Electronics > Metrology

Creating some high-precision resistors, for calibration purposes

(1/4) > >>

RandallMcRee:
I hope to create some precision resistors that I and other members can use for generally checking their 6.5, 7.5 and hopefully 8.5 digit meters. What I have available to me are Alpha Electronics MPP and MPQ types which are 0.2ppm, typical (see data sheet).  I plan to put 4-16 together to create a stable, robust value with good temperature characteristics.

Now, the real question: is it a good idea to have a switch in the resistor path? You see, I have this nice 4 deck, 12 position rotary switch. This would allow four wire measurements on 12 different resistors (or two different configurations of 6 resistors). Since a four wire measurement would be required that should nullify the contact resistance of the switch, right? It would be nice, obviously, to not have to pull one resistor out and pop another in, over and over. But if the repeatability is not there the switch would defeat the purpose. What are your thoughts?

Thanks,
Randall

alm:
Given the accuracy levels you are targeting, I'd worry about thermal EMFs for any external connections or switches. So I would be concerned about the thermal EMF of the switch. Contact resistance is no big deal for four wire measurements that you would be using. However, low thermal EMF binding posts are not cheap either.

RandallMcRee:
I do have four low-thermal binding posts that I purchased from TiN in the group-buy some time ago. I could use those.

The switch and resistors would all be in the same small box. That would negate thermal effects, presumably.

Thanks for the thoughts,
Randall

WattsThat:
A low TCR does not a standard make. What’s the long term drift of a foil based epoxy encapsulated smd part? Do you wonder why it’s not on the datasheet? I do.

There’s a reason why 100ppm is tightest tolerance you can order. Add a possible soldering shift of 300 to 500 ppm and you don’t have much of a standard left.

And you’re worried about a switch in the path?

alm:

--- Quote from: WattsThat on November 07, 2022, 11:12:18 pm ---A low TCR does not a standard make. What’s the long term drift of a foil based epoxy encapsulated smd part? Do you wonder why it’s not on the datasheet? I do.

--- End quote ---
Good point about the packaging. Humidity will get into the resistors and affect the value, and SMD makes them susceptible to stress as the PCB swells and contracts due to temperature and humidity. The performance probably won't rival hermetic through-hole foil resistors or precision wire-wound resistors.


--- Quote from: WattsThat on November 07, 2022, 11:12:18 pm ---There’s a reason why 100ppm is tightest tolerance you can order. Add a possible soldering shift of 300 to 500 ppm and you don’t have much of a standard left.

--- End quote ---
Or maybe 100 ppm is the lowest tolerance they can economically do production testing on? And what does soldering shift have to do with this? Surely you calibrate a resistance standard after assembly?

Navigation

[0] Message Index

[#] Next page