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From: whit@milton.acs.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Resistor types
Summary: Temperature effects, ageing, values available, will differ
Message-ID: <1370@milton.acs.washington.edu>
Date: 13 Jan 90 01:53:12 GMT
References: <1990Jan12.231715.28296@phri.nyu.edu>
Reply-To: whit@milton.acs.washington.edu (John Whitmore)
Organization: University of Washington, Seattle
Lines: 46

In article <1990Jan12.231715.28296@phri.nyu.edu> roy@phri.nyu.edu (Roy Smith) writes:
>
>	What are the significant differences between the various resistor
>technologies for "typical" resistors, say 1/4, 1/2, or 1W in 1-ohm through 10
>Megohm values?  Looking through the Newark or Mouser catalogs, you can find
>carbon film, carbon composition, metal film, and metal oxide, not to mention
>wierdo stuff like silocone coated, vitreous enamel, and wire wound.

	There are three kinds of materials, electrically speaking.
These are metals (high conductivity, positive thermal coeffecient of 
resistance), semiconductors (selectable conductivity, negative thermal
coefficient of resistance), and insulators.
     The best accurate resistors are (still) wirewound, because the 
alloys can be controlled more accurately in a small melt than in an
evaporated film; a pure metal has 0 resistance at 0 temperature
(absolute, of course) if one ignores crystalline imperfections, so alloys
are always used for critical applications.  For reliability, wirewound
resistors often come with fireproof coatings (glass or silicone), since
they will operate normally at high temperatures as long as the coating
doesn't burn off.  Wirewound resistors are expensive for high resistance
values, mainly used in the 0.1 ohm to 10,000 ohm range.
	Metal film resistors use less metal (because the film thickness
is expensive to increase), but achieve good accuracy AND can be trimmed
after manufacture to some final value.  Running a metal film resistor
too hot can generate a local hotspot to burn the small amount of metal,
so film resistors are not preferred if transient surges are present.
Film resistors are easily matched, so most metering resistors are
of this type.  Achieving low temperature coefficients is an art
that manufacturers do NOT discuss in detail, except to note that
they do better than their competition...
	Carbon is a semiconductor; at low temperatures (liquid nitrogen)
a carbon resistor makes a GREAT sensor; when the resistor hits the nitrogen,
it stops conducting.  Carbon composition is a mature technology,
capable of very reliable 5% or even 2% accuracy resistors, and surges
create heating of the bulk (no hot spots).  Carbon film resistors are
inexpensive, more accurate than composition, and apparently are
taking over the market; surges can burn them up, so a 1/4 watt composition
resistor can take 2 W for a millisecond, and a 1/4 watt film
resistor can NOT.  Not only is carbon temperature sensitive, it
is also pressure sensitive; no one uses it for low noise applications
if there is an alternative.  Composition values above 1Mohm are 
often considered unreliable (value changes unpredictably with ageing),
while carbon film is OK up to circa 22Mohm.

I am known for my brilliance,                John Whitmore
 by those who do not know me well.