Low ohm precision resistor standard and testing

[HighVoltage]

And here are the measurements at 5A with the Keithley 2460

[gamalot]

Congrats on your new toy! 

I don't have any Keithley sourcemeter and I don't really understand why it shows 0.100015 ohms instead of 100.015x milliohms.

---

Please ignore my stupid question, just read the 2450 datasheet ...... 

[coppice]

This is my newest edition to the precision low value resistor current shunts.
After years of searching, finally one showed up on the used market.

This one is a Burster, Made in Germany

- Type: 1281-Q
- Built: 1990
- R: 100 mOhm
- Tol. : 0.02 %
- Tk : <10ppm/K
- Pmax: 10 Watt

The resistor seems to be spot on, when measured with the Keithley 2450 SMU.
Here are some pictures and the Datasheet (German from 1990)

I'm surprised that such an elaborately constructed resistor doesn't use material with a better temperature coefficient.

Very few low value resistors, especially ones in the micro-ohm range, are all that precise. The market is usually for an approximate resistor with a first class temperature coefficient, and the error is just calibrated away at the system level.

[HighVoltage]

I don't have any Keithley sourcemeter and I don't really understand why it shows 0.100015 ohms instead of 100.015x milliohms.

Well, this is the default setting of the 2450.
You could apply an mx+b function to the measurement to show mOhm instead of Ohm

[HighVoltage]

I'm surprised that such an elaborately constructed resistor doesn't use material with a better temperature coefficient.

This Burster 1281 is made of standard MANGANIN (CuMn12N) and has a T.C. 10ppm/K.

There was an older type Burster 1284 (Not in production anymore), that was made of ZERANIN (CuMn7Sn) and has a T.C. <3ppm/K. Allegedly, ZERANIN is no longer available from the German manufacturer, Isabellenhuette.

See datasheets enclosed for both materials.

[coppice]

I'm surprised that such an elaborately constructed resistor doesn't use material with a better temperature coefficient.

This Burster 1281 is made of standard MANGANIN (CuMn12N) and has a T.C. 10ppm/K.

There was an older type Burster 1284 (Not in production anymore), that was made of ZERANIN (CuMn7Sn) and has a T.C. <3ppm/K. Allegedly, ZERANIN is no longer available from the German manufacturer, Isabellenhuette.

See datasheets enclosed for both materials.

People tend to refer to any low coefficient material as manganin, so its often hard to tell exactly what things are made of. However, there are a lot of shunts with a demonstrable coefficient of around 4ppm/K across a number of samples. These materials don't have a specific temperature coefficient, as there are tolerances on the mix of metals in the alloy. You need to check if 10ppm/K is typical or maximum.

[Conrad Hoffman]

Interesting. Most of the better-than-manganin wires I've seen are not solderable and have to be spot welded. Too bad they don't make that anymore.

[HighVoltage]

Interesting. Most of the better-than-manganin wires I've seen are not solderable and have to be spot welded. Too bad they don't make that anymore.

The ZERANIN datasheet claims that the material can be soldered.

Based on their datasheet:
The Burster models from 25 uOhm to 100 mOhm use a MANGANIN sheet metal
The Burster models from 200 mOhm to 100 Ohm use a ZERANIN wire

I do not know, how they are internally connecting the sheet metal or the wire but I would assume that it is mounted directly to the 4mm binding posts without welding or soldering.

Maybe one day I get a bad one and can take it apart.

[coppice]

Interesting. Most of the better-than-manganin wires I've seen are not solderable and have to be spot welded. Too bad they don't make that anymore.

Why is solderability a big issue? most shunts I've encountered are welded to copper supports. Either butt welded between 2 chunks of copper, or flat welded to the surface of copper. Why would anyone spot weld these things? That doesn't sound like a route to stable results. Solderability may be an issue on parts of the shunt where the sensing pair of a 4 wire attachment occurs. Most seem to solder to the shunt material, but there might well be plating there, to create solder compatibility.

[Conrad Hoffman]

For shunts, sure, but with smaller wire spot welding is easy and practical. I've never seen the wire I'm thinking of (NiCr "800" series like Kanthal Nikrothal LX) in a format suited to shunts- ribbon or flat stock.

[HighVoltage]

Newest addition to the Burster family of low ohm resistors, the BURSTER 1284

- BURSTER Type: 1284-0,1
- Built: 1996
- R: 100 mOhm
- Tol. : 0.01 %
- Tk : <3ppm/K
- Pmax: 45 Watt
- Imax: 200A
- Material: ZERANIN


This should be a true metrology grade low ohm resistor.
It seems like the ZERANIN is a sheet metal, squeezed between the two heat sinks.
I will use it for some high current measurements for up to 100A.

This thing is huge in comparison to the small Burster 1240 or even the 1281

[HighVoltage]

And here are some first measurements at 1A

[Smokey]

You guys were talking about measuring the resistors at max current operating temperature earlier. 
If you had an accurate thermal oven and let the thing settle between readings, would it be valid to characterize the tempco over the full temperature range but at one current much lower currents than maximum?  Or is there something special about actually pushing the real current through the shunt?

[TizianoHV]

I would expect a different temperature distribution. A thermal chamber will heat up uniformly. The nominal current might create temperature gradients. And those will for sure influence thermal EMF. For most of the cases it will give you a reasonable value.

One trick I use to evaluate thermal EMF in function of the current is to use AC test currents to heat up the shunt and a DC voltmeter to measure EMF (the voltmeter will filter all the AC component and measure only the offset). That's good when you don't have a DC high current supply since you can use any cheap transformer rewound for high currents (I used a free microwave oven transformer, good up to 400A).
Then I measure the resistance at lower DC currents and temperatures.

The photo shows some tests of a DIY low value resistor (I made it to measure pulsed currents, thats why I'm using a BNC). In the first plot there are temperature and EMF in function of AC current. In the second plot there's DC resistance (at just one temperature).


Next step would be to use both DC and AC current to heat up the shunt and measure its resistance at the same time, but I didn't had time for that, jet.

[Kleinstein]

The thermal EMF part depends on the diretion of the current, as it adds a voltage that is little (low thermal EMF materials also have a low Peltier coefficient, as they are linked) effected by the direction of the current.
The difference from heating from the outside and heat from the resistor itself is more from mechanical stress caused by temperature gradients.

[mendip_discovery]

DC EMF can be dealt with by reversing the connections. But we have spoken about that before.

I have a few shunt resistors and I wonder if they could be improved by adding some heatsinks, using thermal pads to insulate the electrical part from the heatsink but keep the thermal connection. It's also a handy way to also apply a temperature sensor.