RE: Power resistor limits was:Big Terry Filter Limits?

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

Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pkomen-at-zianet-dot-com>

Hello Jim,

Remember that the Terry Filter is protecting an NST.  An NST is not an ideal
power source.  The current through the resister is limited so the actual
voltage across the resister is much lower than the rated output of the NST.
60ma through a 1Kohm resister gives 60V and 3.6W (doesn't sound right
but...).  I suspect that there is more going on than this because the big
resisters in my filter get pretty warm, more than I would expect 3.6W to do.
A nick in the winding probably won't mean the death of the resister in NST
service.

Pete Komen

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Thursday, January 24, 2002 12:00 PM
To: tesla-at-pupman-dot-com
Subject: Re: Power resistor limits was:Big Terry Filter Limits?

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>


In some discussions with engineers at resistor manufacturers, they tell me
that the practical voltage rating of big wirewound power resistors is
really proportional to the length. ("a few kV/inch" was one statement)
Obviously, you can't apply the max voltage continuously because you'd
likely exceed the power rating (10kV across a 1K ohm resistor dissipates
100 kW!). The other thing to be aware of is that wirewound resistors have a
spectacular failure mode in HV use (which is why the "official" rating is
quite low).  Typically, these resistors fail because of a nick, dent,
manufacturing irregularity in the resisitive wire which creates a small hot
spot.  When that fuses, a small spark forms, which rapidly propagates back
up the wire.

That said, since these failures are so-called "workmanship" induced (i.e.
not inherently part of the design), if a particular resistor withstands a
particular voltage, it's likely to continue doing so, unless physically
abused.

If you absolutely need reliable HV resistors, then you'll need the big
resistive bar kind, as made by Sandvik (formerly Cesewid, formerly
Carborundum, formerly Globar, etc.) or Maxwell (who use a sort of folded
resitive bulk media) that don't have any "fine dimensions" in them that can
provide single point failures.