Shunt resistor issue - Page 1
Forum section: electronics / general

author: LukaszRed
author_rank: Contributor
timestamp: July 11, 2016, 12:24:56 pm
content: Hello,We have designed a shunt resistor using manganin and ETP copper. We have a task to get a shunt resistance 200uOhms but we are having a weird readings.In reference to our spreadsheet for shunt resistance calculation (attached) we should get 200uOhms. We have few points of measurements, please find results of resistance check between point pairs below:- 1-1 ~ 200uOhms- 2-2 ~ 200uOhms- 3-3 ~ 215uOhms- 4-4 ~ 230uOhms- 5-5 ~ 210uOhmsWe checked it on CropicoD07 at 1A.Nominal current for this component is 100A.Is there any other reliable method of checking the shunt resistance without nominal current?Or shall we measure it by checking voltage drop at nominal current between pins? Does anyone know how to measure shunt resistance properly and what are the factors affecting our readings (especially on top of the pins)?Cheers,Luk

author: TiN
author_rank: Super Contributor
timestamp: July 11, 2016, 01:13:22 pm
content: Well, if you inject test current at same point as you measure then your error comes from copper metal resistance.What is the result if you have current injected at points 5-5 but reading taken at 4-4 and 1-1 ?

author: kfitch42
author_rank: Frequent Contributor
timestamp: July 11, 2016, 01:25:33 pm
content: I am a software guy so take what I say with a grain of salt, but my first impression is that when measuring from the "top of the pins" ( I assume you mean "3" and "4"), it has to go through more "squares". All of my knowledge on this topic comes from:

author: LukaszRed
author_rank: Contributor
timestamp: July 11, 2016, 03:30:06 pm
content: Quote from: TiN on July 11, 2016, 01:13:22 pmWe measure only the voltage drop values in this test. Then converse the mV into mOhms and readings seem to be fine.

author: rob77
author_rank: Super Contributor
timestamp: July 11, 2016, 04:12:48 pm
content: Quote from: LukaszRed on July 11, 2016, 03:30:06 pmbut the question is where you inject the current.you have to do a 4 wire measurement - use 2 wires to inject the current to points 5 - 5 and then use another 2 wires to measure the voltage drop across point 4-4 and 1-1.EDIT:forgot to mention.. 200mOhm for 100Amp nominal is way too much.... 0.2 Ohm @ 100Amp means 20V drop and therefore 2kW power dissipation...

author: StillTrying
author_rank: Super Contributor
timestamp: July 11, 2016, 04:32:22 pm
content: The only way to measure its resistance is to inject the current at 5 - 5, and read the voltage at 4 - 4 or 3 - 3. When you're getting close to the correct resistance turn the current up to 10A for a bit better accuracy.Your figures don't make sense to me, if you mean milli ohms it's going to melt, if you mean micro ohms the voltage is going to be too small to easily measure.

author: TiN
author_rank: Super Contributor
timestamp: July 11, 2016, 04:44:00 pm
content: Edit (messed up calcs)It's 0.2V drop at 100A, so 20W. While 200mV not best for measurements, it's still doable with decent accuracy.With 200uOhm (0.0002 ohm) It's 0.02V drop at 100A, so 2W.Calc.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: July 11, 2016, 05:48:18 pm
content: If 0.2V is the drop at 100A, then your shunt is 2m? not 200m?! The most direct method to measure this resistance is a low ohm bridge, such as the L&N model 1600/1610 which allows accurate measurements at high currents. The indirect method of measuring voltage drop across the shunt is also valid as long as the current source is stable and measured accurately although current measurements tend to be a bit less accurate than direct resistance measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2016, 06:38:32 pm
content: The construction of the shunt looks like there is still quite some copper between the voltage sensing leads. So there will be some contribution to the TC as well. Normally the voltage sensing contacts should come from the resistor material, not the copper. So the construction does not look really good unless you need the copper part for compensation. Also the layout should be more symmetric, to ensure equal temperature are the transitions copper to Manganin.At high currents like 100 A a drop of 200 mV is already quite high. Often shunts use a lower nominal dropout (e.g. 50 mV).

