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| Testing and calibrating a milliohm meter |
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| OM222O:
I designed and built a DIY milliohm meter a while back and just finished writing the software for it. the response times and overall accuracy of each building block is to spec and performs really nicely, so I decided to test some resistors with it. I only have some basic 1% resistors (no idea what the tempco is). I first measured the resistors with a multimeter and compared them to the results with the milliohm meter. the rough value is the same but there seems to be quite a lot of drift which I'm not sure if is just the tempco of the resistors or if it's caused by errors in my design. the multimeter doesn't show as much variation but there is some drift there too, maybe it's just averaging more samples and filtering the results better? I wanted to get some reference resistors to test with and the theoretical range is 10u\$\Omega\$ to 200K\$\Omega\$. What is the best resistor type to get for low tempco? I searched on mouser and I'm not sure how to select the filters. some of the values are just a fixed number (e.g: 100ppm/c) and some are 2 or more values (e.g: -2000 ppm/c , 0ppm/c). Also setting the filter to 0ppm/c only brings up results for 0\$\Omega\$ resistors which isn't really useful! I don't want them to cost over 10$ per resistors either, so please leave recommendations as to what resistors are better suited for testing and calibration of my device without breaking the bank. |
| Vgkid:
What are the full scale ranges of your milliohm meter, budget, accuracy(as the resistance gets lower, the cost goes up). How is the measurement circuit implemented/parts used. All effect what is needed to do the job. |
| OM222O:
I already specified the theoretical full scale range which is 10u\$\Omega\$ to 200K\$\Omega\$, but to be honest I doubt I will be able to get the 10u\$\Omega\$ range working! I also can't find any decent tempco resistors for testing that range. my issue is not with the design itself, rather finding suitable reference resistors for testing and calibration. like I said before, I don't want each of the resistors to cost more than 10$ either, since I need about 10 or 12 of them to cover all of the ranges. |
| MosherIV:
From the fact that you are talking about tempco, the design must be using a fixed constant current probably something like 1 Amp. Yes this will cause significant heating and hence the measured resistance value changes as the resistor heats up. I would try to keep the maximum current to 100mA and try to amlpify the small voltage across the resistor. 10uΩ is very ambicious. At that range, contact resistance is going to be a problem even with 4 wire measurement technique. Do you have access to another bench dmm with 4 wire resistance measurement? If so you ca compare your results with that instead of using lots of reference resistors. You would have to measure each test resistor before you try it anyway, unless you have known reference resistors. |
| OM222O:
--- Quote from: MosherIV on September 29, 2019, 06:43:47 pm ---From the fact that you are talking about tempco, the design must be using a fixed constant current probably something like 1 Amp. Yes this will cause significant heating and hence the measured resistance value changes as the resistor heats up. I would try to keep the maximum current to 100mA and try to amlpify the small voltage across the resistor. 10uΩ is very ambicious. At that range, contact resistance is going to be a problem even with 4 wire measurement technique. Do you have access to another bench dmm with 4 wire resistance measurement? If so you ca compare your results with that instead of using lots of reference resistors. You would have to measure each test resistor before you try it anyway, unless you have known reference resistors. --- End quote --- Exactly! I use a constant current source and a universal shunt that can change between 1 and 100K ohms and a 24 bit ADC to measure some values. this keeps everything relative and avoids things such as ADC offset, voltage reference errors, etc. For the issue of 1A heating up the components, I only take 20 samples and average them with a 90SPS rate and allow 30mS of settling time, so this whole process takes about 500ms and there is a cooldown timer of about 5 to 10 seconds depending on the current used which I tested over a long period of time and it seemed pretty stable and nothing overheated. 10uΩ seems a bit far fetched, I agree! but from the tests I can see I have at the very least 100uΩ accuracy (not just resolution). Unfortunately I don't have access to any other lab equipment that are more accurate than what I've built :-DD |O so I can't pre test the resistors and compare them, hence wanting to buy different reference resistors for different ranges and calibrating against them. here is a picture showing measurements of a 1Ω resistor and of a 10mΩ resistor. The error in that range is particularly bad (about 5%) but I noticed my 1ohm shunt that is used to generate the 1A current is also bad, so changing it should fix the errors (even testing it with the 100mA range yields results that are under 1%) and I tested all the other ranges with 1% resistors and they were all within 0.3% which was a really good sign! |
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