RE: What wattage resistors for NST protection?

To: tesla-at-pupman-dot-com
Subject: RE: What wattage resistors for NST protection?
From: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Tue, 15 Feb 2000 20:09:54 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

There is not a direct connection between the Watts processed by an NST, and
the required wattage of series protection resistors.  The resistance value
of the resistor also figures prominently.

In _theory_, the wattage required would be the NST's output current squared,
times the resistance.  If you have 2000 Ohm resistors and a 60 mA NST, the
wattage would be 7.2 Watts. However, in my experience, the actual current is
considerably higher than the faceplate value when operating into a Tesla
Coil load, perhaps due to using a Larger Than Resonant tank capacitor.  I
use a single 16/60 NST with .02uF cap, and my 1.6K 113 Watt resistors get
mighty hot.  If I up the cap to .03uF, in the less-than-a minute time it
takes for the 20A mains breaker to pop, the resistors get even hotter than
"mighty".

The actual wattage dissipated by MMC bleeder resistors is much lower than
the 1/2 watt rating, and again has nothing to do with the NST's rating.  It
is necessary to use high wattage resistor bodies to not violate the maximum
standoff voltage rating too severely.  A lower wattage body might arc across
the terminals.

Regards, Gary Lau
Waltham, MA USA

>Original Poster: "William Parn" <parn-at-fgm-dot-com> 
>
>Greets,
>
>What wattage resistors are required for NST protection?
>I think I have been seeing wattage resistors that
>are a lot lower than the actuall watts of the NST.
>Can someone explain how this works.
>
>I have also been reading about 1/2 watt bleed resistors
>on each cap in an MMC.  How does this work when the wattage
>on a 15kv-at-60ma could be 900 at some given point in time.
>
>Cheers,
>Bill