diy low ohm meter schematic

[uranium235]

does anyone have a diy low ohm meter schematic diagram??
with the following features?

1.easy calibration
2.measures 1 -0.001 ohm 1% accuracy
3.use minimal and easy to find components
4. can be used as an adaptor for DMM

i already searched the net still i cant find any good diagram

[TheWelly888]

Will this do?

http://baec.tripod.com/DEC90/ohmmeter.htm

No, I haven't tried it myself, all I did was google "low ohm meter" and found this in 10 seconds!

[uranium235]

ive already seen the site before, im having problem on CA3140 i cant find it locally in our area.

[TheWelly888]

You don't have to use a CA3140 op-amp in that circuit. What you can do is to work out why the CA3140 was chosen for this application and have a look at the data for it and find the nearest equivalent.

[Balaur]

A 4-wire mili-ohmmeter is quite easy to design or to improvise.

You just need a serious (10mA-100mA-1A depending on voltmeter sensitivity) constant current supply and then measure the voltage on the resistor with a multimeter. The presented schematic was implementing the same approach.

Off course, you can use any serious opamp for a 10x, 100x or whatever gain to minimize the required current. Put all this in a cheap panel voltmeter or a PIC with a HD447880 display and presto you have a miliohmmeter.

Cheers,
Dan

[Fragrance]

does anyone have a diy low ohm meter schematic diagram??
with the following features?

1.easy calibration
2.measures 1 -0.001 ohm 1% accuracy
3.use minimal and easy to find components
4. can be used as an adaptor for DMM

i already searched the net still i cant find any good diagram

highly accurate and used any 4 1/2 digit DMM i made it using it with my DMM

http://electronics-diy.com/electronic_schematic.php?id=951

[kripton2035]

plenty of examples here : http://kripton2035.free.fr/continuity-repos.html

the one I prefer is : http://kripton2035.free.fr/Continuity%20Meters/continuity-an220.html

[KerryW]

I use CA3140s as a drop in replacement for for TLO81.  I don't know how close they are, but they are close enough for the circuits I build.

Edit:  After reading the circuit description, I see that the CA3140 gets a lot closer to the negative rail than the TLO81, so you can't use one.  Sorry about that.

[oldway]

To measure resistance as low as 1 mR with precision, I would use a 10A current source for the lowest range.
(four wires measurement of course !)

[Kalvin]

To measure resistance as low as 1 mR with precision, I would use a 10A current source for the lowest range.
(four wires measurement of course !)

Few milliamperes is sufficient: https://www.eevblog.com/forum/projects/hx711-based-milliohm-meter/msg782192/#msg782192

[Dbldutch]

I have built the milli-volt meter from the following link as a well working prototype. In the meantime a user developed a pcb, and I am building that one. Parts are on order as we speak.

Have a look here : http://www.scullcom.uk/category/projects/milliohm-meter/
There is also a youtube series describing the design.

You can also gave a look at my own blog where I am trying to complete a BOM :
http://www.paulvdiyblogs.net/2016/09/building-milli-ohm-meter.html
It's a work in progress ahem process, but well worth it.

Enjoy!

[oldway]

To measure resistance as low as 1 mR with precision, I would use a 10A current source for the lowest range.
(four wires measurement of course !)

Few milliamperes is sufficient: https://www.eevblog.com/forum/projects/hx711-based-milliohm-meter/msg782192/#msg782192

How could you measure 1mR with 1% precision if your project has:
-  Initial drift was 3 milliohms
- After initial drift, the variance was +/- 0.5 milliohms
- Measurement current is in range 5mA - 10mA.

[Kalvin]

To measure resistance as low as 1 mR with precision, I would use a 10A current source for the lowest range.
(four wires measurement of course !)

Few milliamperes is sufficient: https://www.eevblog.com/forum/projects/hx711-based-milliohm-meter/msg782192/#msg782192

How could you measure 1mR with 1% precision if your project has:
-  Initial drift was 3 milliohms
- After initial drift, the variance was +/- 0.5 milliohms
- Measurement current is in range 5mA - 10mA.

The idea of my posting was that 10A is not needed in order to be able to measure milliohm resolution, only few milliamperes is sufficient. Dannyf did some further development in the discussion thread and he was able to get the accuracy good enough for milliampere accuracy. The HX711 has some temperature drift issues what was found out, but the temperature effect was few milliohms at worst, and the error due to the temperature drift got less after the HX711 was kept in constant temperature.

The OP wanted to have a milliohm resistor measurement device with a range 1 milliohm ... 1 ohm with 1% accuracy. If the accuracy of 1% means the accuracy over full measurement range of 1 ohm, then the required accuracy should be better than +/- 5 milliohms which can be achieved even with the cheap, non-compensated HX711. If one replaces the HX711 with a better ADC or builds a simple temperature control for the HX711, the measurement accuracy can be easily in range of +/- 0.1 milliohms, which is pretty good. However, the dannyf's idea to use a cheap HX711 ADC with ratiometric measurement and small measurement current with a high resolution A/D-converter was a clever solution.

