shunt measurement in accuracy  - Page 1
Forum section: electronics / beginners

author: generic_username
author_rank: Regular Contributor
timestamp: January 03, 2023, 08:11:45 am
content: Hi,somewhat a noobish question but I can't seem to find my error in a current shunt measurement.So I have 1mR Resistor (1%/5W) in SeriesI'm using an Keysight 34465A 6.5 Digit DMMMeanwell 12V PSU (RSP3000-12)Electronic load EA-EL 9080-200Shematic with Paint skills attachedpurpose of this little measurement was to see if the electronic load was working as intended as an other measurement I took was looking a little odd and decided to check if there's a Problem with the instrument.So I decided to make a quick measurement with an 1mR shunt and solderd a 6mm² cable to the shunt with banana plugs to my DMM and set the electronic load to 10A. to my surprise the voltage was around8.2mV9.2mV which obviously would be awfully wrong. then it popped up in my head that the DMM can do current measurements as well (head -> wall) so measured with the DMM at 10Amps and 9.9... something Amps on the display, so electronic load works as intended, at least for 10A.but it kind of bugs me why the shunt measurement is way off. And it seems I can't really find a clue what I did wrong. The accuracy of the used tools should give better results, so I guess it's my setup that produced that error.Any Idea what went wrong?cheers

author: Psi
author_rank: Super Contributor
timestamp: January 03, 2023, 08:19:27 am
content: Did you have the multimeter on the lowest range for best accuracy measuring 10mV ?Were your power supply and DMM connections to the 1mR resistor isolated from each other. eg, they need to be totally separate and touch the resistor at the same point as close to the resistor as possible. You cant have either of them 'sharing' the cable for the other one.What type of 5W 1mR resistor is this? can you post a photo of how you have it connected?When you tested it, what did you have the electronic load set to. Should be set to constant current 10A.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 03, 2023, 08:30:45 am
content: Did you apply correct kelvin-type wiring with the wires going to the DMM together or twisted?

author: generic_username
author_rank: Regular Contributor
timestamp: January 03, 2023, 09:01:07 am
content: thank you for the repliesResistor is SMD Type, so I did my best to solder the wires as close as possible to the resistor (see picture) blue = power, red measurement cables going to the DMM.I tried my best to make a low resistance connction so.As I didn't had anything other at hand I soldered the Resistor to an old PCB where at least on Lead was not connected to something, other lead was cut as close as possible to the connection so it's not connected to anything.The instruments are on the same power line (PSU, DMM, electronic load) so I checked the CMR of the Multimeter according to the Datasheet it's 160dB for DC or is this not the right parameter?@ PSI the range was on auto normally it workes quite well but maybe it chose the wrong range, didn't thought about that.I actually did mess up a little when writing 8.2mV this is a typo it should actually be 9.2mV

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2023, 09:50:05 am
content: It looks as if your soldered connection points to the shunt have crept in from the ends, reducing its effective length and resistance.

author: generic_username
author_rank: Regular Contributor
timestamp: January 03, 2023, 10:14:48 am
content: so took a better picture, you might be right I thought it was a different type of Resistor joint (those with the U connection, don't know the name)I might as well try another one and see if that was the problem.thx

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2023, 10:41:18 am
content: Yes, the footprint connection points are very critical when it comes to meeting a 1% spec. Looking at the new photo, I'd estimate that the resulting resistance is around, err, 0.92mR.I can't see the ends under the solder, but those shunts normally have separate current and voltage connection points on the footprint (Kelvin connection).

author: MrAl
author_rank: Super Contributor
timestamp: January 03, 2023, 10:50:43 am
content: Hello,You should really have a 4 lead resistor for this. 0.001 Ohm is so low almost anything in the path will alter the resistance, and did it change any when it was heated for soldering.0.001 Ohm is hard to deal with for something like this. You probably can not make connections that are good enough to use that as a shunt. If there is an extra resistance of just a mere 0.0001 Ohms, it's off by 10 percent already.Try using a 0.010 Ohm just to see how that goes. See if the precision picks up by several percent. Then check out some 4 wire resistors.

author: Psi
author_rank: Super Contributor
timestamp: January 03, 2023, 11:44:37 am
content: i would bring in the wires separately and attach them to opposite side of the resistor in a way so they dont touch each other at all. They are only joined by the metal of the SMT resistorSee how I sense resistors are commonly made, so you have the big high current connection points and smaller sense connection points as well.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 03, 2023, 12:45:26 pm
content: For benchtop measurements it's probably easier to get a chassis mounted shunt.Eg:https://www.digikey.nl/en/products/detail/riedon/RSA-100-100/4967078