Re: Chokes vs Safety Gaps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Again I have to re-iterate that in my experience, chokes of any sort
between the gap and transformer are a bad idea.

On 11 Feb 2004, at 21:29, Tesla list wrote:

 > Original poster: Mark Broker <mbroker-at-thegeekgroup-dot-org>
 >
 > I hadn't seen the NSTWinding paper.  But I think the real frequencies
 > of interest are the ones over 1MHz, which are generated by the spark
 > gap.  Indeed, the Terry Filter is "tuned" to these higher harmonics,
 > not the fundamental resonant frequency.
 >
 > It has been a while since I read the RLC paper, but I'm not so sure
 > anymore about the results.  I think that any NST filter should be two
 > center-tapped filters, a la the "Terry Filter."  Putting a resistor
 > (albeit a large wirewound) on one leg of the NST, an inductor on the
 > other leg, and a capacitor between the two that isn't tied to ground
 > is not indicative of a real world filter.  I would be interested to
 > see if the test results changed significantly if a "double pole"
 > filter was used.  (ducking to avoid a swing from Terry :p ).
 >
 > I would expect a fairly good filter design to feature an RLC filter
 > with the inductors straddled by two resistors (NST-C-R-L-R-TC).  The
 > resistors would kill the Q of the resonant circuits comprised of both
 > the filter cap and inductor and main cap and inductor.  IMO this is
 > practically executed by using a large wirewound resistor ;)

RLC? That is exactly what I tried to do about 8 years ago. I
constructed a balanced-about-ground multipole filter with a
Butterworth characteristic based on the macro component values that
apparently counted and killed a NST in less than a minute (the only
one I ever lost incidentally). It was also the only time I ever used
a safety gap and that was firing at low energy (so wide main gap
setting, resonant rise on the primary etc. can be totally discounted).
What I discovered - I hadn't counted the parasitic components present
(like stray C's in the inductors e.g.). It looked like it was doing
the right thing under LV test conditions on the scope.

      A couple of years later I had another graphic demonstration of
what these parasitic components can do. I ran a 1.5kW coil with long
leads coming from the transformer I had been running it on for years
with very short leads. The result: the transformer arced over
internally and I had to partially rewind it.

      Hence my preference for running with very short leads between
transformer and *calibration-set static gap* and not bothering about
including safety gaps and lossy resistors etc. I have never
experienced the hint of a problem with this recipe. I used a metered
EHT supply to set the gap prior to firing so I knew what voltage it
was firing at (within a kV) under normal AC conditions.

Malcolm

 > I would never run an NST-powered Tesla Coil without a safety gap, even
 > with a static gap.  I also strongly encourage people to build at least
 > an RC filter a la the "Terry Filter" minus the MOVs.
 >
 > Cheers,
 >
 > Mark Broker
 > Chief Engineer, The Geek Group
 >
 >
 > On Wed, 11 Feb 2004 08:05:52 -0700, Tesla list <tesla-at-pupman-dot-com>
 > wrote:
 >
 > >Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> Terry
 > >Fritz has already done what seem to me the definitive experiments on
 > >this. He pinged a depotted NST with spark-gap pulse waveforms and
 > >examined the voltage distribution across the windings. He tried this
 > >with various chokes and resistors in series with the NST, and found
 > >(IIRC) that chokes were worse than useless. He wrote a couple of
 > >papers that you can find on hot-streamer-dot-com.
 > >
 > >The "Terry Filter" is the result of these experiments.
 > >
 > >Steve C.
 > >
 > >http://hot-streamer-dot-com/TeslaCoils/MyPapers/NSTWindingStress/NSTWindi
 > >ngStress.html
 > >
 > >http://hot-streamer-dot-com/TeslaCoils/MyPapers/rcfilter/rcfilter.html
 > >
 > >http://hot-streamer-dot-com/TeslaCoils/MyPapers/rlcfilter/rlcfilter.html
 > >
 >
 >
 >
 >
 >