roy@phri.nyu.edu (Roy Smith) (01/13/90)
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. Given
three 1k, 1/4W, 5% resistors, one carbon film, one carbon composition, and
one metal film, why would I choose one or the other for a particular
application?
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"whit@milton.acs.washington.edu (John Whitmore) (01/13/90)
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.
kenny@m.cs.uiuc.edu (01/15/90)
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? whit@milton.acs.washington.edu (John Whitmore) gives a fine discussion of the materials, but omits one point that I consider important. Film type resistors generally have the resistive material deposited on an insulating rod, and then cut in a helical pattern using either screw machinery or a laser. These, and wirewounds, both are quite inductive. If low inductance is essential, composition types are preferable. Kevin Kenny KE9TV
forbes@aries.scs.uiuc.edu (Jeff Forbes) (01/15/90)
In article <21000055@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes: > >Film type resistors generally have the resistive material deposited >on an insulating rod, and then cut in a helical pattern using either >screw machinery or a laser. These, and wirewounds, both are quite >inductive. If low inductance is essential, composition types are >preferable. Non-inductive film resistors are available. I believe that a zig-zag pattern is prepared on a plastic film then attached to a body for support and heat sink. I believe that they are only available in larger power sizes, which is where wire wound is mostly used. Jeff
myers@hpfcdj.HP.COM (Bob Myers) (01/17/90)
>>Film type resistors generally have the resistive material deposited >>on an insulating rod, and then cut in a helical pattern using either >>screw machinery or a laser. These, and wirewounds, both are quite >>inductive. If low inductance is essential, composition types are >>preferable. >Non-inductive film resistors are available. I believe that a zig-zag pattern >is prepared on a plastic film then attached to a body for support and heat >sink. I believe that they are only available in larger power sizes, which >is where wire wound is mostly used. It's also possible to get a low-inductance wirewound; ask if your resistor manufacturer produces any with a "bifilar" winding, which is simply a wirewound with the wire turned around and wound back over itself (or next to the original turns) in the opposite direction, such that the fields cancel (equal and opposite currents). Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
shaffer@net-sun1.dcrt.nih.gov (Micheal Alan Shaffer) (01/17/90)
<231715@<1990Jan12> <21000055@m.cs.uiuc.edu> Distribution: world How about low noise? Which are best in this regard? Mike shaffer@net-sun1.dcrt.nih.gov
davea@hpscdc.scd.hp.com (Dave Angelini) (01/18/90)
The best low noise resistor is made by Vishay. They are thick film or bulk metal I think. Some of their 1/4 watt values cost over fourty dollars in quanity. Wonderful for small signal amplifiers!
brianr@phred.UUCP (Brian Reese) (01/19/90)
In article <1990Jan15.042813.15689@ux1.cso.uiuc.edu> forbes@aries.scs.uiuc.edu (Jeff Forbes) writes: >Non-inductive film resistors are available. I believe that a zig-zag pattern >is prepared on a plastic film then attached to a body for support and heat >sink. I believe that they are only available in larger power sizes, which >is where wire wound is mostly used. Non-inductive (actually super low-inductive) wire wound resistors are also available. The wire is wrapped clockwise in one direction, then counter- clockwise in the other direction in an effort to cancel the inductive effect. (Dale calls this an Aryton-Perry winding) Brian -- Brian Reese uw-beaver!pilchuck!seahcx!phred!brianr Physio Control Corp., Redmond, Wa. brianr@phred.UUCP "Sticks and stones may break my bones, but whips and chains excite me!" All opinions are manf. suggested and are subject to change without notice.
spt@peace.waikato.ac.nz (Simon Travaglia) (01/19/90)
In article <1370@milton.acs.washington.edu> whit@milton.acs.washington.edu (John Whitmore) writes: >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. > > Wirewound resistors are expensive for high resistance >values, mainly used in the 0.1 ohm to 10,000 ohm range. The only negative aspect of wirewounds is that they are rather bulky (but then you should expect that if you have an application that requires them) > Carbon is a semiconductor; at low temperatures (liquid nitrogen) >a carbon resistor makes a GREAT sensor; when the resistor hits the nitrogen, I used to have a mate who Loved carbon resistors because you can "tune" them by filing them with a small file. (He used to play with cheap&nasty music circuits, and used a file to tune up the carbon resistors (3c .vs. about 20 for a trim pot) I don't know how this would effect the "hot spot" problem tho, it may create one. -- SimonT, spt@grace.waikato.ac.nz, Univ of Waikato, Private Bag, Hamilton NZ. This signature is completely biodegradable and useless, except for the tiny disclaimer, which is still biodegradable, but not totally useless. Disclaimer: It was a typing mistake and I've been fired for it.