Calibrating a 100A DC shunt

[enut11]

First test with new setup. Time was limited to 2min so may not be directly comparable with previous results. DUT (BUL100A) is on 34401A DMM.

Load resistor was much cooler (~50C cf 100C previously). I don't see any real difference now between the REF (MP100A) and the DUT (BUL100A).

The ratio chart is also close to flat indicating similar tempcos.

Need to think about these results.
enut11

EDIT: Fundamental error here. I need to run these tests for at least 4 min in order to use the 2002 calibration data to compute the test current and hence the DUT resistance. 

[HighVoltage]

Isabellenhütte makes some very good shunt resistors, the RUG-Z
They can be loaded to 250W and they are very stable.
I have a few of their 1 mOhm versions and I am very happy with them.

At 100A and 1 mOhm you just have 10W power dissipation in the resistor.
The connection is done with M6 screws.


 

[MegaVolt]

I don't remember if this was discussed already, but you could build up to 100 A. Say you had a DMM that you trust to 10A. Take x identical shunts, put them in series with DMM, and measure voltage drop at 10A. Then put them all in parallel, if necessary compensate differences between shunts with a series resistor, and send X*10A through it. Measure voltage across each shunt add and up for total current. Use this to calibrate one or more X*10A shunts or DCCT. Repeat until you're at 100A.

At high currents, the resistance of the wires that help connect the shunts in parallel will kill the entire metrology

[alm]

At high currents, the resistance of the wires that help connect the shunts in parallel will kill the entire metrology

Why? Each shunt is carrying 10A and using the same 4w sensing. Measure voltage across each parallel shunt and add up currents. Leakage current is probably not a concern at these voltages/currents. The only concern I'd have is shunts not sharing current equally thus being far away from the point at which they were calibrated, but that's why I suggested series resistors to compensate.

[MegaVolt]

Why? Each shunt is carrying 10A and using the same 4w sensing. Measure voltage across each parallel shunt and add up currents. Leakage current is probably not a concern at these voltages/currents. The only concern I'd have is shunts not sharing current equally thus being far away from the point at which they were calibrated, but that's why I suggested series resistors to compensate.

Yes. If you take measurements on each shunt, this should work if the current at this point in time is considered constant (the noise is not great).

[enut11]

Found this listing on eBay, $33AUD. Anyone care to comment on suitability for shunt current testing? What else do I need to set this up?

https://www.ebay.com.au/itm/134745360474?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230811123856%26meid%3Da633a8ca142a48568297bdf8b264e1cf%26pid%3D101770%26rk%3D1%26rkt%3D1%26itm%3D134745360474%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2%26brand%3DUnbranded&_trksid=p4375194.c101770.m146925&_trkparms=parentrq%3Ad4b84d1718b0a8dae88e6978ffff78aa%7Cpageci%3A72d9534e-83f7-11ee-acdd-a6db99b57149%7Ciid%3A1%7Cvlpname%3Avlp_homepage

[alm]

Found this listing on eBay, $33AUD. Anyone care to comment on suitability for shunt current testing? What else do I need to set this up?

https://www.ebay.com.au/itm/134745360474 [snip tracking parameters that were making the URL ridiculously long]

Here is the datasheet. I don't quite see the point of a device with 1% accuracy and linearity error specs. Don't you have shunts that are better than this?

From the datasheet, it looks like the sensor just needs symmetric power rails from +/- 12V to +/- 15V, and has a voltage output of +/- 4V. But I don't see the point unless you need galvanic isolation or the wide bandwidth.

[enut11]

Thanks @alm. Poor accuracy and poor linearity. No go. Cheap for a reason.

[enut11]

Test results to duplicate Reply #1, MP100A (2002 calibrated Ref) vs 100A from Bulgaria (DUT).

Leap of faith here: I am assuming that a 21 year old calibration certificate is still valid!

Calculated resistance of the DUT is now 0.607 mOhm, ie 1.2% high?

REF/DUT ratio  chart looks good with a small downward slope.

Still, not sure if I'm making progress here or just running around in circles.  Maybe the next test, 2002 Cal shunt vs Murata, will shed some light.
enut11

[enut11]

This test compares the 2002 Calibrated shunt with the Murata 100A specified at better than 0.25%.

