how to measure low resistances accurately -tutorial-

[ali6x944]

hello everybody,
i found this really nifty trick in the form to measure low resistances accurately with a multimeter, it's called 4-wire measurement or Kelvin sensing, this will be a summery for that, check it first .
Here: https://www.eevblog.com/forum/testgear/question-11059a-kelvin-probe-set/msg821592/#msg821592
it is basically a will known value current source that has also known voltage -low voltage like 2v would be great-  connected in series to the resistance you want to measure, and a multimeter connected in parallel to the resister , the meter should be set to the dc voltage measuring rang , it should be ideally set to the mili or micro volt set, but mili volt set work for most time, now measure the voltage drop across the resister.
now it becomes straight forward ohms law:

IR=V
SO,
V/I=R
 and because we know the current flow through the resister and we measured the voltage drop across the resister,  we use the equation above to get the resistance.

   
so
example:

[Brumby]

It's a very well known measurement technique.  There's been a discussion on getting Kelvin probes somewhere here recently....

Here: https://www.eevblog.com/forum/testgear/question-11059a-kelvin-probe-set/msg821592/#msg821592

[ali6x944]

OH...
this is just a summery for that.
thanks man for noticing me 

[Brumby]

It's part of the broader subject of remote sensing.

Precision power supplies, for example, will have a set of 'sense' wires that you connect to the device under test (DUT) which detect the actual voltage at that point, rather than internally.  This bypasses the problem of the voltage drop in the leads from the power supply.

[ali6x944]

wow! that is amazing! i didn't know that, thanks man
but i was thinking how to use an op amp to sense the voltage instead of a multimeter?

[uncle_bob]

Hi

One caution it a "tutorial" sense:

The order of lead attach to the load is quite important in a Kelvin setup. The isolation of the leads is a critical part of all this. The "assumptions" include cool stuff like zero current in the sense leads and zero voltage delta between the load and the sense point.

-----

If you go the sense lead on the power supply route - be careful there as well. A fully isolated sense lead (like I just recommended above) is *not* a good idea on a power supply. There you want some sort of protection so the supply does not go nuts if a connection breaks.

Bob

[ali6x944]

thanks uncle_bob