Selectable 4-Terminal Resistance Standard

[Nogtail]

I've repaired a few multimeters recently and needed something to check the resistance ranges. I wanted a reference with values between 10Ω and 1MΩ with 0.01% tolerance, which should cover most multimeter ranges.

All the resistance standards I've seen use separate terminals for each resistor. I'm lazy and didn't want to move cables around to select resistors, so I used a rotary switch instead. Using a switch also ended up costing a bit less - gold-plated Pomona banana jacks aren't cheap!

The switch introduces some extra resistance, but from my measurements, it's only around 10mΩ, which is fairly negligible. I'll be using 4-wire sensing most of the time, which should eliminate switch resistance.

Are there any obvious downsides anyone can see with the design/construction? My main worry was mechanical stress on the switch influencing the resistors. I mounted the resistors a couple of millimetres off the PCB to give them some mechanical isolation. So far I haven't been able to measure any difference in resistance when applying force to the switch - at least with a 6.5 digit meter. I'm working on an SMD version, so perhaps it would be more of an issue there.

[enut11]

Very professional looking instrument. Well done. In my view, 4W is only needed below 1Kohm. However, I acknowledge the simplicity and ease of use of your design. The test leads in most hand-held multimeters introduce more error than calibrated resistors so it should be fine. Would you share how you made it look so good?
enut11

[Gyro]

A very neat implementation - I can say that because I went for the air wired approach! ( https://www.eevblog.com/forum/testgear/my-shunt-collection-(ucurrent-passive)/ ). Remember that you can also use the resistance standard as a multi range current shunt with lower voltage burden than typical DMMs (but without the protection). In my case, I went for 10R to 1k as I was targeting current shunt use.

[slavoy]

Very neat work 
May I ask how you made the labels on the enclosure? Is this a PCB?

[Nogtail]

Very professional looking instrument. Well done. In my view, 4W is only needed below 1Kohm. However, I acknowledge the simplicity and ease of use of your design. The test leads in most hand-held multimeters introduce more error than calibrated resistors so it should be fine. Would you share how you made it look so good?
enut11

Thanks! 4W definitely isn't needed for the upper couple of values, but it came for free with the design. It's convenient to have though, as you can leave the multimeter in 4W mode for all ranges.

I designed the front panel in Inkscape then imported it as a vector into KiCad. I really like the look of matte black PCBs with gold plating, but matte solder mask is a huge pain to get clean. Usually I'd throw boards in the ultrasonic cleaner after soldering and let it sort them out, but I didn't want to risk affecting the precision resistors so I cleaned them by hand.

A very neat implementation - I can say that because I went for the air wired approach! ( https://www.eevblog.com/forum/testgear/my-shunt-collection-(ucurrent-passive)/ ). Remember that you can also use the resistance standard as a multi range current shunt with lower voltage burden than typical DMMs (but without the protection). In my case, I went for 10R to 1k as I was targeting current shunt use.

Your 10R-1k box looks very similar, I'm surprised I haven't stumbled across it already! I'd like to build myself a 10M/100M box in the future - probably with an air wired approach like yours, or at least some isolation slots to cut down on PCB resistances/contamination throwing off the values. I was planning on using the some of the higher values as current shunts to get measurements of photodiodes in low light environments.

Very neat work 
May I ask how you made the labels on the enclosure? Is this a PCB?

Yep, it's a PCB as a top panel for the enclosure.