Electronics > Metrology

DIY 20A 0,1R Current shunt, Metrology grade(?)

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Hmm... easy to go down the rabbit hole...
It started I bought a defective Datron 4600 (Transconductance amplifier 11A DC-20kHz) to be accompanied to my Datron 4705 (bought with broken 100V and 1000V range).
The symptom with the 4600 was that after like 2h it turned off and lit the red "PSU" LED.
After changing all electrolytic capacitors in the switched mode PSU to new Low-ESR 105C A-brand, the 4600 has worked fine. Opening the PSU was difficult, box is all welded (amazing design), only access is from one of the short sides.
But having the 4600 fixed, problem arised with adjustment... using normal Amp-meters are not accurate enough. First thought of buying used FLUKE Y5020 0.1R 20A, but having seen teardown pictures here on the forum https://www.eevblog.com/forum/buysellwanted/ice-teas-score-thread/msg3439280/#msg3439280 I was not impressed, no hermetic resistors, it looked like a very expensive heating fan...
I then thought of buying a used Burster 1282-0.1R, but it is only specified at 50Hz, I E-mailed Burster and asked for additional data at different frequencies, but it was unfortunately not available. https://www.burster.de/fileadmin/user_upload/redaktion/Documents/Products/Data-Sheets/Section_1/1282_EN.pdf

So what to do...
Why not buy 10pcs 1R Hermetic Julie Research and put in parallel?
The specs are amazing, 1R +/-0.005% 6W TC +/-2.5ppm/C   8)
Datasheet on Xdevs: https://xdevs.com/doc/JRL/JRL1/CH48T4HK.pdf
It is quite expensive buying 10 pieces, but less expensive than used Fluke Y5020 or Burster 1282.
Said and done, ordered 10 pieces from USA.
Bought 60mm aluminum tube (thickness 3mm) from a car modding site that was just perfect fitting the resistors on.
Found a very nice grey cube steel box (Hammond 1415D) 152.4x152.4x154.23mm
In the back I put a 80mm 12V fan, and a PCB with LM317 for adjusting the fan speed.

But how to connect Sense?
I thought of 3 options, but decided to go for option 2, using sense connection on ONE resistor.
Did an experiment with option 1 (using Sense at the connection points of the resistors), but the result was an absolute disaster, sensed voltage was increasing constantly... Copper has terrible TC, but I thought that since the leads are quite short, and the temperature rise should not be that high, it would be OK... but not!
Next problem was how to arrange the connection with the resistors, I then found that old coins with diameter 27mm and high copper content was perfect, so sacrified and grinded down two 71 year old Swedish 5-Öre coins that were in bad shape from the beginning.

Next I will send the shunt for calibration   :)
Will be interesting to see the AC performance, I am hoping for good performance up to 20kHz...

For frequencies higher than a few Hz this paper may be interesting:
Probably already a bit on the academic side but for 20kHz worth reading...



Did you actually measure tempco of paralleled setup. I've tried to combine multiple of these JRLs into higher power 1R device, but due to copper leads TCR the resulting tempco was much worse than any single resistor.

I didn't figure out way to improve it yet, but did log results on xDevs draft.

Lots of good work and probably a very accurate .1 Ohm resistor at DC. At high audio frequencies it may depart from the target accuracy. The resistors have inductance; I measure 150 nH at 100 KHz and 50 nH at 1 KHz on my sample but I would not take those numbers as absolutes. The overall construction is more important and you want the current flow to be as coaxial as possible. Maybe drill a hole in one coin and wire with coax going through the coin to the other side.

What you are really looking for is a Fluke A40A current shunt. There are a few equivalents but all are quite rare. If you were nearby you could borrow mine (which sees virtually zero use). The A40B is closer to what you have built. https://us.flukecal.com/products/electrical-calibration/electrical-standards/a40b-series-precision-dc-and-ac-current-shunts 

I suspect that TCR issue with the copper is also an issue on the precision decade resistors.  How did they overcome that limitation? Did they? https://www.ietlabs.com/esi-tegam-db62-decade-box.html  says 20 ppm up to 10 Ohms.

As long as the device is used as a current shunt, kelvin sensing would mitigate (most of) the problems related to copper leads. The Kelvin connections can be summed with some low Ohm resistors (it looks like Fluke did just that with the A40B series).
Once one tries to put multiple shunts in series to increase R it gets difficult.
Thermal EMF is a different story though...


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