DMM Test Lead Resistance

[TMM]

The TL21 alligator clips aren't proper kelvin clips because both jaws are permanently electrically connected and both the pair of wires is just soldered to the same spot. The resistance between the jaws and DUT isn't compensated for. Thankfully the alligator clips are very stiffly sprung so any resistance is minimal.

I modified mine with proper kelvin clips and longer leads.


The TL22 tweezers have the same problem. That said, i haven't seen any proper kelvin tweezers.

[walter.aranda]

When using the 0.1 ohm resistor for high current measurements, I measure voltage drop across it -- not resistance.

I wasn't talking about measuring voltage across the resistor or using it to measure current, but measuring its resistance. Or any other resistor where < 1 ohm resolution / accuracy is required.

 

But it was in my introduction.  Neither one of us are on the same page.  If you go back and look at my intro, I stated that the 0.1 ohm resistor is used for high-current measurements.  Nothing more.  I'm not looking at measured resistance accuracy when I'm measuring voltage across a precision resistor in order to compute current.  When computing current this way with a Hi-Z input DMM, I don't care about wire lead resistance at all. 

I'm interested in your statement about the shunt when taking high current measurements. If the leads introduce resistance, and as current increases, won't the lead resistance begin affecting current measurement accuracy?  I'm referring to a test where the DMM is inserted in series with the circuit for the current measurement.

Imagine the 0.1 ohm resistor with the DMM measuring the voltage across it as the DMM in current mode. Then put two 0.1 ohm resistors in series with the shunt. Will this affect the voltage across the shunt? It will increase the burden voltage, which might affect the current through the shunt, but the DMM will still correctly indicate the current through the shunt.

Agreed that it's still measuring the accuracy across the shunt.  Let's assume for a moment that our test leads were to deteriorate to say...100 ohms resistance.  Let's run our DMM in series -- and current is 10A DC.  The DMM in current mode is measuring the right current no matter the lead resistance.  However, my point from the beginning (and that is the title of this thread) is that the lead resistance can affect the circuit by reducing the in-circuit current and give an erroneous reading as to the circuit current if the DMM were not there.

Circling back...Because the lead resistance can reduce circuit current, I place a 0.1 ohm resistor in series with the circuit.  This value means the circuit is being perturbed as little as possible for the purpose of the current measurement.  In doing so, I've removed test lead resistance out of the equation.

Hoping to align us on the same page...


Paul 

   

Hello, sorry, I did not quite understand your comment on how to eliminate the current reading error caused by the cables of a multimeter. In the hypothetical case where a multimeter has a series resistance of 0.2 ohms in the cables and it is desired to measure the current through a 0.1-ohm load connected to a 1V source, how would you eliminate the current reading error?