#237: 4 Wire Resistance Measurement | Kelvin connection

in today's short video we're going to take a look at the four wire resistance measurement also known as making a Kelvin connection it will talk about when and why you'd want to use the 4 wire connection versus the more common two wire resistance measurement but first it helps to understand how a DMM makes resistance measurements in the first place to understand the benefits of using a 4 wire connection digital multimeters make resistance measurements by forcing a known current through the resistor that you're testing and then measuring the voltage that appears at the terminals of the DMM and calculating the resistor using Ohm's law the amount of current that is forced through the resistor is a function of the range that the meter is in for very low current ranges like a 10 ohm range or a 1 ohm range if your meter supports such a low resistance measurement the current that gets forced could be several milliamps it might be you know one milliamp five milliamps 10 milliamps in that neighborhood this 2 wire measurement works perfectly well when the resistor you're testing is you know several ohms or more however when you're dealing with very little values less than a gnome or two there can be errors and inaccuracies that creep in using this two wire measurement let's take a look at why the probes that you use to connect to your resistor will have some resistance issues associated with them and it could be you know a few hundred millions or even an ohm or so per lead and then the contact resistance of where you make the probe contact to the resistor you're measuring that can also have some resistance associated with it if those resistors are on the same order of what you're trying to measure that can certainly lead to a lot of error because that same current is going to pass through all of that so the measured resistance is going to be the sum of the device you're testing plus the lead resistance and the contact resistance now to illustrate this let's just take a look at the probes coming from my fluke multimeter here if I simply short them together I see a reading of between 100 and 200 milli ohms sometimes going below 100 milli ohms sometimes going above and it really is a function of you know how clean these robes are how tightly I'm squeezing them together you'll see I can get down to about points zero six nine point zero seven ohms by really tightly squeezing them together if I just make a connection here I can see that I'm more like a between a hundred and two hundred billions so if I'm trying to make a very low resistance measurement like say for the shunt of a meter that I'm putting together that could be a significant source of error now many multimeters will have a math or Delta function that would allow you to zero out the measurement that you're getting just from the probes themselves and subtract that from the total reading but that doesn't fix the fact that you're going to get some variation due to the amount of pressure and the cleanliness of the probes themselves so it's only a kind of a partial fix and it can really affect the accuracy let's take a quick look at why so as we said using the Delta zero or null function doesn't do anything to remove the variability of the contact resistance it can also make the accuracy worse and here's why now these numbers don't really apply to my meter here but just using them here for argument's sake let's say that we have out a lead resistance of 0.7 ohms per lead the contact resistance making contact to the d-o-t is point two ohms on each end and the D UT we're trying to measure is 0.3 ohms and the accuracy the DMM is rated at point 1 plus or minus point one percent the resistance that we'd measure would be two times the lead resistance plus two times the contact resistance plus the D UT which would give us 2.1 ohms multiplied by the accuracy we're going to have plus or minus point zero zero two one ohms in terms of our accuracy tolerance but since the d UT is point three ohms even if we subtracted out the lead in contact resistance we doesn't affect the amount of error that's crept in because of the way we've multiplied tolerance against the total so we'd have 0.3 ohms plus or minus point zero zero two one which is an accuracy of plus or minus point seven percent seven times worse than the spec of the DMM now the route of these problems with the two wire measurement is that the voltage that's measured across the det includes the voltage dropped across the lead and contact resistors so the way to fix that is to use a 4 wire connection the 4 wire connection separates out the connection to the test current and the connections to read the voltage across the d UT now by doing this the lead resistance and contact resistance really drop out of the equation to a very large extent because it almost doesn't matter what this lead in contact resistance is anymore because even if we're dropping a lot of voltage across these guys with the test current we're measuring the voltage at the D u T so if we dropping a half a volt across here I don't really care because I'm measuring the voltage here also even though these leads and contact resistors are going to be about the same as the leads that's carrying the current the amount of current that flows back through the DMM is very very small on the order of nano amps or Pico amps so the voltage drop across the lead resistors lead resistance and the contact resistance is very very small and typically negligible so this 4 wire connection is very useful when you want to measure very low values of resistors typically a few ohms or less so quick example here let's say I'm trying to measure a diety resistance of 300 billion ohms or 0.3 ohms the meter is using a test current of 1 milliamp the bolt egde that's dropped across our D u T will be you know 300 micro volts or 0.3 milli volts and and then but the current that's going into the DMM which typically has a 10 mega ohm input impedance is only 30 Pico amps so that 30 Pico amps going through the lead resistors up here will lead to voltage drops in the nano volt range which will typically be very very small compared to the voltage being used to calculate what that resistor is so it's a very effective technique for measuring very low value resistors let's take a quick look at a measurement example we've got a piece of nichrome resistance wire here that has a resistance of a few hundred billions per inch put two bends in there so we can make some measurements between those bends be consisted about what we're doing here let's start by making a two-wire measurement so I'll put one probe in the notch on this side or bend in this side put the other probe in the bend on this side let's take a look at our measurement we can see that it's varying a little bit four hundred seventy five hundred five twenty-two pending on how much pressure I'm putting on these things and scrubbing it around on the probes I'm going to a lot variation in my contact resistance which is affecting the resistance measurement that I'm making and I've also you know not taking into account the probe lead resistance so let's take a look at making that same measurement with a four wire measurement I've connected up a second set of leads into the four wire sense inputs with some grabber Clips here and we'll clip on to wear those little bends are and the wire here one side and now the other that'll be our measurement here will tell to do a four wire measurement and then grab the probes and just connect up to really anywhere beyond where those sense clips are done and make our measurement here so now I can see it got a pretty consistent measurement even if I move these probes around if I disconnect and reconnect again I'm still seeing about that same 450 millions of resistance between those knotch connections because now the lead resistance and my contact resistance don't become part of the measurement and they were only measuring the voltage drop across our test section of the nichrome wire so that's really all there is to a four wire measurement is making a remote sensing or a Kelvin connection across the device under test so all you're measuring is the voltage drop across that with a given test current so it could it's a very effective way of minimizing errors in ensuring high accuracy when measuring low values of resistance so I hope you enjoyed this video learned a little something about the difference between a two wire resistance measurement and a four wire resistance measurement and when and why you'd want to use the 4 wire measurement for your low value resistance measurements if you liked this video give me a thumbs up if you haven't subscribed already please do so and tell your friends and thanks again for watching and we'll see you next time

Channel: w2aew

Views: 48,870

Rating: 4.9918232 out of 5

Keywords: W2AEW, Tek, Tektronix, Fluke, Keithley, DMM, ohmmeter, resistance, measurement, 2 wire, 4 wire, kelvin, kelvin connectin, kelvin measurement, current, voltage, ohms law, accuracy, low value resistance, shunt resistor

Id: kqkluABmJhQ

Length: 9min 4sec (544 seconds)

Published: Sat May 14 2016

Reddit Comments

[u/kc4umo]

Very nice explanation as always.