diy low ohm meter schematic

[NiHaoMike]

It would probably help to use (very) low frequency AC to null out the effects of thermocouple offset.

[Marco]

It would probably help to use (very) low frequency AC to null out the effects of thermocouple offset.

If it's a sine or a repetitive 3 current bipolar measurement, say -x, +x and +2x Ampere, you could not only subtract out thermocouple offset but also measure Josephson voltage for your room temperature superconductor

[PCB.Wiz]

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

1. is taken care of by using 4.
You are unclear on spec, do you want 1% of full-scale, so 1 ohm measures 10mOhms, or do you want to measure 1mOhm to 10 microohms.
1A into 1mOhm is 10uV at 1% scale.


Since you already have a calibrated, decent? meter, the missing part is higher injection currents than a DMM usually uses on resistance.

A good place to start, would be to create a precision, stable current source of 100mA and 1A and maybe 10mA.
If you have a bench power supply, that gives you more choice to control the circuit power loss, which will be a significant issue.
If you pick a low dropout regulator, with a low reference voltage, and low Vmin, you can slash the power dissipation needed.

Wirewound parts at 1% and ±50ppm/°C are plentiful.

Failing that, a 2-3A USB wallwart means you have 5W to spread over the (choose large) dropper elements, at the 1A test current.


Depending on how good your DMM is, you could also add a fixed gain opamp, low noise and low offset. OpAmps keep getting better !

eg a TLV387 has 10uV vos and 8.5nV/sqhz and 0.01uV/'C with 0.1% gain resistors gives you a solid foundation.

With typical numbers of 1uV Vos, you may even find 100mA of test current is sufficient for your tolerance specs.
 

[Doctorandus_P]

There are indeed different ways of measuring small resistances.
The easiest is to use a high current, as a higher current generates a bigger voltage drop and thus a larger test signal.

There are also meters designed for measuring contact resistances, and these deliberately use a very small test current (micro amps or less?)
But a micro amp at a milli ohm is a nano volt. And I'm not sure that is even measurable by normal folks. Small temperature differences can easily swap this due to thermocouple effects.
The "Low Level Measurements Handbook" is a good read for what the real life limits are of what can be measured. When measuring small currents for example, at some point you are counting single electrons. Once you start measuring the time between single electrons passing, you're well into the VooDoo range.

I also like the method of the article posted in circuitcellar #314 (Schematic attached). It uses simple hardware and smart software to get very usable results.

[intabits]

Here's what I made to measure low resistances:-

[Terry Bites]

www.analog.com/en/design-notes/batterypowered-circuit-measures-milli-ohms-and-micro-ohms.html Easy

[Shock]

Terry Bites, are you using that last design or see any video on it?

[Gyro]

It's the AD abstract of the acquired old Maxim AN106. See reply #17.

Here's the version of it that I built...  https://www.eevblog.com/forum/projects/design-review-ac-milliohm-meter/msg4589653/#msg4589653

[Shock]

That enclosure looks the real deal.