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How do I make my HV chokes?




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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.

>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.

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 ;-)

Gary Lau
Waltham, MA USA