![]() Since the source/ground wires are low impedance wires where all we care about is current. This means that our sense wires and our source/ground wires all have their own contact resistance. The Kelvin probe isn’t just a “fancy” alligator clip, but the two jaws of it are actually electrically isolated. Ie, one has the sense wires connected to the source/ground in the connector, not in the device under test. (And also thermal effects if one’s connections are close enough to each other to have “about the same” temperature.)īut I have seen a lot of people take a “Kelvin probe” and then connect both wires to the same side of the connector… I have seen plenty of people wire up kelvin probes incorrectly through the years…Īnd this video mostly goes through the theoretical side of things.īut the idea behind the 4 wire measurement is that one can remove the effects of contact resistance. Once you have a meter then you can worry about whether Kelvin probes are necessary or compatible with your meter If that’s important for your design then you probably need an LCR meter. 5%, 10%, 20% or more are common tolerances. That’s definitely not true when it comes to caps and inductors. For resistors, you can generally get 1% or even 0.1% resistors pretty cheap. Any LCR meter is better than no LCR meter. Measuring to 0.1%, Kelvin would be necessary. If you’re measuring a 1K resistor to 1% accuracy then a Kelvin configuration would only be necessary if your test leads are more than 10 Ohms. If you need an accurate measurement of a tenth-ohm brushless motor winding, yes you need it since your test leads have a significant resistance compared to a tenth of an ohm. If the resistance/impedance of the test leads are significant relative to your part and your desired accuracy, sure. What he doesn’t mention is that a Kelvin configuration is probably not necessary for hobbyist use. Inexpensive as far as semi-lab-grade meters go. The DER lcr meter he uses is an extremely good buy for an accurate and wide-range unit. Posted in Tool Hacks Tagged kelvin clips, kelvin probes, ohmmeter, resistance Post navigation If you don’t want to probe in real life, you can always do it in virtual reality. You do need a meter that will do the actual four-wire measurement, although a power supply and a voltmeter will do the job, too. We’ve seen these probes built from scratch, too. While the sense wire pair will also have resistance, the sensing current can be very small which means there will be correspondingly less error in the measurement. Pulling supply current through the probe wires - which have some resistance - causes a voltage drop that affects the measurement. If you wonder why that’s better than two probes, it all comes down to resistance in the test probes. With a four-wire measurement, one pair of wires carries current to the device under test and the other pair of wires measure the voltage drop. You are measuring a voltage and a current. We liked advice from his old instructor: you aren’t really ever measuring a resistance. Kelvin probes are a special type of probe for making accurate resistance measurements using four wires and, in fact, the probe’s jaws are electrically isolated from each other. But if you watch recent video, you’ll learn they are really Kelvin probes.
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