A question on 4-wire

[raptor1956]

I have a new Rigol DM3058E DMM and it has 4-wire resistance measurement but the manual is not crystal clear on which set of leads is which.  The front panel has the "Input Hi" and "Input Lo" connection that are used for v, i, capacitance and 2-wire resistance and it has connections called "Sense Hi" and "Sense Lo" that you use to connect the second set of leads when doing 4-wire resistance measurements.  Now, it stands to reason that the "Sense Hi" and "Sense Lo" connections are just that -- the sense or voltage read inputs while the "Input Hi" and "Input Lo" connections are the current source during 4-wire operation but when I look in the manuals to confirm that I find nothing that explicitly confirms this.  So, is my understanding correct?

It's an interesting thing to try to test, but my first effort failed.  I used a 1.2m length of 30ga wirewrap wire that has a resistance, per the tables, of about 339 mOhm/m so I cut it in two then tied them together so I now have places to connect leads at both ends and in the middle.  I started out with both black leads at one end and both red leads at the other and got something over 400 mOhm as you'd expect, and when I move the "Sense Hi" lead to the center the reported 4-wire resistance dropped about in half as you'd expect.  Then I moved the "Sense Hi" lead back to the end with the other red lead then move the other red lead to the middle and the result was pretty much the same.

So, thinking about it it actually makes sense that the readings would be about the same no matter if the current source or voltage sense was moved to the middle and that leaves me with the question of how one might go about actually testing to confirm.  I tried using another DMM to look for the voltage drop but don't see anything in DC or AC down to mV.

So, what's going on here?


Brian

[raptor1956]

I just figured it out ...

Using the second meter in current mode I tested both sets of leads and the "Input Hi" and "Input Lo" leads are definitely the current source -- 1mA.  I get no current when running the "Sense Hi" leads through the current meter.

And the 1mA current explains why I didn't see the voltage with my meters as it was in uV's -- about 400uV.


Brian

[electr_peter]

DMM behaviour in 4 wire resistance is completely normal described. Sense terminals are used to measure the voltage while Force terminals provide the current. Resistance is calculated as ratio of V over I.

In contrast, 2 wire resistance measurement uses Force terminals to both force current and measure voltage drop. Sense terminals are not used in this case.

Such behaviour is common to most DMMs with 4 wire resistance measurement capability. Sense terminals are used for very limited amount of DMM functions - 4 wire resistance, voltage ratio.

[raptor1956]

Yes, I'm familiar with how it's supposed to work and I was just checking because the manual for the Rigol DM3058E wasn't particularity explicit with which leads do what and how and where to connect them to the DUT.  I correctly reasoned that the voltage sense was what they labeled the "Sense Hi" and "Sense Lo" terminals, but because they were not explicit in so stating in the manual I wanted to confirm for myself.  My first test, looking back at it, wasn't going to resolve the issue, but looking for the current did.

You can use two DMM's without 4-wire capabilities to measure resistance using 4-wire methods so long as you have a power supply that can provide the current.  Use one DMM to measure the current and the other to measure the voltage drop.  So long as the PS provides a stable current it doesn't much matter what the current is so long as is high enough to provide a measurable current and voltage drop and the meters have the necessary resolution to measure it AND that the current not be so high as to cause enough heating to effect the resistance.

You can even measure 4-wire resistance with just one ordinary DMM and a precision power supply.  Have the power supply output an accurate current and use the DMM to measure the voltage drop.


Brian