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How do I make my HV chokes?
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From: bmack [SMTP:bmack-at-frontiernet-dot-net]
Sent: Tuesday, February 24, 1998 9:36 PM
To: Tesla List
Subject: Re: How do I make my HV chokes?
How about a common mode choke?
Has anyone tried this? One possible configuration might be two
spirals separated by poly' with a small cap across the NST side.
Comments?
Jim McVey
> From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
> Sent: Sunday, February 22, 1998 3:41 PM
> To: Tesla List
> Subject: Re: How do I make my HV chokes?
>
> Gary, all,
> I have scanned my memory banks after reading your post:
>
> > From: Gary Lau 20-Feb-1998 1052 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> > Sent: Friday, February 20, 1998 10:40 AM
> > To: tesla-at-pupman-dot-com
> > Subject: Re: How do I make my HV chokes?
> >
> > >>Gary Lau wrote:
> > >> I have seen advice advocating L-and/or-R-only protection networks
posted
> > >> to this list many times but have not seen any rationale for using
these
> > >> configurations beyond anecdotal "I've been using this and had no
failures
> > >> yet". Can anyone defend series-only protection networks in a more
> > >> analytical manor?
> >
> > >From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
> > >Although the NST HV winding does have a large L,
> > >it also has a significant parasitic shunt capacitance,
> > >on the order of hundreds of picofarads.
> > >Perhaps these series LR schemes work against this
> > >shunt capacitance to form an LC low-pass.
> >
> > Sorry, I don't buy it. The parasitic shunt capacitance is a
distributed
> > capacitance. At the outermost xfmr secondary turn, there is
effectively
> > no capacitance to shunt HF energy to ground, and the outermost turn is
as
> > likely a location to carbon track to the core as any other. A lumped
shunt
> > capacitance, outside the xfmr case, is required to keep the HF nasties
> > out of the NST.
>
> I seem to recall Ed Harris measuring a shunt capacitance at the
> terminals on the order of a couple of nF.
>
> The R may be needed to keep the filter from self-resonating
> > >if the NST's self-C is not lossy enough.
> >
> > The R is needed to keep the choke from self-resonating (actually just
> > reducing the Q and resonant-rise) against either it's self-C or
external
> > shunt-C. I believe the concept of the shunt-C wanting to be "lossy" is
> > wrong. Lossy implies a non-reactive parasitic component, i.e.
> > resistance. In a low pass filter, we want a simple, reactive shunt
> > capacitor. Any series resistance in the shunt-C diminishes it's
effectiveness.
>
> Interesting argument. Yet, you agree that there must be some Q-
> killing loss in the circuit. You need resistance to achieve a
> Butterworth response in a filter. Can't series R be modelled as an
> equivalent shunt element and vice-versa?
>
> > Having ranted on this topic now, I must confess that despite following
my
> > professed beliefs and meticulously constructing an L-R-C-safety gap
> > network, and measuring and confirming that my chokes aren't saturating,
I
> > still killed a 15KV/60mA NST, later reviving it by depotting and
> > repotting in Vasoline. Could it be that NST's in tar are doomed
> > regardless, but detarred, rebuilt ones have better survival rates?
> > Sorry for this anecdotal theory ;-)
>
> My theory (which I have yet to test properly) after examining safety
> gap operation: The chokes themselves and any other inductance between
> the gap and the transformer terminals is the real killer. As soon as
> the gap fires you have energized chokes with energy stored in them
> and nowhere else to go except back to the transformer. My money says
> that positioning the transformer as close to the main gap as you can
> with only some series R and some "lossy" shunt C should do the trick.
> I will condemn one of my own neons to checking this out soon.
>
> I mused on the placement of the main gap vs cap across the
> transformer terminals too. This whole thing seems to be a bit each
> way. If you position the gap across the transformer, the mechanism
> noted above is at work. If you position the cap across the
> transformer, the transformer appears as a shunting element in the
> resonant circuit formed when the gap fires. I'm opting for the gap in
> this position so at least the tank operation won't be affected.
>
> Malcolm
>
>