TTT341 Kelvin Probes

A look at Kelvin Probes and their value in electronic measurement.

[@advancedleveldiagnostics]

Great explanation! Thanks for sharing!

[@DmitryLukin]

Thank you very much for your best (transparent and easy understandable) explanation!

[@buildstoys]

Thanks for the thorough explanation Tom.

[@gregorythomas333]

Excellent video...my grandpa only used Kelvin probes so that is what I learned on :)

[@iPatroni]

Great explanation. Thank you Tom.

[@jimnewman5596]

A great video Tom, learned something new today. I take it that this is used mostly in low resistance, low impedance circuits.

[@tomtektest8042]

Thanks for the comment and question. Actually Kelvin Probes are used anytime you want accurate measurements. I do not intend to get into it, but the use of Kelvin Probes is essential not only to measure resistance, but also to resolve the series resistance (ESR) from the parallel resistance (Leakage) as well as to compute the Dissipation (D) and Quality factor (Q) of a component. If you go to the IET Labs website, they have an excellent set of papers that explain this much better than I could.

[@richardgray8593]

I needed to see this. Thanks.

[@seshachary5580]

very educative. thank you regards

[@larrysmall3521]

Another place you will see this is on high current regulated power supplies. They use remote sense to measure and regulate the voltage at the load instead of the supply. This can be significant if the load current changes during different modes of operation.

[@bradclancy6069]

Good refresher, great video! 100% we, don’t measure resistance or impedance(resistance and capacitances). I argue, we don’t measure currant either. Again we measure voltage and calculate amperage. Earthier with a shunt or inductive coils. Would you agree, sir?

[@Deathkloud]

I love your voice. It sounds like a smart winnie the pooh.

[@tomtektest8042]

Thanks for the comment. You may not be old enough to remember the actor Sebastian Cabot, but many of my friends (old enough) say I sound like him. He did the voice of Winnie the Pooh in early films and my granddaughter used to ask if I was really Winnie. I wish.

[@neilphilip2320]

Your videos are SO informative - many thanks - having just acquired a 4-wire device I now know what the 4-Omega button is for! Do you have one on measuring current gain in transistors (accurately) ? I need to match about 6 of them for a project. But thanks for giving us all some of your time and ATB... Neil in England

[@hepcat67]

Hi Tom: Thanks for another excellent video. Could you perhaps do a short video showing how to retrofit the leads on the DE-5000 to the Kelvin leads? One thing I noticed on the DE-5000 is that if I tried to extend the leads, it wouldn't calibrate. Thanks.

[@tomtektest8042]

There is already a better video than I could make on this. Search KZbin for "DE-5000 Kelvin Clip Upgrade" and click on the video by Kevin Gittemeier from about 2 years ago. I used it before I did the change and it was very helpful. Good Luck.

[@hepcat67]

@@tomtektest8042 Okay, thanks.

[@threethousandyen]

What is the problem with the DE-5000 alligator clips compared to the replacements you installed? Do they not conduct as well or is it something more ergonomic?

[@wayneparris3439]

I purchased that meter several months ago, I have also watched a few videos where the probs are changed. As the probes come with BNC connectors and the standard way to install them on the meter is to cut off the BNC connectors and hard wire them into the adaptor box for the meter, I was going to just install 4 panel mount BNC connectors into the meter adaptor box. This way I can also use the probs on other equipment or replacement if needed is as simple as just changing the probs at the BNC. Thoughts?

[@resiggy13]

I guess I still don't understand what your getting that's different with the extra 2 wires here. At the end of the day the length of the wires between the meter and the device is still the same so how are you effectively moving the measuring point closer to the device under test? Is it just that the extra 2 wires are of an already well known resistance and therefore can be mathematically/electronically factored out of the equation? If so, why couldn't that same technique be done with just two well known wires? I must be missing something.

[@tomtektest8042]

Kelvin Probes use on pair of wires to provide the current, which is the same throughout a series circuit, and a second pair of wires (with extremely low current flow, therefore extremely low voltage drop) to measure the voltage at the Device Under Test (DUT). This provides the most accurate values obtainable for both Voltage and Current at the DUT. Then, all you have to do is divide the Current into the Voltage to get the Resistance (or Impedance if you use an AC signal). This is especially important when you are trying to resolve ESR or Dissipation in a Capacitor or Resistance and Q in an inductor. Try some of the papers on the IET Labs website, they are much better at explaining Kelvin Probes (and why you can't just use the rel key on your multimeter).

[@drtidrow]

Remember, an ideal voltage measurement device should have infinite input impedance. Obviously we can't have that in the real world, but the input impedance of most meters these days is well into the megohm range, which means that the current through the sense wires will be very small (nanoamps or less), and hardly affected by the lead impedance, at least for DC to low audio frequencies.

[@ugetridofit]

Why can't we just use two wire, hold the probes together and zero out the lead length resistance first before we measure the resistor. Many meters have the ability to zero out cable length.

[@tomtektest8042]

That works to some degree if you are just measuring DC resistance. If you want to measure a resistor, capacitor, or inductor at AC, then you need Kelvin probes. Perhaps I will do a video on this in the future, but in the meantime you can go to the IET Labs (formerly GenRad) website for many good papers, including some on Kelvin probes that explain this better than I could. Look under the GenRad tab.

[@felixcat4346]

For those of us not living on Pluto, just press the rel button on your DMM.