author: BravoV
author_rank: Super Contributor
timestamp: July 11, 2016, 06:52:54 pm
content: Quote from: Kleinstein on July 11, 2016, 06:38:32 pmYeah, I was wondering about those huge resistance as well.Attached example few of my really old shunt resistors for high current sensing, not for high accuracy though. A TO220 body for size reference.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 11, 2016, 07:10:16 pm
content: Is this a constant current of 100A?Or is this just a pulse?What is the application?

author: MK14
author_rank: Super Contributor
timestamp: July 11, 2016, 07:23:41 pm
content: Is it 200 milli Ohms like it says in the OP ?Or is it 200 Micro Ohms, like it seems to say in the OP's PICTURE(S) ?tl;drDoes he mean m = Micro or m = milli ?(I accept that technically milli is correct, but some people get confused).

author: StillTrying
author_rank: Super Contributor
timestamp: July 11, 2016, 11:17:45 pm
content: Quote from: DiligentMinds.com on July 11, 2016, 08:55:13 pmDid you mean too low?Quote from: DiligentMinds.com on July 11, 2016, 08:55:13 pmAny bets? Wethem off.

author: StillTrying
author_rank: Super Contributor
timestamp: July 12, 2016, 12:14:49 am
content: Quote from: HighVoltage on July 11, 2016, 07:10:16 pmIf the shunt's resistance is 200mohm it will be both.

author: zlymex
author_rank: Frequent Contributor
timestamp: July 12, 2016, 01:34:06 am
content: What is the precision required?The voltage drop of normal industrial shunts(0.5%) is 50mV, 60mV or 75mV according to countries.For precision shunts, the voltage drop is higher such as 200mV.For very precision shunts, the voltage drop is even higher such as 800mV for Fluke A40B.(Yes, 100A times 0.8V equals 80W of power dissipation)According to some regulations, shunts must be verified at 10%, 20%, 40%, 60% and 100% of the rated current. That is to say, test at 100% of the rated current is required and that current could be applied for certain amount of time so that the temperature reached equilibrium (because the shunt resistance might be quite different at different current owing to TCR)

author: LukaszRed
author_rank: Contributor
timestamp: July 12, 2016, 08:38:55 am
content: Thanks everyone for replies. Sorry for confusion in the main post. Below some clarifications:Yes it is MICRO OHMS (uOhms) I fixed first post.We applied a constant current of 100A and checked the voltage drop using Fluke Mulitohmometer.Just to not get sidetracked, I agree that method of measuring the voltage drop at nominal current is pretty reliable. Even if we measure it on the "pins" or the 1-1 points we are getting similar results and they seem to be fine. However we wanted to avoid 100A application since we don't have a proper test station to check it.We used Cropico D07 4-bridge and results on the pins are much different from readings we are getting in the 1-1 points, as explained in the 1st post. We would like to understand why is that happening and how to make it the right way without applying the nominal current.For similar components our customers usually say they want to get e.g. 200uOhms or 20mV drop at nominal current measured on the pins. We want to sort out the correct method of measuring it.We are not electricians and we want to investigate it to understand the issue. I hope I did explain it right. Again thanks for great response, a lot of useful information from you guys!

author: HighVoltage
author_rank: Super Contributor
timestamp: July 12, 2016, 10:48:37 am
content: You really should get a low voltage high current DC power supply and test at 100A, since this is what your customer wants.And then you can measure properly on all the spots of your shunt.One big thing is symmetry and yours does not look much symmetric.Here are some pictures of how I did a 100A test with an Agilent N5741A power supply on some super capacitors.Also, for 100A you need some heavy wires to connect to your shunt.How are you planning to connect it?

author: StillTrying
author_rank: Super Contributor
timestamp: July 12, 2016, 11:22:26 am
content: I think you're right to keep the current points 5-5 centred and far apartto give the current time to spread evenly.The right hand 5 as a point seems very close, I'm sure if you moved it to the top right or bottom right corner the reading would change, so keep it near the right hand center edge, I assume it will eventually clamped all along that edge which you might have to arrange for the maximum accuracy.There will be no test current flowing and so no volt drop from point 2 and a half, to point 4, so that's where to measure.When getting a 3 digit reading like 'abc', the last digit is always somewhat unreliable, so testing at 10A should give you an extra digit 'abcd' where 'd' is now the unreliable digit, but all abc are now accurate.I don't think it's a very good shape either, looks like there will be too much of a concentration of current at the 2 bottom center corners to me.There's nothing in the picture to show us the size or thickness of it, which would have helped with the milli/micro confusion, - I adjusted the contrast looking for a clear fingerprint!Having said all that I think you're just about there.

