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bz

wrote in
oups.com:

On 18 May, 15:37, Ancient_Hacker wrote:
On May 18, 7:19 am, "N Cook" wrote:

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
onhttp://home.graffiti.net/diverse:graffiti.net/

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.

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.

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.

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.

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.

remove ch100-5 to avoid spam trap