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

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

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.

Tesla list wrote:
> 
> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi Mike,
> 
> I am thinking of the big long ceramics.  The 100 watt ones are 6 1/2 inches
> long and the 250 watt ones are really big (10.5 x 1.125).  No problem with
> voltage on them.  Stay away from the metal enclosed ones.  I like the ones
> at the bottom here:
> 
> http://info.digikey-dot-com/T021/V5/592.pdf
> 
> like the L100J1.0K-ND or the L225J1.0K-ND.  Unfortunately, it looks like
> digikey is out of stock :-(
> 
> Cheers,
> 
>         Terry
> 
> At 07:36 PM 1/23/2002 -0500, you wrote:
> >Hi Terry,
> >Mike Church here again, What dictates the stand off voltage in these
> >resistors ?
> >Is it the actual length of them or construction ? The reason I'm asking is I
> >would like to obtain a couple and I need to know what to ask for exactly.
> >Another thought is that if the more watts they are rated at (100 or 250)
> >,does this mean that they are going to waste this much more of my avilable
> >power comming from my four 15/60's in heat?
> >
> >I guess what I need is something more along the lines of the big long
ceramic
> >type resistors other than the smaller variety 3/8" x 1" with the colored
> >bands on them.
> >
> >Sorry for all the questions
> >Thanks again,Mike Church (CHURCHMON)-at-aol
> >