author: LukaszRed
author_rank: Contributor
timestamp: July 12, 2016, 12:34:08 pm
content: Quote from: StillTrying on July 12, 2016, 11:22:26 amShunt size is attached. This dimensions should give you around 167-170uOhms resistance measured at 4-4 position. If we measured it on pins we reached required 200uOhms. We used Cropico D07 at 1A for it. if we would connect it to nominal current and measure the voltage drop we would probably get that rough 170uOhms in any position.We would like to understand the issue with Cropico at 1A.

author: StillTrying
author_rank: Super Contributor
timestamp: July 12, 2016, 01:10:18 pm
content: Quote from: LukaszRed on July 12, 2016, 12:34:08 pmIt wouldn't take long to test the Cropico on a strip of clean copper, or a long shiny bolt, at 1A and 10A. I'm sure you know that the current connections go at the ends and the voltage measuring connections go in between them but not touching them.If you've not stabilized the readings at 3-3 and 4-4 I'm running out of ideas!

author: LukaszRed
author_rank: Contributor
timestamp: July 12, 2016, 03:45:58 pm
content: Quote from: DiligentMinds.com on July 12, 2016, 03:20:23 pmWhat do you mean? Do you mean to move the pin to the opposite side? If yes, then this is not possible I am afraid. We recorded a video today how we measure it using Cropico unit. I will upload it later and share it with you guys.

author: LukaszRed
author_rank: Contributor
timestamp: July 12, 2016, 05:23:08 pm
content: There is a video how do we measure it using Cropico below:https://youtu.be/GH7mglHR8jcShunt dimensions: Width: 7.3mm / Length 9.35mm / Thickness 1.65mm

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 12, 2016, 06:20:39 pm
content: Quote from: LukaszRed on July 12, 2016, 05:23:08 pmThat is not the way to measure the "working" resistance of a current shunt. You can not use this kind of connection and expect a meaningful result. You should connect the current and voltage wires at relevant points, otherwise it is not at all clear what you are trying to measure.CheersAlex.

author: StillTrying
author_rank: Super Contributor
timestamp: July 12, 2016, 06:22:44 pm
content: I think we've found the problem(s).Looks like you have LS06, but should be using LS05, but I can't see a picture of them.LS05 Executive lead set consisting of 4 x 1 metre leads with banana plugs, 4 xcrocodile clips 4 x test prods and 4 x Kelvin clips (KC1) jaw opening4mmLS06-P Kelvin lead set comprising of miniature Kelvin clips (KC2) with 1 metreleads attached terminated with banana plugs Jaw opening 6mm.Suitable for fine wires.

author: LukaszRed
author_rank: Contributor
timestamp: July 13, 2016, 08:11:37 am
content: Quote from: StillTrying on July 12, 2016, 06:22:44 pmWe are using LS05 from what I seehttp://instrotech.com/cropico-ls05-p-574.html

author: LukaszRed
author_rank: Contributor
timestamp: July 13, 2016, 08:15:46 am
content: Quote from: DiligentMinds.com on July 12, 2016, 08:12:19 pmThe thing is that it is Cropico unit which applies potential and current exactly in the clamping spot. It means if we clamp it down to pins it will apply 10A in the pins position (3-3). If we apply it as you describe big copper tabs (5-5) then readings are different but we are not sure why. Current still flows through the resistor and shouldn't we get similar readings?

author: altaic
author_rank: Unknown
timestamp: July 13, 2016, 09:31:01 am
content: Quote from: LukaszRed on July 13, 2016, 08:15:46 amPlease readhttps://en.wikipedia.org/wiki/Four-terminal_sensing. You're not measuring resistance; you're measuring the minute voltage drop across something you're flowing masses of current through, and deducing what the resistance of the thing is given the known current. Force terminals flow the massive current (big chunky ends, #5). Sense terminals measure the minute voltage drop (little tabs with resistance boosting holes #1-4).

