From: "Ed Price"
Subject: Re: Resistors for r.f. applications
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 03 Jan 2003 00:15:34 GMT
NNTP-Posting-Date: Thu, 02 Jan 2003 19:15:34 EST
Organization: Cox Communications
"John Woodgate" wrote in message
> I tried a web search for resistors that stay resistors, and about the
> same value, up to at least 10 MHz, but found it surprisingly difficult
> to find anything. I need some 50 ohm 0.5 W and some low values like 1
> ohm, very low power.
> Suggestions (polite), please.
You must have already decided that you want minimum inductance, so maybe you
have to sacrifice size control. About the least inductive resistors are
silicon carbide, in the physical form of tubes, cylinders or pellets. These
are the old Carborundum devices (company later known as Cseiwid, but then
sold to ?). Power is no problem; you can push current through them till they
glow, and the temperature coefficient of resistance isn't too bad.
Unfortunately, the volume resistivity isn't too variable, so you need to
play with the geometry to get the resistance you want. A silicon carbide
resistor can be mechanically abraded to adjust the resistance, or, you can
abrade the silvered contact areas.
These resistors are a great solution for RF loads or for big pulse
generators. You can slam 10's of kA through them, and they don't break,
change value or arc.
The second best choice are carbon composition resistors, probably in the 2
watt size. These are lower inductance than wire-wound, and can survive
current surges that would vaporize a metal-film. Unfortunately, repeated
heavy current surges make them drift, and their resin bodies don't dissipate
heat all that well.