What structure/type of resistors would be best for purest R resistors to make up and be measured with an accurate calibrated measurement bridge. Normal off the shelf resistors that is, secondary standard level of accuracy is not required. Required a few, odd values like 374 ohm +/- 2 ohm , butactual values known accurately to +/- 0.01%, in the range 20 ohm to 100K
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
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You can buy 0.01% resistors, but they're not cheap!
Alternatively, there used to be a guy on eBay that sold resistor kits-- He'd measure some wirewound resistors with a GOOD ohmmeter, and mark them with their measured values, to 0.01% or so IIRC.
"N Cook" wrote in news:f2k5kr$8kh$ snipped-for-privacy@inews.gazeta.pl:
Over what frequency range will the bridge be used? Over what range of resistances?
If you are only working with audio frequencies, you can use carbon resistors, wire wound resistors and metal film resistors.
If you are working at higher frequencies, in the RF region, you must make sure your resistors are non inductive. Even some metal film resistors have spiral patterns cut in the metal film, making the resistor into an RF choke at some frequencies.
For ultra low resistance readings, 4 wire kelvin connections are necessary to reduce the error due to the resistance of the leads.
What is the temperature range under which you will be doing calibrations? All resistive devices have some kind of temperature coefficient of resistance.
What is your application?
-- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+ser@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
choke
only 100Hz to 10K and 15 to 25 degree C
"N Cook" wrote in news:f2kl2r$kg2$ snipped-for-privacy@inews.gazeta.pl:
....
Go to radio shack and buy some resistors with the necessary precision. For your application, the construction will not matter.
If you need odd values, make them from series/parallel combinations of standard values.
-- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+ser@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
Ancient_Hacker wrote in news: snipped-for-privacy@o5g2000hsb.googlegroups.com:
I'd want to know what "good ohmmeter" he was using before I spent any money on his "precision" resistors.
-- Jim Yanik jyanik at kua.net
on
The worst R type for drift is carbon, so forget about those. Picking cheap Rs using a meter isnt really effective: you may know its value when measured, but theyre sold as rough tolerance precisely because they cant be relied on to stay that value. Temperature and time cause wander.
As already mentioned, metal film are routinely spiral cut, so are no use for hf, but should be good for 10kHz, and are relatively stable.
The main thing is to pick an R with a very small tolerance - not only does this tell you what value you're getting, but also that the R should remain within that small tolerance.
NT
snipped-for-privacy@care2.com wrote in news: snipped-for-privacy@e65g2000hsc.googlegroups.com:
Resistors used for calibration purposes should NOT be used for ANY other purposes.
NEVER subject them to any current other than that of your ohm meter.
In such a case, long term chemical stability would be your main concern.
Even most carbon resistors would probably be ok provided you store them under normal 'comfort level' conditions (0-80 percent RH, 65-85 degrees F).
Ceramic encapsulated metal film or wire resistors should be more resistant to environmental factors than composite body carbon resistors.
When used within certain environmental and power limits that may or may not be apparent to the user.
HIGH REL or MIL SPEC resistors usually have tighter limits and longer lifetimes under load than 'ordinary resistors'.
-- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+ser@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
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