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

[Micke]

HighVoltage: Thanks for offer, good to know if the need arises for LEM Ultrastab, have looked at them earlier, really nice devices!
I use a Wavetek/Datron 4600 as current source (capable of 11A DC-20kHz)
Unless the combo current shunts and 4600 drift in opposite direction, both are very stable even if outputting 10A continuously!

DavidKo: Thanks for the advice, changing cable to twisted pair is feasible, might try it in the future, changing the coin (connection area) to bigger is more difficult. Strange how hard it is with shunts for AC, that already above 50/60Hz errors are growing fast!
Would love to see teardown of the koaxial design Fluke A40 series!
I did find teardown of the JRL CH48T4HK resistor long time ago somewhere (did not save URL), the trimming looks terrible! 

[DavidKo]

I have checked the datasheet of JRL CH48T4HK and these pictures confirms my suspicion. They are intended to be used on DC since no AC is mentioned. They have a nice coil inside. I expect that you will have better results with 0.001Ohm standard resistor (old USSR are quite cheap and what I have seen on ebay, they are rated for 10A with 32A maximum). Zlymex shown and inside of such a resistor so you do not need to worry about the inductance.

[HighVoltage]

If you look for some really good milli Ohm resistors, also look at:

Isabellenhütte
RUG-Z-R002-0.1-TK1

They come is stages of 0.5 mOhm to 100 Ohm
Datasheet attached

However, I have not tested them for AC!

[JohnG]

For what it is worth, keeping the inductance low is a necessary, but not sufficient condition. It is also required that the inductance does not vary with frequency in the range of interest.

The first is a simple matter of frequency response. 100 nH of inductance gives a corner frequency of 1.6 kHz, where the voltage magnitude error will be about 30%. 100 nH is about 10 cm of wire in a circular loop, more or less (it is a weak function of wire diameter in such a situation. If you coil it, the inductance can increase an order of magnitude, but if you minimize the loop area, it can be much smaller.

The second is more insidious. If the shunt element skin depth starts approaching the shunt element dimensions, the resistive portion of the impedance will start to increase due to eddy current effects. i.e. skin effect and proximity effect. Note that long, flat conductors can mitigate this if configured like a parallel plate waveguide, or they can make it worst if located in a single plane with adjacent edges. If the problem is bad, one way it will manifest items is a decrease in the series inductance as frequency increases, due to a change in the current distribution. Coaxial shunts are good in this regard as the shunts effective have no "width" perpendicular to the current flow, so you only need to worry about the conductor thickness, and of course the shunt interconnects.

I've attached a paper. I know that there are earlier and more comprehensive references, but it's what I could find quickly and is a decent intro.

John

[laichh]

My 10 mili-ohm DIY shunt, DC & AC calibrated by Ohm-Labs.

Shunt element: https://www.alpha-elec.co.jp/w2img/202210101035252016071918051067010_PSB(EN)_final_06Oct2021.pdf

[HighVoltage]

My 10 mili-ohm DIY shunt, DC & AC calibrated by Ohm-Labs.

Very interesting DIY Shunt.
Can you share the calibration certificate by Ohms Labs?

[DavidKo]

I have noticed some cheap shunts when looking for shunt recommended by HighVoltage
10W Isabellenhutte R001 ±0.5% <10nH, 75ppm ~GBP25
50W RXM50 R001 A ±0.05% nothing said about inductance, 30ppm ~GBP190

[HighVoltage]

I have noticed some cheap shunts when looking for shunt recommended by HighVoltage
10W Isabellenhutte R001 ±0.5% <10nH, 75ppm ~GBP25
50W RXM50 R001 A ±0.05% nothing said about inductance, 30ppm ~GBP190

Interesting, I was not aware of the RXM50 series resistors.
Thanks!

The RUG-Z resistors by Isabellenhütte are much more expensive:
https://uk.rs-online.com/web/p/chassis-mount-resistors/4904827?sra=pmpn

£930.74 + VAT

[laichh]

My 10 mili-ohm DIY shunt, DC & AC calibrated by Ohm-Labs.

Very interesting DIY Shunt.
Can you share the calibration certificate by Ohms Labs?

Here you are.

[ap]

The value of the RUG-Z is that it is available in a 1ppm/K version. AFAIK no other high power single resistor is available with such tight TK. Distrelec offers them at a lower price, still expensve but worth it if one does not want to use a Ultrastab and needs a metrology high power solution.
I use one in my lab with DIY solution fan case similar to the one shown above (but linear and external PSU for EMI reasons) for up to 10 Amps (0.1R) and I have also seen it drift very moderately over the years.