Very accurate low resistance measurement problem

[nzkunal]

measure low resistance

Hi,
I need to design a very accurate ohmeter for a copper wire tube. At present I put 0.5A through the wire (26.4-28.4 ohms). The voltage is then shifted down throrough a potential divider. The tester seems to drift with time. I am currently running some tests to monitor the affect of the current on resistance. The reason I am using the high current is so I can get a higher resolution on my ADC. I believe this is more accurate than putting less current through and then amplifying it later in the circuit lower SNR etc.

Does anyone know of any techniques using bridges etc to measure low resistance values (round 20-30 ohms) with an accuracy of round +/-0.05 ohms?
Thanks in advance

 

[flatulent]

low resistance measurement methods

For low resistances the four probe method is frequently used. Constant current if forced through the sample and the pickup is two probes *attached to the sample between the current probes) going to a high resistance measurement circuit. This removes the wiring and contact resistance/potential effect problem.

 

[dumbfrog]

copper resistivity measurement

:lol: kelvin:

https://www.allaboutcircuits.com/vol_1/chpt_8/9.html

 

[papyaki]

resistance measurement adc

Hi,

Some years ago, I have designed a relay tester able to measure relay's contact resistance, using the four wire method described by Flatulent.

Measured values were arroud 0.02 ohms with an accuracy of 5% under a constant current value of 100mA.

I think this method is the best one to do such measurment. Other methods under high current are tributary of wire heating over time.

To improve the accuracy, you can also make several measurment and average them.

 

[chinara]

very accurate low resistance measurements

hi,flatulent:
It is very interesting to heard you about the method, measuring the low resistance. But I could not understand what you mean. Would you be so kindly to explain more detaily?

 

[RegUser_2]

forums low resistance measurement

hi,flatulent:
It is very interesting to heard you about the method, measuring the low resistance. But I could not understand what you mean. Would you be so kindly to explain more detaily?

It's a standart 4 wire measurement : 2 wires are used to supply constant curret and the other two are used to connect your voltmeter to the load whose impedance U want to measure. The point here is to avoid including the resistance of the bias wires and measure the voltage drop only across the load.

Naturally when the measured impedance is very low and the possible maximuum bias current it is limited, then the measurement is prone to errors due to drifts in the voltage drop readout and EDF due to various materials of the probe and the material whose resistance U want to measure. Those errors can be greatly reduced by using 4-wire measurement with an AC bias current at frequency above the readout 1/f noise knee and a lock-in readout.

 

[RegUser_2]

low resistance measurement

Hi,
I need to design a very accurate ohmeter for a copper wire tube. At present I put 0.5A through the wire (26.4-28.4 ohms). The voltage is then shifted down throrough a potential divider. The tester seems to drift with time. I am currently running some tests to monitor the affect of the current on resistance. The reason I am using the high current is so I can get a higher resolution on my ADC. I believe this is more accurate than putting less current through and then amplifying it later in the circuit lower SNR etc.

Does anyone know of any techniques using bridges etc to measure low resistance values (round 20-30 ohms) with an accuracy of round +/-0.05 ohms?
Thanks in advance

Let's try to estimate proper readout for your case, assuming an 4-wire measurement tecgnicue.

1. If the noise-equivalent resistance U want to measure is 50 mOhm, and some chose an instrumental amplifier as a front and, with noise appr 20 nVrms/RtHz above the 1/f knee of say 100 Hz and say the bandwidth of measurement is 1 Hz, then the rms value of the preamp noise will be 20 nVrms and the pp value will be 2*pi or about 185 nV. From here some could estimate the minimum bias current to be 185E-9/5E-2 = 400 uA

2. In order to provide 400 uA constsnt current through the 30 Ohm resistance , the easiest way is for some use reference voltage and large resistance in series. In order to minimize the mains pickup, actually 2 such bias resistors should be used in each input of the instrimental amplifier, to increase the CMRR. In general the value of this bias resistors should be at least ten times higher than the maximum Rx of 30 Ohm -> Rl = 300 Ohm (2x150 Ohm). The minimum value of the voltage reference will be 400 uA * 300 Ohm = 120 mV. This is too low voltage for a standart voltage reference. If some takes a 1.2 V bangap reference, then the bias resistances can be increased tenfold what will make the bias current even more "constant" vs variations in the Rx -> we endup with 2x1.5 kOhm bias resistors.

3. The gain of the readout is defined by your data acquisition. The DAQ resolution should be larger than the dynamic range of your measurement which U defined as 30 Ohm/50 mOhm = 600, or U'll need 10 bid ADC. If some assumes that a microcontroller is used for DAQ (for instance the micropower MSP430F149 is very good choice), then if the refference of the ADC is it's power of 3.3 V, the maximum gain of the preamplifier will be 3.3V/(400 uA * 30 Ohm) = 3V/12 mV < 300.

4. All the estimates were done assuming that an AC bias current with frequency of 100 Hz is applied, what is done by chopping the 1.2 bandgap refference using electronic switch controlled by the uC. The lock-in loop is done in software by multiplying the ADC readout by +/-1 syncronously with the bias chopper drive and integrating over 10 bias periods. to have null at 10 Hz and relatively flat responce at 1 Hz.

5. I'm not sure that applying 0.5 A current buy's you much because then the contact impedance heats the copper and U have a thermal dependence of the coper resistance. A as small bias currens as possible allways are recommended not to garble the measurement with additional efects.

 

[VVRuban]

lock-in voltmeter resistance measure

Hi

Here is interesting APPNOTES from ADI about micro-OHM measurement.
Maybe it will be useful for you.

Best regards.

 

[devrimaksin]

very accurate resistance

Standard 4 wire measurement should work

 

[nicleo]

resistance measurement in wiring

If you think that 4-wire resistance measurement method is not accurate enough for your application, you can consider SIX-wire resistance measurement method as follows:

[url]http://www.bitpipe.com/detail/RES/1005157565_312.html[/url]
**broken link removed**

By the way, I'm really impressed by the explanation given by RegUser_2. I'm looking forward to hear more practical experience (in low resistance measurement) from him. Hopefully he can share more with us in this forum. Thanks in advance.

 

[nsakan]

Basicaly, the four probe method is very good. But instead of steady DC current measurement consider using the modulates or sinewave current and a lock-in technique (phase locked measurement).

There was a nice project on the net to build a simple lock-in miliohmmeter based on AVR microcontroler on:

http://cappels.org/dproj/dlmom/dlmom.html

The site is currently unavailable, but I think that I have the entire project on some of my backup CD's. If You are not able to find it on the net send me an e-mail on

nsakan@phy.bg.ac.yu

 

[rauol]

The Drift in measurement can be due to the temperature coefficient of the Copper wire, if you are using copper wire as the sensing element, better change it, copper has a temp coeff of 4000ppm degree C.
Any change in temperature due to the power dessipation in the copper can change its resistance.

 

[nsakan]

Exactly because of the problem with high current and copper heating it is recomeneded to use som kind of low current measurement.

The best sollution is the lock-in measurement and it makes a extremley good aproasch for lot of measurement systems. Basicaly with a high quality lock-in amplifier You coug measure signals that are burried deep inside of a noise (more than 100 dB in electronical units :->, or signals comparable witn nV for rest of us). The lockin nanoohmmeter that I have mentioned has both advantages, it is simple to build, and along wit that it is a lowcost-one on one hand, and on the other hand it gives You the oprtunity to have the verry precise measurement, in this case of electrical resistance.

Sincerely Yours,
Nenad Sakan