What is a Kelvin connection?
A Kelvin connection is a means of making electrical potential contact with a current carrying component in such a way that eliminates or greatly reduces the effect of contact resistance. This is especially important when dealing with low millivolt measurements as in shunt resistors where contact resistance is a significant and unknown variable. The best means is to use a (4) terminal device in which two terminals conduct the high current, while the other two are used expressly for measuring voltage. When working with two terminal devices, clip leads are sometimes connected as close as possible to the resistance element for measuring the potential—these are referred to as “Kelvin clips.” In this case, clip leads are used for making temporary measurements, but a two lead device may be converted to a (4) lead device simply by replacing the clips with soldered leads.
A milliohmmeter is a classic instrument that uses the (4) lead Kelvin connection. Check out this link:
Power supply voltage sense
To eliminate the effect of lead resistance in high current power supplies that causes voltage droop, separate sense voltage terminals connect directly to the external load for the purpose of voltage feedback. Again, we have (4) terminals; (2) for power and (2) for sense. This is a common feature in high current, low voltage power supplies. The sense leads conduct very low current (generally in the uA range) so the sense lead resistance does not add significant voltage error.
How about batteries?
Since my next article requires accurate measurement of battery terminal voltage, I searched electrocircuits to see if there was an article on the Kelvin connection. The search came up blank so I am now writing this article as a prerequisite. Yes, battery voltage may also be best measured via the Kelvin connection.
What about circuit boards?
Circuit boards may suffer from the same problem if the components are not selected properly and /or the layout is faulty. In the case of low resistance /high current resistors, there are numerous (4) lead through-hole resistors available. There seem to be no SMD (4) lead resistors available, but this is not a serious issue because SMD and thru-hole devices are often mixed in circuit board design.—few circuit boards use 100% SMD components.
Circuit board layout
This is a PCB that I cooked up for sake of argument. It demonstrates how to connect a differential amplifier on the right side of the board to a high current shunt resistor on the left side. I could have used a (4) terminal resistor, but I wanted to show the best means of connecting to a (2) terminal resistor. First note how the narrow sense traces run separate from the wide power traces—if connected directly to the power trace on the right side of the board, the voltage developed across the power trace resistance would add error—this is also one example of a “ground loop.” Also, note how connection is made on the “neutral” side of the pad under the component—in this way, the resistance of the pad itself does not introduce error because minimal current flows in this area of the pad.
Out of my tool box
Beat up 0.001Ω. 50A, 50mV meter shunt
0.01Ω Shunt resistor with hook clips connected
Examples of (4) lead resistors available from DigiKey
Stackpole resistor TCB3FB10L0-ND
Vishay /Dale resistor MRS129801-ND
Vishay /Dale resistor WSMSB-.0004-ND
Murata meter shunt 811-1096-ND
For the future
Running engine battery voltage detector /automatic power switch
Undocumented words and idioms (for our ESL friends)
Beat up: idiomatic expression—well worn, well used, old, perhaps mishandled