100A Meter Shunt - Page 1
Forum section: electronics / general

author: German_EE
author_rank: Super Contributor
timestamp: December 28, 2019, 10:30:13 am
content: I am currently playing around with meter shunts that work at the 100A range, I actually need about half that but I'm designing something that I want to last. One thing I am not sure of is how to trim and calibrate the thing, assuming it's fitted to a 6.5 digit meter then I need to vary the resistance by nano ohms to get a meaningful calibration down to 1mA or so..How?My best guess is to aim for (say) 200mV @ 100A and then feed the signal into a series of course and fine potentiometers until I get exactly 100.000mV with a 100A load.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2019, 11:03:29 am
content: With these high current shunts a mayor difficulty is the heating due to the test current. So in the 100 A range the burden voltage is usually chosen smaller, more like 50 mV or 75 mV. For this reason it could also be a good idea to use a shunt that is made for a higher current than the current one is actually interested in. So if you want to measure 100 A with good accuracy take a 200 A shunt and thus have 1/4 the heating it is made for. The voltage would than be smaller, but with modern meters the small voltage is usually the smaller problem than self heating.Adjustment is easy in software. If really needed one could use an external divider.

author: Rerouter
author_rank: Super Contributor
timestamp: December 28, 2019, 11:45:19 am
content: to get 6.5 digits out of a shunt is not easy,Thermal expansion of the shunt changing its resistance (It is non linear)Mechanical coupling, As any stress / flex changing the shape will change the resistance (e.g. a motor running near by making it vibrate)Junction voltages induced across any metal-metal junctions (you can potentially remove this by spot welding measurement wires made of the same material to the shunt so there is no junction, and running them far enough away that the gradient is minimised)Variable air currents / vortex sheddingIn these cases a sealed oil bath shunt is what would probably be used for stability as you are after 10ppm accuracy as your goal post.If its not some fancy metrology shunt you will also probably need to qualify it yourself, e.g. set up a 1mA test current you can switch on and off, and use that to map out the curve of it at different temperatures.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 28, 2019, 07:27:53 pm
content: Why do you need to measure at 6.5 digits, it make no sense as other replied ??

author: German_EE
author_rank: Super Contributor
timestamp: December 28, 2019, 07:59:57 pm
content: Simply because the meter shunt will be plugged into a 6.5 digit bench meter. I could plug it into one of my handheld 4.5 digit meters but where's the fun in that?Experimentation so far has shown changes in recorded values due to self heating, although at these resistances that self heating is pretty low- I suppose that one answer to this is a shunt with a very high thermal mass.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 28, 2019, 08:01:07 pm
content: Quote from: German_EE on December 28, 2019, 10:30:13 am50mv shunts were either ground down on one edge or cuts were made into the edge to raise the resistance to the desired value. If you look at some ebay listings, the cuts are plainly visible - even on new shunts.Ed

author: HighVoltage
author_rank: Super Contributor
timestamp: December 29, 2019, 12:47:16 pm
content: That is a very difficult goal you have for 100A and 6 1/2 digits.Forget all the small shunts which are rated 100A, they will heat up far too much.I started with such 100A / 60 mV shunt and failed.Now I am using a 1000A shunt and a nanovoltmeter for precision 100A measurement.Here are some of my shunts from over the years of measuring 100A

author: BravoV
author_rank: Super Contributor
timestamp: December 29, 2019, 01:03:17 pm
content: @HighVoltage, what the tempco of those shunts ?

author: BravoV
author_rank: Super Contributor
timestamp: December 29, 2019, 01:22:14 pm
content: Alternative source, PCN Japanese current shunt resistor ->http://www.pcn.co.jp/pcn-English/Images/seihin-02.html#02-04Look for RH-250M series, they're rated max at 200A ->DatasheetI scored few of them used while ago, they're "relatively" cheap as its not well known or popular, as new one cost > $200.Shot with TO-220 chip for size comparison.Few 1 mOhm, 10 mOhm and 1K Ohm with 0.1% accuracy and 50 ppm/K, check the datasheet.Black wires are the sense lines.Better version with solid sense terminals instead of wires like above examples, a 10 mOhm, 0.1% accuracy, 50 ppm/K, rated at 50 Watt with max current @200A.Better accuracty at 0.02% (Q grade).

author: HighVoltage
author_rank: Super Contributor
timestamp: December 29, 2019, 02:19:12 pm
content: BRAVO, I did not know this Japanese brand.In addition to my above shunts, I also have a few of these...RUG-Z-R001-0.1-TK1- Power Resistor 1mOhm 0.1% 250W, made by IsabellenhütteThey are probably one of the best shunt resistors, money can buy.Rated for 250W, so theoretically good for 500A, I have used them only for up to 100ARetail: Euro 570 (+VAT)I got a few of them for like Euro 10 each ..

author: HighVoltage
author_rank: Super Contributor
timestamp: December 29, 2019, 02:27:22 pm
content: Data for Isabellenhütte RUG-Z

author: BravoV
author_rank: Super Contributor
timestamp: December 29, 2019, 02:28:13 pm
content: Yep, that RUG-Z series is the ducks gut, as Dave like to say.