Tempco tests on power resistor current shunts

[enut11]

The USB temperature sensor was not touching the relay so it is possible that, especially during the heating cycle, the resistance and temperature got out of sync. The temp sensor is rather bulky and there is not much room in the oven. Perhaps if you slide some of the data back and forward you will find a better correlation.

I am trying a different logger, one that can handle 2 DMMs, one for resistance and the other for temp. This time the temperature sensor will be a thermistor which is much more reliable.
enut11

[enut11]

@berke
Here is the 70R relay (TXT) file recorded with 2 DMMs, one for resistance and one temp. It is summer here and quite hot in my office and
 oven cooling takes a long time so I cut it short a bit.

I am still trying to get an improved setup for my current shunt tests. This latest setup uses a 34401A for 4W resistance and a 34461A for temperature via a 5K thermistor. Logger is the Agilent Digital Multimeter Connectivity Utility V1.0.2.0

The utility does not allow a timed sample rate so the best way to slow it down and taking too many samples is to select 100 NPLC. This also smooths out the data points.

[tszaboo]

For such a tempco test, I have the following suggestion: Have the starting point as the high/low temperature, and don't do temperature control. If you have a controller, it will do on/off control and upsets your measurement. Just isolate the setup, place a thermal mass in it if necessary, measure temperature at the beginning and switch of cooling/heating, and let it reach room temperature. The results will be much less noisy and easier to curve fit them.
And that being said, regular wirewound resistors will easily be ~200ppm/K, if accuracy matters you need high precision ones made exactly for this.

[enut11]

You are correct @tszaboo and it shows up on this run on a Microohm 2R 10W resistor. You can clearly see the oven influence up to about record #850 when the oven was turned off.

After that, the resistance change (0.00015 ohm) does not look right for the temperature drop (8.9C). I need to look closer at this.
I am using a 10mA CC from a HP6177C to boost the signal to the DMM. Measured directly, the 2R was only 2% of the lowest 100 ohm range on the meter (HP34401A).
enut11

[berke]

@berke
Here is the 70R relay (TXT) file recorded with 2 DMMs, one for resistance and one temp. It is summer here and quite hot in my office and oven cooling takes a long time so I cut it short a bit.

@enut11, you are right about the lag, if I take into account about 39 seconds of lag for the temperature (hand-optimized) I get a nice linear correlation.

But it comes to 3.79 ohm per degree (~11%) which way too high to just be the copper.  What's going on here?

[enut11]

With that data I attached the thermistor to the side of the relay so it should track better than the previous run.

May be the current source? I am using a HP6177C at 10mA. Here is the warmup plot fed directly into my 34461A DMM.
[EDIT] forget the above line the relay 70R was measured directly by the DMM 

[berke]

Did you put any thermal paste?  Otherwise it's just playing convection lottery.

[1audio]

How significant are thermocouple effects here? They could swamp the resistance changes if not accounted for. I believe some of the low resistance meters (HP for example) use AC excitation to avoid those effects.

[enut11]

I have no idea nor how to check it. I am using these Chinese Kelvin clips.

[enut11]

Did you put any thermal paste?  Otherwise it's just playing convection lottery.

No thermal paste. Too messy in an oven. I am going to rely on time as an equaliser.
enut11

[1audio]

both the resistors and the connections have a multitude of different metals and Seebeck potentials at every connection. Heating them and having the meter at room temperature ensures there will be voltages. They will be in the millivolts so not a big issue when the test voltage is 1V but very significant with a low Ohm resistor. Read more in the intro for the HP4328 here: http://ftb.ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_4328A

[enut11]

@berke
Here is the 70R relay (TXT) file recorded with 2 DMMs, one for resistance and one temp. It is summer here and quite hot in my office and oven cooling takes a long time so I cut it short a bit.

@enut11, you are right about the lag, if I take into account about 39 seconds of lag for the temperature (hand-optimized) I get a nice linear correlation.

But it comes to 3.79 ohm per degree (~11%) which way too high to just be the copper.  What's going on here?

Hi @berke. Pls check your 70R relay calcs again. I calculate about 1/10 of your results.
enut11

[Kleinstein]

There is some effect of thermal EMF. Some resistors are rather good in this respect and some are not. For example Konstantan has a thermal EMF of some 40 µV/K relative to copper, while good manganin is usually specified with < 1 µV/K. NiCr variants are usually also good.
Ideally there would be not temperature difference across the resistor, but with self heating of the resistor and asymmetric thermal design a temperature difference proportional to the power will develope. For DC operation this would result in a linear change of the apperant resistance with the voltage, that could become relevant for low value resistors.
As a small effect there is also the peltier effect at the contacts that causes an extra heat flow from one side to the other. With metals, especiall with low thermal EMF this effect should be relatively small though.