Calculated resistance of the DUT (Murata 100A) is 1.001 mOhm. Spot on, but is this just a coincidence?

REF/DUT ratio chart looks OK but with a downward slope. Also, been getting a weird oscillation during the tests??

[enut11]

Third of a trio of comparisons, this one MUR100A vs BUL100A at 20A.

Calculated DUT (BUL100A) resistance is .606 mOhm, ie 1% high.

Ratio of REF/DUT is a rising plot. May be due to the small size/mass of the Murata 100A which is less than half of the BUL100A.

[enut11]

Can we draw any conclusions from this project?

It is based on the assumption that the 2002 100A shunt is still in cal.

One result does seem encouraging, the 2002 MP100A vs MUR100A test. They seem to validate each other.

Until I get a better reference, I think I have done as much as I can. I played with test current reversals but the results were inconclusive. None of the shunts heated up by more than a couple of degrees - not surprising as they only had to deal with less than 0.5W.
enut11

[beanflying]

This test compares the 2002 Calibrated shunt with the Murata 100A specified at better than 0.25%.

Calculated resistance of the DUT (Murata 100A) is 1.001 mOhm. Spot on, but is this just a coincidence?

REF/DUT ratio chart looks OK but with a downward slope. Also, been getting a weird oscillation during the tests??

Interesting play time  Like any metrology you are now left wondering what about if? Time to chase the next level of certainty because 'settling' on what you have just isn't ok

I should rig up the 100A one when you get it back to me (NO Rush I am booked until the New Year) and hook it up to my 3-15V 60A bench supply and see what sort of comparative numbers I can get.

[enut11]

Hi @beanflying. Quite right. There is never an end to it. This can be good or bad but always something new to discover. Now curious about those damn LEM sensors!

A bonus for me was discovering how stable PC PSUs can be. At 20A on the 5v outlet, after 2min settling in time, the Gigabyte voltage stayed within a millivolt or so for 5 min at least. Not bad for $5.
enut11

[mzzj]

BUL100 and MP100 seem to have smaller drift or they drift in a same way after applying current. I'd be inclined to guess that MUR100 has highest drift since it is  physically lot smaller. But impossible to say without some extra equipment and measurements.

MP100 vs MUR100 ratio changes 0.08% during the test and it is more than I'd have expected as the shunts are operated at only 1/5th of nominal current.

What you might be able to do is add cooling fan right on top of the MUR100 and see if the BUL100/MUR100 ratio would be more stable.

[trobbins]

Another way to possibly get some more insight is to initially have two test shunts in series but with one shunt 'shorted' whilst the other shunt warms up and stabilises, and then unshort the first shunt, and monitor its change against the other shunt.  Then vice-versa, as a way to characterise each shunt's warm-up period.

[Berni]

The easy way to test for shunt temp co. is to wrap it up in some cloth for insulation, stick a thermocpule inside then log the thermocpuple as you run the test. That way you can produce a chart for the shunt resistance versus temperature.

This way you can extract the ppm/°C temp co out of it. These characteristics are often not quite flat, so it also gives you an idea of how much variation in temp co there is. In order to check that you got the right measurement you can repeat the procedure with different conditions and they should all follow the same curve, regardless of the test current.

The way i did this test is i used a big boatanchor 500A lab PSU to send a large test current trough the shunt, measuring it as it heats up (Using a Danfysik flux gate to accurately measure the current). Getting it up to about 100°C. Then i switch the cables over to a Rigol DP832 where a script over SCIPI toggles the PSU between 9A and 1A while a 8.5 digit DMM mesures the shunt voltage. This toggling lets me null stray thermal voltages. Since the test current is much smaller the shunt cools down and i am continously mesuring it while it does so. Towards the end i unwrap the insulation about it to let it cool faster towards room temp.

So this way i get a test at different currents and temperatures in one run. At the same time also testing out that the shunt can survive the full rated current.

Also do use cable shoes to connect to the large screw terminals. For low resistance shunts the resistance of the block on the end matters, so you will get different results depending on where exactly on the block the current enters. Similar for where the sense wires connect (the block doesn't have the same voltage everywhere on it).

[alm]

Can we draw any conclusions from this project?

