Electronics > Beginners
Multimeter Vichy VC97 - measuring resistance
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Jwillis:

--- Quote from: BravoV on September 09, 2019, 08:51:25 am ---Ordinary multi-meter that only has 2 wires (even the expensive ones) can not reliably measure sub-ohm resistance, as the probe's wires it self will change if you move around.

As pointed, only 4 wires measurement as in most bench DMM or LCR meter that have 4 wires can do that reliably.

Example of 4W, at the alligator pair clips, each side has different wire, so 2 clips = 2 pairs = 4 wires.



--- End quote ---

Just to be sure BravoV .The kelvin clip 4 wire meter measures the voltage add and the current across the resistor and the unit calculates the value. Would that be a correct in the simplest terms ?
tooki:

--- Quote from: Jwillis on September 09, 2019, 07:22:45 pm ---Just to be sure BravoV .The kelvin clip 4 wire meter measures the voltage add and the current across the resistor and the unit calculates the value. Would that be a correct in the simplest terms ?

--- End quote ---
Not exactly. One pair of wires pushes a constant current through the resistor under test, and the other pair measures the voltage drop. Because the voltage measurement uses the meter's high input impedance, essentially no current flows through the sense lines, eliminating the lead resistance.

See 4:20 in:
https://youtu.be/L9q5vwCESEQ?t=260
BravoV:

--- Quote from: Jwillis on September 09, 2019, 07:22:45 pm ---Just to be sure BravoV .The kelvin clip 4 wire meter measures the voltage add and the current across the resistor and the unit calculates the value. Would that be a correct in the simplest terms ?

--- End quote ---

Basically at the 4 wires, 2 wires "force" a known constant current into the DUT, then the other 2 wires are basically a volt meter measure the voltage drop across it.

And then using the basic formula -> Resistance = Voltage / Current , to get the final resistance result.

So at most 4W DMM, you will see the terminals sometimes labelled "force" and "sense" , the "force" terminals mean the ones that force a constant current thru the DUT, and then the "sense" terminals are basically a voltmeter measuring the voltage drop.

As a hobbyist, in the past started in similar situation as op, with a limitation using single cheap DMM, without using the fancy 4 wires measurement tools, I made my self an adjustable constant current source power supply with 100ma and 1A capability just to use it for measuring low resistance, such as low ohm resistors, or sometimes measuring a cable like ordinary power cable which I suspected whether it was made from cheap fake copper inside.

PS : Just posted after Tooki, his post explains it better.
bob91343:
Just for fun I built a sub-Ohm setup.  It uses one IC as a current source and any old DVM to measure the result.  I get reliable readings well below 1 Ohm.  If I remember correctly it resolves 0.01 Ohm.

I don't use it because I have better units.  But I still have it and switch it on occasionally.  It's so small I have trouble finding it.
BravoV:

--- Quote from: bob91343 on September 10, 2019, 04:04:46 am ---Just for fun I built a sub-Ohm setup.  It uses one IC as a current source and any old DVM to measure the result.  I get reliable readings well below 1 Ohm.  If I remember correctly it resolves 0.01 Ohm.

I don't use it because I have better units.  But I still have it and switch it on occasionally.  It's so small I have trouble finding it.

--- End quote ---

Damn, you just remind me of my old DIY constant current rig, and I lost it too and it has a sentimental value for me.  :'(

Fun fact for others that starting to venture into low sub Ohm measurement, if you can measure it "reliably", then try cheap fake copper power wire or thin one, once hook up firmly and steady at both ends, then sort of move, drag or pull the wire gently, sometimes you will see the wire resistance varies quite significantly.  :-DD
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