author: StillTrying
author_rank: Super Contributor
timestamp: July 13, 2016, 09:55:01 am
content: Yesterday, I was going to post. "I'm shocked after watching the video, it'll be interesting to see what happens next". Today - I'll stick by that for now!

author: HighVoltage
author_rank: Super Contributor
timestamp: July 13, 2016, 10:40:40 am
content: It seems to me, that the OP is not listening to what we have to say.It also seems that there is a fundamental lack of understanding the very basics.You probably need to have some local electrical engineer showing you step by step of how to do it.Your sense terminals need to be at a very different spot than your force terminals.You can NOT use the standard clips of the Cropico, that will NOT work at all.

author: LukaszRed
author_rank: Contributor
timestamp: July 13, 2016, 12:45:29 pm
content: Hello,To avoid confusion I just want to tell that we know how to measure the shunt resistance in the proper way. The question we would like to have an answer for is:Why the results on pins using cropico unit are so different from results in 1-1 position? I know that all of you will say that "you should not measure it that way!" "I am shocked how you measure it!" etc. But this is not the answer that actually helps, because we already know that it is not the right way to do such test. We are not familiar with the knowledge behind it because we are not electriciansand we don't understand why the readings on the pins are much different than in other spots of the component. What is causing that fluctuation of resistance?We have also prepared quick report for you guys, to calm down some of the voices. We are just looking for the answers** Forgot to mention in the report. We used fluke multimeter to measure voltage drop.http://www.fluke.com/fluke/uken/digital-multimeters/fluke-45.htm?pid=56082

author: zlymex
author_rank: Frequent Contributor
timestamp: July 13, 2016, 02:00:48 pm
content: To represent graphically/intuitively, I draw isopotential lines or equal potential lines.Any point in a line has the same potential.The voltage drop(hence the resistance) of any two points is equal to the number of lines in between(times the step if precise).The shape and distance of these lines vary very much with the actual materials, often very difficult to calculate especially the shape is irregular, but can be measured by a probe pen.The principle of the drawing is: current likes the shortest path(least resistance path).I draw these lines very rough, but somethings are certain such as:- the resistance of 1-1 is smaller than 2-2- the resistance of 2-2 is smaller than 3-3- the resistance of 3-3 is nearly equal to 4-4 (there must be a measurement error if not)Edit: the last three lines should be:- the voltage drop of 1-1 is smaller than 2-2- the voltage drop of 2-2 is smaller than 3-3- the voltage drop of 3-3 is nearly equal to 4-4 (there must be a measurement error if not)

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 13, 2016, 02:28:36 pm
content: Quote from: zlymex on July 13, 2016, 02:00:48 pmThe resistance of each "pin" is about 25uOhm, together - 50uOhm. As simple as this.CheersAlex

author: zlymex
author_rank: Frequent Contributor
timestamp: July 13, 2016, 03:27:38 pm
content: Quote from: Alex Nikitin on July 13, 2016, 02:28:36 pmOh yes, I should say 'voltage drop' in place of 'resistance'.

author: Gyro
author_rank: Super Contributor
timestamp: July 13, 2016, 07:32:29 pm
content: @LukaszRed: Please look at the picture of the DO7Plus Manual page that you posted in Reply #31. Look at the picture, then note the..."HINT:It is important to connect the I (current) leads outside the U (potential) leads"You need to do that... Do what the manual says if you want to get accurate readings.I notice from your picture of your probes that the leads are plugged in with 4mm plugs. Unplug these and buy some small individual clips so that you can make the measurement properly as the manual tells you to do!Edit:Or if there are enough of you, then just hold the 4mm plugs on the correct points on the shunt... I leads at points 5 and U leads at points 4. It really isn't difficult to do the correct thing.

