DMM test lead resistance Standard type and Kelvin type compared - Page 1
Forum section: electronics / testgear

author: sarel.wagner
author_rank: Regular Contributor
timestamp: July 15, 2017, 01:57:13 pm
content: I was curious about my kelvin leads, seeing that the Der EE DE-5000 LCR meter has what I would term pseudo kelvin leads on its little adapter. The dual wires are terminated on the back of the crocodile clips so the length of the crock clip is measured with the DUT resistance. So I have various test leads, with the kelvin leads from Pomona (Also pseudo Kelvin) being my commercial leads. I am busy building my own proper Kelvin leads.This is only a quick test to show the resistive differences between the leads.https://youtu.be/0ZZhoYDOzZQThe build and comparison of my own leads will follow later after delivery of the wires.Regards

author: alm
author_rank: Super Contributor
timestamp: July 15, 2017, 02:04:10 pm
content: What is the point of measuring Kelvin probes with a two wire connection? If you want to properly test those leads, you should have connected one banana plug to sense high on the LCR meter, the other banana plug to force high, and short the sense low and force low through the tip. Then you are measuring what the resistance would be in Kelvin configuration.

author: sarel.wagner
author_rank: Regular Contributor
timestamp: July 15, 2017, 02:08:31 pm
content: Hi, thanks for asking. The DE-5000 is actually measuring in 4 wire mode. So the value of the resistance displayed is from a 4 wire measurement. I was interested in what the relative wire resistance for each wire in the pair were.So centre conductor were 0.421ohm and the Gnd were 0.048ohm. Standard generic no name test lead resistance were 0.086ohm.Rgrds

author: alm
author_rank: Super Contributor
timestamp: July 15, 2017, 02:14:58 pm
content: While that may be true, it has nothing to do with my question. If you short the force and sense leads, then all resistance from that point on will count towards the measured value. If the probe was connected the way it was designed, only the resistance from the last 30mm of the probe (according to thePomona 6303 datasheet) counts towards the measured resistance.

author: sarel.wagner
author_rank: Regular Contributor
timestamp: July 15, 2017, 02:29:35 pm
content: Right you are as well Sir and I do agree. Point being I was not at all measuring that, it was done already by another youtuber. I am planning on building proper Kelvin leads and was curious about how the commercial one by Pomana was constructed, as it were from coaxial cable. We know that the resistance differs between the coaxial inner and outer conductors. It is significant as to what conductor is having the higher resistance between sense and force side.The measured difference between the Gnd conductor being on the force terminal is consistently 0.006ohm lower than if the Gnd conductor is on the sense terminal of my 5.5digit Benchtop 4 wire DMM using said Pomona pseudo Kelvin test leads.Rgrds

author: SeanB
author_rank: Super Contributor
timestamp: July 15, 2017, 03:40:02 pm
content: Ideally the resistance of the force and sense leads should make little difference in a true Kelvin connection, but in many cases there is also some interaction ,as many times the force and sense leads will have a resistor inside the meter connecting them, or there is some dependence on the lead resistance causing common mode offsets. Building your own I would go for as low a resistance as possible on both force and sense, so as to minimise this affecting the measurement, plus making them a shielded twisted pair as well to reduce EMI issues from being picked up inside the instrument as well. Shield ground for both sides should be connected together at the instrument and connected to it's internal ground shield.

author: sarel.wagner
author_rank: Regular Contributor
timestamp: July 15, 2017, 04:06:07 pm
content: Exactly as I plan to do Sean. I want to see if a difference is detectable between PTFE coated and PVC covered wire as well, in 4 wire mode, twisted shielded pair vs non TSP.Rgrds