Micro ohmeter by arduino

hi guys i need your help here i'm trying to make an ohmmeter based on arduino, i used the kelvin 4 wires method by injecting current 1A DC in the resistance ( µohms ) and get the voltage drop but the probelm is that the voltage i get is by micro volts and i want that to be readable by arduino i tried AOP but nothing works, any ideas ! and tnx

elec_tao: hi guys i need your help here i'm trying to make an ohmmeter based on arduino, i used the kelvin 4 wires method by injecting current 1A DC in the resistance ( µohms ) and get the voltage drop but the probelm is that the voltage i get is by micro volts and i want that to be readable by arduino i tried AOP but nothing works, any ideas ! and tnx

If you are getting microvolts and you need volts, that should tell you that an amplifier is needed.

Paul

I don't want to be rude but if you can't figure out how to do that you are way over your head!

I've got an old lab-quality HP multimeter with a 4-wire option on my workbench at work and I wouldn't trust it to read "micro-Ohms". But, I haven't checked the specs. And, maybe you have a good-solid way of making the connections to the DUT?

I wouldn't try to build something like that myself!

The "Arduino-side" is super easy if you can provide a DC voltage... The ADC is 10-bits (0-1023) so with the optional 1.1V* reference so you can read down-to 1mV, with 1mV (or 1/1000) resolution which is enough resolution for most measurement applications.

  • The 1.1V reference is very-stable but it has a tolerance so you may need to calibrate (easy to do in software if you know the correction factor).

Yeah that's true I want to add and ADC with 24bits so I can read the micro volt. is it possible ???

I'd start with a kOhm meter, with handy currents (1mA) and voltages (5V for 5k). Then fix all problems before you start downgrading towards lower resistances.

I also wonder what's the test resistor you try to measure?

If you only want micro accuracy (1ppm), you need a sufficiently equipped ADC, noise suppression, humidity control and temperature compensation.

I want the test to measure contact resistance in circuit breaker !!

AFAIK contact resistance is bound to some specific current. Not all contacts support 1A, others require more current for stable readings. Also the switched voltage may be of interest, where a high voltage will burn oxide or other dirt from the contacts. And the way of closing the contacts plays a role as well. Do you want to measure entire switches, from their wire connectors, or compare different contact materials?

All in all a big and sophisticated project with many pitfalls if you want to get reliable and reproducable readings.

I myself have never found the need to measure contact resistance in a circuit breaker and I've dealt with many over the last 50 years.

Any testing is usual to do a "remote heat image" test.

Electric utilities measure contact resistance with constant high current.

Example: constant current of 10 - 100 amps while measuring the contact voltage drop.

You'll have problems in measuring reed contacts with constant 10A :-]

At 100A it may be easier to measure the emitted thermal radiation instead of a voltage drop.

elec_tao: Yeah that's true I want to add and ADC with 24bits so I can read the micro volt. is it possible ???

As long as you can get your noise levels down to 24 bit - or 0.06 ppm(!) - then you stand a chance to measure something useful with your 24-bit ADC. That alone is a major challenge. This are noise levels where you will pick up anything from mains hum to the electric fields your muscles produce. So don't move or breath too hard when you're trying to get down to those final bits.

Another problem: the wires connecting that resistance to your Arduino will have a resistance that's many orders of magnitude higher. The connections you make wiring up have resistance, also possibly in the mΩ orders of magnitude, unless you solder everything, in which case you're down to just the resistance of the wires themselves. Somehow you still have to compensate for that. A differential reading may help, but only if you get the wires similar enough.

The four wire technique overcomes the problems of lead resistance , so that particular area shouldn’t be the problem .