Coming up with any absolute value beyond what you already know is impossible since you don't know for sure how much each resistor has drifted. But you could try to estimate the uncertainty of your relative measurements based on things you can measure, like the noise in the setup (based on standard deviation of the voltage values), temperature coefficient (see post above), accuracy of the DMMs you use to measure the voltage (probably negligible contribution) and repeatability of the comparison between the shunts. Try reversing the current to see the effect of this, and varying connections (see the Ohm-labs white paper someone posted early in the thread). The result of this should be that you can see the ratio between the resistors is 1.xxxx +/- 0.xxx as 95% confidence interval (k=2). That should tell you how accurately you know this ratio. A lot of metrology is about ratios, not absolute values. Absolute values are just ratios relative to some higher standard.

[PartialDischarge]

Finally received the Ultrastabs from ebay. They work great.
Used 6 x 10.5Ohm parallel resistors (YR1B10R5CC) as load, to get a total of 100mV/100A at the output. Used all coaxial cables to the sensor (+15, -15, output).

Offset was about 5uV ( 5mA), so could easily measure 100mA as 100uV after nulling this offset.

[tszaboo]

Isabellenhütte makes some very good shunt resistors, the RUG-Z
They can be loaded to 250W and they are very stable.
I have a few of their 1 mOhm versions and I am very happy with them.

At 100A and 1 mOhm you just have 10W power dissipation in the resistor.
The connection is done with M6 screws.

These shunts are great. How did you get them? I wanted to acquire one at home since I used them about a decade ago for calibrating things as the resistance reference.

[HighVoltage]

Isabellenhütte makes some very good shunt resistors, the RUG-Z
They can be loaded to 250W and they are very stable.
I have a few of their 1 mOhm versions and I am very happy with them.

At 100A and 1 mOhm you just have 10W power dissipation in the resistor.
The connection is done with M6 screws.

These shunts are great. How did you get them? I wanted to acquire one at home since I used them about a decade ago for calibrating things as the resistance reference.

They are usually very expensive:
https://www.distrelec.de/de/leistungswiderstand-1mohm-250w-isabellenhuette-rug-r001-tk1/p/16057580?queryFromSuggest=true&itemList=suggested_search
(EUR 1.251,88)

I was very lucky a few years ago, when they were offered at a huge discount at Distrelec

[enut11]

A lot of discussion above is about the tempco effects of different shunts, being related to either size or construction material.
In my tests, even at 20A, the shunt elements rarely changed by more than a couple of degrees C. Dissipation power= 20x20x0.001=0.4W
Seems to me there are other effects at present unknown.
enut11

[PartialDischarge]

I have a gwinstek mili ohm meter and in my experience the real problem with low valued shunts is where you take the measurement, errors become bigger wrt to the nominal value of the shunt.

[Berni]

That is because you are using a very low resistance shunt at low currents. So you also get a tiny signal out of the shunt that is more difficult to accurately measure (especially due to thermocpuple voltages)

Once you go >300A the pesky square in P= R * I^2 starts to become a problem since even a very low resistance shunt will be dissipating 10 to 100W of power, so things will certainly get rather hot even if it is a huge shunt.

Well made shunts can have impressively low temp co. but i have also seen some cheep chinese shunts that had horrifically bad temp co. like 1000ppm/°C or something. The alloy mix for the resistive material has to be carefully tuned just right to get that low temp co. This is why good shunts can be quite expensive.

[tszaboo]

A lot of discussion above is about the tempco effects of different shunts, being related to either size or construction material.
In my tests, even at 20A, the shunt elements rarely changed by more than a couple of degrees C. Dissipation power= 20x20x0.001=0.4W
Seems to me there are other effects at present unknown.
enut11

A few things. Dissipation goes up with square of the current. So 100A is 25 times as much dissipation. And nonlinearity comes with this.
Often times for four wire shunts, the actual value of the high current path is a lot higher than the nominal value. You have the nominal value between the 4 leads and some extra for the leads leading there.
When measuring 20A*0.001mOhm = 20mV, thermocouple effects will effect matter. I had a design where the leads were connected to different sized PCB planes, and the shunt was measuring current differently in either direction.
The better ones also have a voltage coefficient defined, that I'm not even sure what is the mechanism for that.