DIY 10K low TC home lab referance

[HighVoltage]

.... 10R Trimmer ....

Nicest machine i own is a 34410A, plus another classic 34401A.

When I started with precision resistors many years ago, I also had "only" a 34401A and a 34410A as my best instrument and a low Ohm series trimmer worked perfectly and I was really happy with my first 10k "standard".

It's only when you go down the rabbit hole of more accurate ppm and higher precision of a 3458A that a series trimmer might not be the best solution.

[pizzigri]

10 ohm random contact resistance would make your project futile if you used 10 ohm trimpot in series. Better way would be something like 100M fixed resistor and 10k trimmer in parallei with your precision resistor.

Uhh but what kind of resistors should those be? A 100M resistor with high accuracies and low tempco is likely going to be hugely expensive. Can you suggest a bom and example I could follow? And Ulf, I will never be able to afford a 8.5 meter, especially for the huge hurdle and cost of calibration.... I am just trying to have a better “standard” than the single Vishay resistor I built some time back without having any possibility to bave a real idea of its value.

[KT88]

There are (at least) two reasons for a parallel trimming arrangement:
1. any series elements hamper the thermal conditions at the contacts of the main resistor like added TCs and different thermal conductivity...
2. If the trimming range is say 10ppm only this part is affected bei variations of the large trimming resisor (100ppm of 10ppm for example).

[pizzigri]

However, the precise resistors to be used are going to be really difficult to find.... the proposed 100M +10K resistor +10K frimer is going to yield about 5000 ohms.... to have 10k the calculator at Digikey says I need a 20.1k resistor and 20k trimmer plus the 100M resistor. That really makes it complicated. And I do understand the advantages of parallel resistors, I have gone through the great post with the teardown of resistance standards, but my needs are of the simplest design possible for calibration so not to burden some merciful EEVblog member taht may want to help with the actual calibration and also ease of construction, availability of the resistors and reasonable cost - I would reuse as much as I can what I have, the metal box, Pomona contacts and maybe the Vishay resistor of my standard (which on my 34410 meters as lower than 10k, at least after a couple hours of measuring ).  Both 34410 and 34401 are still in calibration.

[mzzj]

However, the precise resistors to be used are going to be really difficult to find.... the proposed 100M +10K resistor +10K frimer is going to yield about 5000 ohms.... to have 10k the calculator at Digikey says I need a 20.1k resistor and 20k trimmer plus the 100M resistor. That really makes it complicated. And I do understand the advantages of parallel resistors, I have gone through the great post with the teardown of resistance standards, but my needs are of the simplest design possible for calibration so not to burden some merciful EEVblog member taht may want to help with the actual calibration and also ease of construction, availability of the resistors and reasonable cost - I would reuse as much as I can what I have, the metal box, Pomona contacts and maybe the Vishay resistor of my standard (which on my 34410 meters as lower than 10k, at least after a couple hours of measuring ).  Both 34410 and 34401 are still in calibration.

I guess my explanation wasn't enough obvious: Connect 100M and 10k pot in series resulting adjustable resistor from 100M to 100.1M
Now connect that parallel with 10k precision resistor and you get adjustment range from 10k||100M = 9,999k to 10k||100,1M 9,9990001k
That is actually way too small adjustment range even for super accurate resistor( I pulled the example numbers out of hat)

Lets try again: Select 10,0155k precision resistor and connect 6M to 7M variable resistance parallel to that (6M resistor + 1M trimpot)
You get trim range from 9998,8 to 10001,2 ohms.
Now your 1M trimmer 100 ppm tempco has practically zero effect on end result(100ppm trimpot tempco would cause 0,0002 ohm error or 0,02ppm)
6M resistor tempco is also largely irrelevant as 100ppm tempco would have 0.2ppm effect on the final value.

There is also some traps with this approach if you go too deep in the ppm rabbit hole..


 

[pizzigri]

Ahhh i actually got it backwards, now it does make sense!!!
OK so to go on to the practical side... I would need a resistor that gives me a 0.5% tolerance, so that buying like four/five of these resistors I could actually find a 10,0155k value, BUT with a good tempco of say 0.2%. I looked up on Mouser and could not find such a resistor... other possibility, contact Vishay and have them build a custom Z Foil resistor, doable but at unknown cost (and I suppose very steeeeep cost). I find it really difficult to do....
and as I said, it would be used as a reference for my lab, which at best has good 6.5 digit capability.
Im a bit demoralized.