Tempco tests on power resistor current shunts

[enut11]

I have been working on an LCD monitor for my HP 6177C DC Current Source. It has 5 ranges from 20uA to 200mA full scale. It is based on a simple current-shunt principle using a 4.5 digit 200mV LCD readout. Required accuracy of the monitor is 0.01%. Low tempco resistors are needed to maintain cal over a reasonable temperature range.

The shunts are 1R, 10R, 100R, 1K and 10K. I happen to have low tempco 100R, 1K and 10K resistors so the challenge was to find  suitable 1R and 10R shunts.

[enut11]

I could simply buy some low tempco resistors but I thought it might be fun to study the behavior of 'garden variety' power resistors, like the one shown below, that are available at our local electronics parts store.

[enut11]

I built a mini oven based on the temperature controller I use in my LM399 Voltage Ref ovens. A 10K thermistor in a bridge configuration controlling a heater via an opamp/power transistor. This one I made variable from 20C to 35C.

[enut11]

First test was a 1ohm 5W 'white cement' power resistor and, I must admit, it produced a very peculiar result.

A HP 34401A DMM in 4-wire ohms mode was used to record resistance on the 100ohm scale. Logging program was from Forum member Ian Johnson.

The test was over 60 minutes. The first 5 min the oven was off with ambient temperature 25C. Over the next 55min the resistance initially dropped for about 6 min and then more or less stabilised at an intermediate value by the time the temperature reached 35C.

How do you explain this behavior? How do you work out the tempco for the 10 degree test? Is it max to min or max to final value?

Comments anyone?

[Andreas]

Comments anyone?

Had similar experience with unstable resistance value on a power resistor.
Maybe the contact between resistance element and terminals is only crimped and not welded.

with best regards

Andreas

[macaba]

I recently measured TCR on a batch of 2PPM 1206 resistors and encountered a similar effect to what you are seeing (overshoot then decay). Thermal EMF is my best guess, even with OHMF/OCOMP ON/DELAY 1 (on my 3458A). I found that a 30 minute settling time at each temperature setpoint was enough for the decay curve to settle to a flat line, you may find it requires 1-3 hours (I'm guessing) due to the larger thermal mass.

[jonpaul]

no need for this test, use a precision 4 terminal shunt resistor, available to 0.001 or 0.01 ohm 1% Isobellehutte in Germany or Tepco. At currents over 10 A use a meter shunt.

Jon

[mendip_discovery]

no need for this test, use a precision 4 terminal shunt resistor, available to 0.001 or 0.01 ohm 1% Isobellehutte in Germany or Tepco. At currents over 10 A use a meter shunt.

According to RUG-Z PDF which is the calibration standard one they manufacture. With 0.1% and a temp/co of 1uΩ/Ω per °C, it's worth a sniff.

[enut11]

no need for this test, use a precision 4 terminal shunt resistor, available to 0.001 or 0.01 ohm 1% Isobellehutte in Germany or Tepco. At currents over 10 A use a meter shunt.

Jon

Thanks @jonpaul. I am on a (steep) learning curve and resisting the temptation to buy my way out 

[enut11]

Noted: A longer time-frame has been suggested by members.

I recorded the resistance of the 1ohm/5W during a 120 min cooling cycle. More predictable trend but still large excursions in resistance. 
Calculated tempco was 153pp/C. is this typical?

This type of shunt is forever banned from my metrology experiments!

[enut11]

To explain a bit more, as I will be using a 200mV panel meter, the shunts will be:

1R for 200mA
10R for 20mA
100R for 2mA
1K for 200uA
10K for 20uA

Very low value shunts are not required as currents are low and burden voltage is not an issue for the current source.

[KE5FX]

If you don't want to go all the way to an expensive Kelvin resistor expressly designed for low tempco, the metal-finned Dale wirewound resistors make an excellent substitute.

[enut11]

I have an RS 10W finned resistor that I can test, along with a few 2R resistors that I can parallel if they are suitable.

Two of the 2R resistors are labeled: Micro-ohm and Pacific, both are 10W

[enut11]

With the mad holiday season over I have spent some time improving my tempco oven setup. I am using Ian Johnson's PDVS2 logging program, available on this Forum. The logger has provision for up to 2 meters as well as a temperature/humidity sensor. The temperature sensor I am using is the USB TnH SHT-10 V2.
https://www.eevblog.com/forum/metrology/3458a-logging-via-windows-app-revisited/

Below is a sample run on a Fluke 10.05K resistor logged from my HP 34401A. The chart shows a heating and cooling cycle. I am concentrating on the cooling cycle from approx record #400 over 2 hours.
10,050.449R at 36.7C
10,050.665R at 28.8C
Delta R=0.216 ohms
Delta temp = 7.9C

.216/10050x1000000=21.492 total ppm shift
21.492/7.9=2.72 ppm/C
Too good??