For the measurement of thermal effects I would not recommend using clips. The clips can mechanically move and this way change the resistance. For the thermal tests soldering of 2 wires to each end should be the better way. For the 4 wires it is than OK to use connectors (maybe avoid high thermal EMF or thermal gradients).

[enut11]

both the resistors and the connections have a multitude of different metals and Seebeck potentials at every connection. Heating them and having the meter at room temperature ensures there will be voltages. They will be in the millivolts so not a big issue when the test voltage is 1V but very significant with a low Ohm resistor. Read more in the intro for the HP4328 here: http://ftb.ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_4328A

Thanks @1audio. The HP4328A is an impressive instrument. Low level AC to measure resistance is an interesting approach as it avoids extraneous DC temperature effects. Pity a 4328A costs more than a 34401A second-hand.

Thanks @Kleinstein. Next I will solder the 4-wires to the DUT and see if I can measure the difference.

[berke]

Hi @berke. Pls check your 70R relay calcs again. I calculate about 1/10 of your results.
enut11

Indeed, what was going on is that I made a stupid mistake by swapping R and T while reading the file, and not noticing that the resistance is around 30 even though the file is named 70R.

That being fixed I get 0.371% per degree which is what to expect from copper.

[enut11]

@berke, good you found that. So, did the test tell you what you needed to know?
enut11

[berke]

@berke, good that you found that. So, did the test tell you what you needed to know?
enut11

Yes, thank you! It gives me a concrete data tempco data point and an idea of the uncertainty.  According to these guys https://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html the official Cu R tempco is 0.429%.  This means that unless your relay is very atypical the practical value can be within 15% of that.  I don't think that kind of data is easy to come by.

[enut11]

Back to my current shunt tempco tests, here is a MicroOhm 2R 10W resistor.
Seems very good at 11 ppm/C.
Not sure what caused the reversal in resistance at around record #420. The oven heater was off for the whole test.

@berke, I would appreciate if you could verify this.
enut11

[Kleinstein]

The voltage measured for the 2 ohm resistor is quites small. The jump around reading #420  is only some 200 nV, so not really much. If not carefull this would be thermal EMF from the connectors with a heating effect (e.g. a person sitting next to it and than leaving the room). Also the DMM internal current source could show some LF noise. At least the 34401 uses an BJT based OP-amp for the current source, but there is still 1/f noise.

Another point is just a random jump in the actual resistance, e.g. from slip stick like movement between parts or a microscopic crack (e.g. in an oxide layer or paint) moving. Relative to the resistor the effect is not that small, but still possible to be a real effect of a not so stable resistor.
Overall it is not clear, if there is really a temperature effect, or more like random variations that

For the rather low resistors, especially when going below 1 ohm the test current (1 mA) from the DMM may not be enough to really get good results.

[enut11]

Thanks @Kleinstein. I am now using a soldered connection 4W resistance measurement.
Granted, the 2R resistor is only a small fraction of the lowest range on the 34401A and there appears to be a lot of noise in the plots.
I did try a 10mA constant current using a HP6177C but, as shown in Reply #30, the HP takes about 90min to warm up and adds another dimension to the results.
I am really just trying to find the best method available to me so I can study temperature dependence of low ohms current shunts.
enut11

[iMo]

[OT]: @Kleinstein: I wonder how the popcorn noise of the LM399 (in the above 34401A measurement setup) translates into a result. Imagine a 399 jump by, say, 4uV up or down during that measurement..

[Kleinstein]

The ohm measurement in the 34401 and most better DMMs is running ratiometric. So the same voltage reference is used to generate the current and in the ADC. So noise in the reference would ideally have no effect.  Because of different filtering / frequency response there could still be a short (e.g. 1 reading)  transisent effect on jumps from popcorn noise. As the noise is up and down this would be just a part of the noise, no longer time trend or such.

[bsdphk]

Power resistors encapsulated in ceramics suffer from significant mechanical stress as the temperature changes, because the ceramics and the resistance element have different thermal expansion coefficients.

[enut11]

As a sanity check on my methodology, I tested what I call a 'cement trough' power resistor available from the local electronics store.
The tempco of 144 for a cheap resistor looks OK to me although I do not have a specification.

[berke]

@berke, I would appreciate if you could verify this.

Hi,

The resistance varies too little to get a good estimate, but that is consistent with a very small tempco.  As it is the estimate is 16 µohm/°C or 8 ppm/°C.