Where to buy highly precise sense resistors?

[jkrbasu]

Hello again!

For those of you who spend a lot of time calibrating- where do you get your high precision sense resistors? And what's the best tolerance you've seen? I've seen +/- 0.005% in some of Dave's videos, but haven't come across anything like that in the flesh! I'd like to get some in the range of 0.1 to 100 Ohms (four decades)

Cheers-

[SvanGool]

If you insist on the accuracy, you can find a used 0.005% 100 Ohm hermetically sealed (no humidity impact) 2ppm/°C resistor here and you get a 1K resistor for free  .

[ap]

There are several contributors to what you define as precise:
-initial accuracy (sometimes parts do not even meet these, be carefull)
-aging drift (aging effects from humidity, crystal structure changes...)
-thermal drift
So to minimize this, I am using hermetic, low tempco resistors (VPG...), with applied power limited to 10mW, so mainly for the mA measurements, and for lower impedances (higher power), Isabellenhütte devices (1ppm/K); not cheap though. Hermetic power resistors from VPG or wirewounds may be an option. The latter have a higher aging rate. All depends on your budget and accuracy requirements (not only initial accuracy). Also, AC sensing may e.g. make wirewounds a no-go.

[jkrbasu]

Thanks guys! Looks like the Vishay products are the way to go. I'll keep my eyes peeled for these on eBay

[alm]

What is the word on the performance of desoldered VPG resistors from suspect sources (i.e. eBay)?

[SvanGool]

I would take a look here, TiN tested a lot of used resistors and my personal conclusion would be that at least the TCR is not bad. I know he also uses sometimes the source I mentioned above.