Feedback on my calcs would be appreciated
enut11

[berke]

Nice oven.

Do you feel like doing a test using say 100 ohms of copper wire?

I'm working on a driver for a small solenoid (~1.5W) and these buggers get quite hot quite fast.  The manufacturer suggested that the coil resistance can be used to measure its temperature.  Apparently copper has a tempco of roughly 0.4% per K.  I can't non-destructively put a thermocouple inside, but I found the resistance variation tracks the resistance predicted using that model reasonably well, using the case temperature.

I can check your tempcos if you provide CSVs.

[enut11]

Hi @berke. Sorry, but the oven is too small to test 100R of copper wire.

Below is the 10.05K test CSV. Next test will be a 2R 10W resistor.
BTW, I had to rename the file extension to TXT to upload it.

Columns are date/time, resistance, temperature and humidity.
thanks
enut11

[enut11]

Nice oven.

Do you feel like doing a test using say 100 ohms of copper wire?

I'm working on a driver for a small solenoid (~1.5W) and these buggers get quite hot quite fast.  The manufacturer suggested that the coil resistance can be used to measure its temperature.  Apparently copper has a tempco of roughly 0.4% per K.  I can't non-destructively put a thermocouple inside, but I found the resistance variation tracks the resistance predicted using that model reasonably well, using the case temperature.

I can check your tempcos if you provide CSVs.

@berke, I have two small relays, one has 70R coil and the other 170R. I could test the coil tempco of one of those if you want?
enut11

[jonpaul]

Worked at precision lab, Julie Research Laboratoire in 1966..1968.


Your plots look typicall of the setup, réf, meter warm-ups or contact R variations with warmup.

So, copper TC or R is well known,  as are all metals and common wire materials, just look up the TC.

Actual shunts and precision wirewound resistors use special zero TC wire or compensate TC with a PTC/NTC combo.

Enjoy

Jon

[berke]

Below is the 10.05K test CSV. Next test will be a 2R 10W resistor.
BTW, I had to rename the file extension to TXT to upload it.

Thanks, I find a temperature coefficient of minus 22.35e-3 ohm per degree when I throw out the first hour of data.  Otherwise you have two slopes (see bottom left figure.)

EDIT: Sign

[berke]

@berke, I have two small relays, one has 70R coil and the other 170R. I could test the coil tempco of one of those if you want?
enut11

Sure that would give an interesting comparison point, the 70R sounds comparable to what I have.

I can share the Octave script for the plots if you want.

Like jonpaul said the copper tempco is well-known but the practical information about the deviations is what is most interesting.  Also it's not as fun to just look up the value!

[enut11]

Unfortunately the temp/humidity sensor has become intermittent so no more tests until I sort that out

[enut11]

Thanks for the analysis @berke. Quite an impressive set of graphs. How did you generate these?

My simple approach to use delta max and min resistance and temperature yielded a tempco of 2.72 ppm/C. If I interpret your figure of -22.35e-3 correctly this translates to -2.235 ppm/C. If that is the case I was not too far off
enut11

[berke]

You're welcome.

Let's see, on the bottom right plot R goes from 10050 + 0.66 ohm to 10050 + 0.50 ohm from 28.6 to 36 degrees (the labels on the Y axis are messed up, insufficient precision), that gives (0.50-0.66)/(36-28.6)=-21e-3 ohm/degree, dividing by 10050 that gives -2.1 ppm/°C so yes we're in agreement.

I used GNU Octave, a program similar to Matlab except you don't have to fork out $4000 to get an fft() function and another $4000 to be able to load HDF5 files.  Script attached (paths need to be adjusted.)

No worries for the test, if/when you get back to it ping me.

PS. Looks like the forum doesn't like *.m files!  Zipped.

[enut11]

@berke, I have two small relays, one has 70R coil and the other 170R. I could test the coil tempco of one of those if you want?
enut11

Sure that would give an interesting comparison point, the 70R sounds comparable to what I have.

I can share the Octave script for the plots if you want.

Like jonpaul said the copper tempco is well-known but the practical information about the deviations is what is most interesting.  Also it's not as fun to just look up the value!

@berke, attached is a partial test of a 70R relay. The temperature sensor failed again during this test. I switched the oven on at record #150 and off at #550. Hopefully there is some useful data to analyse. During the test I did note a large change in resistance and it could be observed in real time! Note CSV changed to TXT.
enut11

[berke]

@enut11, this data doesn't seem to be good.  I can't find an interval with a decent one-to-one relation between temperature and resistance, see below.