author: The Soulman
author_rank: Super Contributor
timestamp: July 13, 2016, 09:43:43 pm
content: OP is nearly there, just need to connect the four wires coming from your tester to the terminals of the shunt resistor in the same way as in the end product.I'd suggest the current carrying terminals be the numbers 5, and the sense terminals the numbers 4, then you should get repeatable readings.The tester is actually a two part device: a current source that lets a known current flow from one current terminal thru (in this case) the shunt to the second current terminal, and a voltmeter on the two sense terminals to measure the voltage drop across the shunt.Using ohms law (done inside your tester) resistance=voltage/current we get the value of the shunt resistor in ohms, then grind or mill the material and retest and repeat till you get the desired value, by taking away material you can only go up in resistance, once to high... make a new one.Now the shunt has a known value the customer can let a unknown current flow thru the shunt (again using terminals 5) and measure the resulting voltage drop with a voltmeter (again connected to terminals 4), now he can use ohms law in a different way: current=voltage/resistance.Resulting in a customer who knows how much current there is flowing thru his motor/device/apparatus.In a nutt shell, that is how it works.There are a few pitfalls still, but keep posting and people will help you out.

author: The Soulman
author_rank: Super Contributor
timestamp: July 13, 2016, 10:22:08 pm
content: Not for the OP to worry about, just to get some brainwaves flowing and to get the feet of the ground...Quote from: Gyro on July 13, 2016, 07:32:29 pmNow imagine a straight bare copper wire of 10 meters long with a end to end resistance of 30 Ohms and we have a connection terminal on each end called A and a set of connections spaced 1 meter apart somewhere along the line called B.Now if we force 1 amp of current from A to A we get a total voltage drop of 30 volts across the wire and at our measurement terminals B We measure3 volts, all is well.If we put the same 1 amp of current thru terminals B and measure voltage at A, we´d measure 3 voltsthe same.And what if we swap one A terminal for one B terminal, 3 volts again....

author: StillTrying
author_rank: Super Contributor
timestamp: July 14, 2016, 02:35:33 am
content: Right...All my posts are carefully considered believe it or not, shocked was/is the right word.You keep asking why the resistance at 1-1 is different from 2-2/3-3 for some reason, it doesn't matter what it is at 1-1, you can make it anything you want because the lugs are not measuring there. But..The only way to measure resistance is to force a known steady current between 2 carefully selected points and then measure how much force(voltage) was needed to do it. The more force(voltage) that was needed the higher the resistance is.When you force this known steady current though 1-1 or 2-2 the currentdoes notflow like this. shunt1.jpgIf you could see the current flowing between 2 points on a surface it would look more like this. shunt2.jpgMost of the current total flows directly through the middle line between the 2 points, but there are smaller currents taking longer curved paths, each side of the center line, and even smaller currents flowing all over the place. If you need a very accurate measurement, then all these smaller currents add up to quite a few % of the total current.Drawing these currents on your shunts would look vaguely like this. shunt3.jpgRight away, hopefully, you can see that some of the lower parts of the 1-1's current paths are missing - cut off, by the cutout, compared with 2-2. So when you come to force the steady known current between 1-1 it takes a bit more force(voltage) to get it to flow though the paths which are still there. And of course this bit of more force means 1-1 has a bit more resistance.Here's one you prepared earlier. shuntg.jpgIf you moved points 1-1 down towards the bottom edge even slightly, then more of 1-1's current paths would be missing, and show up as more resistance. If you moved points 1-1 up a bit it would give 1-1 more current paths and less resistance, you'll get a different resistance nearly every time.The 4 wire measurement bit to avoid all these errors has already been done.. and done, so just do it for now.All this won't be 100%, but close enough for me!

author: LukaszRed
author_rank: Contributor
timestamp: July 14, 2016, 09:06:59 am
content: @up StillTryingThanks! This is sort of answer that puts more light into some corners. So what you are trying to say that the current density affects the resistance? We agree with that and we did a simulation of that phenomenon and we can confirm that resistance readings change across the shunt.So it looks like when we measure the resistance with cropico we are applying the 10A current to the pins which potentially makes the current density relatively high across the measuring path. That's why we are getting high resistance results. Of course ideally we want to apply the current on the component ends to provide that current flow and only voltage leads to the pins.If I am getting this right I think we are nearly there.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 14, 2016, 10:06:40 am
content: Quote from: LukaszRed on July 14, 2016, 09:06:59 amWhen you measure the resistance with Cropico on the pins you add the resistance of the pins to the measured value. A simple estimate is that each pin has about 25-30uOhm resistance, hence +50uOhm reading.CheersAlex