Re: chokes (fwd)

---------- Forwarded message ----------
Date: Thu, 21 May 98 16:30:03 EDT
From: Gary Lau  21-May-1998 1619 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: chokes (fwd)

>From: Mad Coiler <tesla_coiler-at-hotmail-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: chokes
>I am thinking about making a couple chokes for my NST coils. I have to 
>toroidial cores, about 2.5" diameter. I have a lot of #24 magnet wire. 
>Will this stuff be to small? I dont really know what the current rating 
>is for #24. I might be apllying as much as 180mA in the NST setup.
>Tristan Stewart

The use of chokes in NST protection networks is controversial.  Terry
Fritz has done some outstanding research in actually scoping the
consequences of using various RC and RCL protection networks.  His and my
spice simulations confirm his results.  See
Unfortunately, the various archives are full of advice suggesting the use
of chokes, although specific advice and justification are lacking.

The bottom line was that in RCL networks, they effectively block the HF
tank energy from getting to the NST, but that the choke and bypass caps
will resonate on their own in an equally destructive manor, even when using
series damping resistors.  Instead, a more simple R-C network is a better

Ignoring this, you have a couple of problems with chokes.  I built a
couple of chokes using 3.25" OD 2.25" ID .50" THK cores.  I have
experimentally measures that they begin to saturate at 300 mA.  Since
your core material is likely to be different, your milage will vary.
While your RMS 60 Hz current through the choke may be only 180mA, the
peak current will be higher, especially if the NST is resonant at the
mains.  But this is small compared with the current that will flow when
the main gap fires, discharging the safety cap through the choke.  In
short, core saturation is likely, and will effectively reduce the choke's
inductance.  I'm sure my core saturated, not sure of the consequences
(although I DID blow my NST).

Ignoring this too, great efforts must be made to keep the choke's turns
from arcing to the core, to adjacent turns, and to the far end of the
choke.  I covered my cores with .04" poly, sealing the edges with
hot-melt glue, and covering only about 270 degrees of the core with wire
to keep the ends seperated.  Number 24 AWG wire is thick enough for the
current involved, but magnet wire doesn't have enough insulation.  You'll
have to use thickly insulated wire, preferably coax core with solid poly

I would recommend dumping the chokes and using 300-500 pF bypass caps
with series resistors and safety gaps on each side of an NST.

Good luck,
Gary Lau
Waltham, MA USA