Shunt resistor issue

[LukaszRed]

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 ~ 210uOhms

We 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

[TiN]

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 ?

[kfitch42]

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:

[LukaszRed]

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 ?

We measure only the voltage drop values in this test. Then converse the mV into mOhms and readings seem to be fine.

[rob77]

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 ?

We measure only the voltage drop values in this test. Then converse the mV into mOhms and readings seem to be fine.

but 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...

[StillTrying]

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.

[TiN]

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.

[Edwin G. Pettis]

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.

[Kleinstein]

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). 

[BravoV]

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).

Yeah, 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.

[HighVoltage]

Is this a constant current of 100A?
Or is this just a pulse?
What is the application?

[MK14]

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;dr
Does he mean m = Micro or m = milli ?

(I accept that technically milli is correct, but some people get confused).

[StillTrying]

This is why I keep harping on LANGUAGE.  If you are going to be an engineer, you have to be very precise in how you describe things....

Since 200u-ohms seems to be more reasonable, I will take that as what the OP meant.  Still too high for the best accuracy,

Did you mean too low? 

I guess the OP will have to clear this up for us.

Any bets? We  them off.

[StillTrying]

Is this a constant current of 100A?
Or is this just a pulse?

If the shunt's resistance is 200mohm it will be both.

[zlymex]

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)

[LukaszRed]

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!

[HighVoltage]

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?

[StillTrying]

I think you're right to keep the current points 5-5 centred and far apart to 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.

[LukaszRed]

There's nothing in the picture to show us the size or thickness of it, which would have helped with the milli/micro confusion,

Having said all that I think you're just about there.

Shunt 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.

[StillTrying]

We would like to understand the issue with Cropico at 1A.

It 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!

[LukaszRed]

There's nothing in the picture to show us the size or thickness of it, which would have helped with the milli/micro confusion,

Having said all that I think you're just about there.

Shunt 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.

Your picture of your shunt shows both "sense" lines coming out of the top of the shunt.  Can you flip one of these to the bottom?  I think this would help reduce the TCR [and PCR] contribution of the copper connections.

What 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.

[LukaszRed]

There is a video how do we measure it using Cropico below:
https://youtu.be/GH7mglHR8jc

Shunt dimensions: Width: 7.3mm / Length 9.35mm / Thickness 1.65mm

[Alex Nikitin]

There is a video how do we measure it using Cropico below:

That 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.

Cheers

Alex.

[StillTrying]

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 x
crocodile clips 4 x test prods and 4 x Kelvin clips (KC1) jaw opening
4mm
LS06-P Kelvin lead set comprising of miniature Kelvin clips (KC2) with 1 metre
leads attached terminated with banana plugs Jaw opening 6mm.
Suitable for fine wires.

[LukaszRed]

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 x
crocodile clips 4 x test prods and 4 x Kelvin clips (KC1) jaw opening
4mm
LS06-P Kelvin lead set comprising of miniature Kelvin clips (KC2) with 1 metre
leads attached terminated with banana plugs Jaw opening 6mm.
Suitable for fine wires.

We are using LS05 from what I see http://instrotech.com/cropico-ls05-p-574.html