Edit: Some microcontrollers have pretty high resolution on-chip A/D-converters. For example, the Psoc5 LP devices have a 20-bit differential input delta-sigma ADC. Unfortunately I do not have time atm to give it a try how good milliohm meter could be built using the the Psoc5 LP device, using ratiometric measurement, chopped stimulus current and synchronous detection. If the accuracy of the on-chip reference voltage is not good enough, one can use an external reference voltage for improved performance.

[oldway]

does anyone have a diy low ohm meter schematic diagram??
with the following features?

1.easy calibration
2.measures 1 -0.001 ohm 1% accuracy
3.use minimal and easy to find components
4. can be used as an adaptor for DMM

i already searched the net still i cant find any good diagram

Without using any op amp:
Range 1mR to 10mR ...Current source 10A ...reading DMM 10 to 100 mV
Range 10mR to 100mR ...Current source 1A...reading DMM 10 to 100 mV
Range 100mR to 1R....Current source 0.1A....reading DMM 10 to 100 mV

[Kalvin]

does anyone have a diy low ohm meter schematic diagram??
with the following features?

1.easy calibration
2.measures 1 -0.001 ohm 1% accuracy
3.use minimal and easy to find components
4. can be used as an adaptor for DMM

i already searched the net still i cant find any good diagram

Without using any op amp:
Range 1mR to 10mR ...Current source 10A ...reading DMM 10 to 100 mV
Range 10mR to 100mR ...Current source 1A...reading DMM 10 to 100 mV
Range 100mR to 1R....Current source 0.1A....reading DMM 10 to 100 mV

Take a UNI-T UT61E DMM, select the 220mV range and use a 10mA current source to generate the test current. Measure the voltage across the unknown resistance, and the UT61E will display the resistance directly in milliohms and the display will show 10uV per milliohm. The measurement range is 1 milliohm ... 22 ohms with the resolution of 1 milliohms.

Edit: Whoa, made some trivial mistakes here as I didn't have the UT61E at hand.

[Kleinstein]

To get a really good resolution at a moderate current, one should measure with both polarities of the current. So do a kind of low frequency (e.g. 1 Hz) AC measurement. With DC one might resolve something like 1 µV and even than can have trouble with thermal EMF of the measured resistance. As ther thermal EMF can very well be at the probed resistor there is no real point in improving the measurement part.

With AC one can measure much smaller voltages. Something like 0.1 µV or even 0.01 µV gets reasonable with a low noise amplifier. So it can be a good idea to have a switch to reverse the polarity of the test current. An maybe an extra (low noise) amplifier, as the ADC inputs and even more the DMMs are usually not that low in noise. With an ADC a µC could aso directly do the difference calculation.

[TimFox]

For an AC measurement, a lock-in amplifier (with long averaging time) can be useful to measure the voltage.

[Gyro]

does anyone have a diy low ohm meter schematic diagram??
with the following features?

1.easy calibration
2.measures 1 -0.001 ohm 1% accuracy
3.use minimal and easy to find components
4. can be used as an adaptor for DMM

i already searched the net still i cant find any good diagram

Without using any op amp:
Range 1mR to 10mR ...Current source 10A ...reading DMM 10 to 100 mV
Range 10mR to 100mR ...Current source 1A...reading DMM 10 to 100 mV
Range 100mR to 1R....Current source 0.1A....reading DMM 10 to 100 mV

I still like the elegant little battery operated current source used in the Maxim AN106 milliometed design (attached). There's nothing particularly special about the reference or the opamp, use your own choice. You can trim it to be as accurate as you like. I'm still on the original batteries after long enough to be worrying about their shelf-life!

It's DC of course, but as long as you have decent meter sensitivity and take into account any thermal emfs before and after measuring then it won't go far wrong.

[rob42]

C8051F350 costs about 2.2$, contains 24-bit ADC, PGA/opamp:
https://ru.aliexpress.com/item/C8051F350-GQR-8-k-ISP-Flash-MCU-Family-C8051F350/32709262043.html

Here is design of precision milli/micro ohm meter:
http://bbs.38hot.net/thread-45486-1-1.html

[laffreux]

Here is a simple way to measure sub-ohm...
You just need a Lab power supply and a volt-meter.
It is as accurate as the meter !!!

[kripton2035]

shorty with display to build yourself :
http://kripton2035.free.fr/Projects/shorty-display.html

[Dave]

Here is a simple way to measure sub-ohm...
You just need a Lab power supply and a volt-meter.
It is as accurate as the meter !!!

No?
You either also need to measure the current with a second multimeter, or have a very accurate power supply (SMU territory). The tolerances of both instruments multiply to get the measurement tolerance.

[TimFox]

Or, put a relatively high resistance measured-value power resistor in series with the lab voltage supply.

[Doctorandus_P]

The easiest DMM adapter is to use an LM317 with an (about) 1.25 Ohm resistor to use it as a 1A current source / sink.
It's not so easy for calibration (you have to adjust the 1A capable power resistor), but otherwise it's quick, simple and effective.

Then have some power source (lab supply?) and push that 1A current though your DUT, and measure the voltage drop with your DMM. This is also inherently a 4 wire measurement.

[Marco]

Low ohm meter is an ambiguous term, do you just want to measure small resistances or do you specifically want to measure low resistances at high current. Micro-ohm meters generally do the latter, not because high current is necessary for the resistance measurement but because it's part of the overall measurement which is not just a resistance measurement.