  My previous statement that a capacitor is 
necessary to form a low-pass filter network assumed that we do not have a 
resistive load.

In our application, the "load", what the output of the filter network goes 
to, is the NST secondary, measuring thousands of Henries.  There is also 
resistance in the NST secondary, but the huge inductance dominates.  Adding 
a choke measuring a few micro or milliHenries in series with the NST 
secondary measuring thousands of Henries does not form a low pass filter. 
In order to form a high or low pass filter, you need 2 or more network 
components that have reactive impedances that differ with frequency, i.e.  L 
vs. R, L vs. C, or C vs. R.  Adding just a choke in series with the xfmr 
amounts to an L-L network.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On
> Behalf Of Dave Goodfellow
> Sent: Monday, December 31, 2007 10:25 AM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Terry filters
>
> I have bought into the idea that chokes are not the answer for protecting
> nst's based on the evidence that you (Gary) and others on the list have
> produced. I don't agree, however, with your claim that a series inductor
> does nothing to from a low pass filter. In my old car stereo days, I used
> chokes on woofers to keep the high frequencies out of the speaker.
> Admittedly, when I listened to the woofer with and without a choke, a 
> choke
> alone didn't just drop the high frequency right out of the sound that the
> speaker was fed.  According to my old Fosgate manuals, a choke on it's own
> would attenuate the signal at 6 db per octave. The bigger the choke, the
> lower the frequency the attenuation would begin. This means that if the
> speaker were 8 ohms, and your choke was stated to be 100hz for a 8 ohm
> woofer, it simply meant that it would present more than 8 ohms to the
> amplifier at 100 hz and up, and increase the impedance by 2x's (I think)
> with each doubling of frequency.  If you wanted 12db per octave, a 
> capacitor
> of a specific value would be placed on the downstream side of the choke, 
> to
> the negative feed of the speaker. If one wanted 18db per octave, a second
> choke would be placed in series, down stream of the first choke.
> When I thought that chokes were necessary for Tesla coil building, a 
> ferrite
> core car stereo woofer choke was exactly what I used on the high voltage
> side of the transformer! Naturally, to find a capacitor so large and of
> sufficient voltage to use the car stereo formula was impractical, so I 
> just
> used the choke mounted directly on the hv bushing.
>     Your chokes looked like a good design and your testing is convincing
> that chokes are not helpful. Unwanted capacitance I believe is the culprit
> in making chokes bad. I have found the effects of unwanted capacitance
> working in A/V. As our video signals are becoming higher and higher, 
> cables
> have to become shorter. That tiny bit of capacitance in coax can kill the
> signal when the cables are too long!
>
> Dave Goodfellow
>
> ----- Original Message -----
> From: "Lau, Gary" <Gary.Lau at hp.com>
> To: "'Tesla Coil Mailing List'" <tesla at pupman.com>
> Sent: Monday, December 31, 2007 12:51 AM
> Subject: RE: [TCML] Terry filters
>
>
> I'm sorry Dr, but anecdotal evidence like this is going to set us back to
> using leeches and bloodletting.
>
> No one has ever performed a study on failure rates, ON EQUIVALENT
> HARDWARE,
> comparing R-C filters and L or L-R filter networks.  But there is
> overwhelming theoretical and simulation-based evidence that show that R-C
> filters are the superior topology.
>
> Adding just a series inductor with or without a series resistor does 
> nothing
> to form a low pass filter.  You need a capacitor; this is elementary 
> circuit
> analysis.  On what basis do you conclude that this would "catch especially
> the very nasty high freq transients "?
>
> I maintain that the use of chokes in NST/PT/pig protection networks should
> be abandoned.
>
> Regards, Gary Lau
> MA, USA
>
>
>
> > -----Original Message-----
> > From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On
> > Behalf Of resonance at wildblue.net
> > Sent: Sunday, December 30, 2007 9:21 PM
> > To: Tesla Coil Mailing List
> > Subject: Re: [TCML] Terry filters
> >
> > If you are really on a tight budget using used NSTs and can't afford a
> > full Terry filter, there is an alternative.
> >
> > For almost 20 years I used a simple pair of air core RF choke made of 2"
> > ID x 12 inch long PVC tube.  I wound it for 10 inches with #26 AWG PVC
> > covered wire.  It seemed to catch especially the very nasty high freq
> > transients that could break down the NSTs HV coils as these transients 
> > can
> > form a "spider-web" like effect, essentially "crawling" over the 
> > windings
> > until they connect and then the main power flows causes failure.
> >
> > Using a pair of these simple air core chokes I had only one failure in
> > nearly 30 years.  Some of these coils with NSTs and these simple chokes
> > are still in daily operation at museums for over 15 years.
> >
> > The failure I had was also a used NST so that could have entered into 
> > the
> > factor --- the xmfr might have been ready for a failure from prior neon
> > service.
> >
> > Food for thought anyway.
> >
> > Dr. Resonance
>
> _______________________________________________
> Tesla mailing list
> Tesla at www.pupman.com
> http://www.pupman.com/mailman/listinfo/tesla
>
>
> _______________________________________________
> Tesla mailing list
> Tesla at www.pupman.com
> http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla at www.pupman.com
http://www.pupman.com/mailman/listinfo/tesla