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High current shunt - temperature drifts
CosteC:
Hello everyody
I am in need to measure about 400 A AC (continious) with as good accuracy as possible and fits 6 1/2 digit multimeter (~0.06%). Current is somewhat distorted 50 Hz sine.
Easiest way seems shunt. I have found 0.2 class and 0.25 class shunts:
https://www.lumel.com.pl/en/catalogue/product/shunts-b2-60mv-3 CLASS 0.2 but temperature coefficient seems 100 ppm/K which is rather a lot, but also specified for ambient temperature not shunt temperature.
Other https://docs.rs-online.com/18e7/0900766b813bb936.pdf is CLASS 0.25 and has thermal coefficient of 15 ppm/K which is bit questionable, as manganine coefficient is 0ppm for 25'C but -42 ppm for +100'C Obviously there are slightly different alloys under the same name, varying a bit in performance.
What is your recomendation for such measurements?
Kleinstein:
400 A is quite high. A good shunt would habe to be phyical large to get good power handling capability. One may want a special low noise amplifier to get away with a low burnden voltage. For AC only the DC drift would be less of an issue. 60 mV burden would still be 24 W and the current path outside the active area can add to this.
The shunt would still need to be low TC to handle the heating - no need to care about 100 C, as the shunt should not get that hot at all.
Manganin is still a good choice, because of the low resistivity, low thermal EMF (important for DC) and good contacts.
The 2 shunts shown are way to tiny - for good accuray consider 10 x or more physical size. This could be shunt for a higher current (e.g. 1000 A) and using less burden with a better amplifier. Here AC is much easier than DC.
An alternative to consider would be a good current transformer instead of directly a shunt.
Hydron:
This is definitely a job for a CT rather than a shunt - far safer and easier. You should be able to get one that matches the 0.2% shunt accuracy fairly easily (ratio error, so you have to add the meter AC current error too), or I suspect there are other better options if you want to spend more.
TimFox:
You can use a CT terminated with an appropriate low-valued resistor to develop maybe 100 mV rms across that resistor and drive an AC voltmeter to avoid any error on an internal current shunt in the DMM.
CosteC:
--- Quote from: Hydron on July 21, 2024, 10:56:14 pm ---This is definitely a job for a CT rather than a shunt - far safer and easier. You should be able to get one that matches the 0.2% shunt accuracy fairly easily (ratio error, so you have to add the meter AC current error too), or I suspect there are other better options if you want to spend more.
--- End quote ---
Any proposals for 0.2% class or better CT?
--- Quote from: TimFox on July 21, 2024, 11:01:07 pm ---You can use a CT terminated with an appropriate low-valued resistor to develop maybe 100 mV rms across that resistor and drive an AC voltmeter to avoid any error on an internal current shunt in the DMM.
--- End quote ---
Well. Not adding low value resistor in most of cases will ruin CT accuracy... And I studied DMM current measurement accuracy. It is not-so-great for currents > 100 mA.
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