Kelvin connected resistor footprint - Page 1
Forum section: electronics / projects

author: AlfBaz
author_rank: Super Contributor
timestamp: March 28, 2014, 09:10:47 am
content: I'm looking at this shunt resistor herehttp://australia.rs-online.com/web/p/surface-mount-fixed-resistors/7637205/Pretty high spec, 5mOhm, ±0.1% tolerance, ±15ppm TC, 3WAs can be seen the kelvin connections are part of the main connections as would be expected.The suggested foot print in the data sheet shows 4 separate, isolated pads.This brings me to the question, what are we gaining with the notches in the resistor and what if I just created a foot print with one lone pad either side.Are the notches some form of thermal barrier in an attempt to reduce thermal emf issues or is there something else at play here

author: ve7xen
author_rank: Super Contributor
timestamp: March 28, 2014, 09:37:52 am
content: My naive guess would be that they're for isolating the solder joints easily, so that the load current isn't flowing through the solder junction for the sense terminals. I guess it's negligible, but you don't want to be measuring the solder joint resistance too.Edit: In other words, the black bit is the 5mOhm part. If not for the notches, currents may flow in the silver leads or solder, which would drop additional voltage that you'd see at the sense terminals. If they're not filled in, the notches should force the current across the resistive element. I guess.

author: Rerouter
author_rank: Super Contributor
timestamp: March 28, 2014, 09:39:28 am
content: Kelvin connections work by preventing any voltage drop occurring in the solder joint, so by making it one pad per side you stop any benefits from it being a kelvin connected resistor,

author: fcb
author_rank: Super Contributor
timestamp: March 28, 2014, 09:45:28 am
content: That's quite a good question AlfBaz.I think you can divide the development of these resistors into two areas. (a) the low TC compound that forms the resistor and (b) the packaging.The packaging allows the manufacturer to guarantee the specification without worrying too much about how the customer implements the layout, also the notch makes the part less susceptible to variation due to wonky placement.If you could buy a similar TC two wire resistor with that form factor, then I guess you could implement the same thing with a four pad layout on the board (you would probably need a blob of adhesive to prevent unwanted self centering).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2014, 11:18:20 am
content: If you use just two pads... how do you know that you've put your kelvin traces on the correct sides? Should they be lined up in the middle? Inside down the center? Outside at the edges? Inner or outer corner?Those are the kinds of questions it avoids.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: March 28, 2014, 01:27:06 pm
content: Because if you dont define a voltage measuring point then its upto whoever does the footprint to do it right... and that can make a surprising amount of difference.http://www.analog.com/library/analogdialogue/archives/46-06/shunt_resistors.htmlObviously a worst case scenario with high current density, and low resistance shunt to make stuff misbehave more, but it makes the point.

author: robrenz
author_rank: Super Contributor
timestamp: March 28, 2014, 01:50:13 pm
content: IMO Other than the obvious of virtually eliminating the effects of voltage drop caused by sense lead resistance (assuming avery high impedancevoltage measurement) a Kelvin connection on a resistor is a means tocontrol the exact pick off pointsto measure the voltage drop across. When you get down to these levels (5mOhm) if it were a 2 lead resistor and you used Kelvin connections on the leads just a few thousandths of an inch change in position on the leads could exceed the entire tolerance. No one could agree on what the resistor value was because it isconnection location dependenteven though you are using a kelvin connection on the leads. By controlling the voltage pick off points in theconstruction of the resistorand supplying separate sense connections, all voltage drop variation because of measurement location is gone.If you put the sense and source legs of that resistor on the same pads you have completely destroyed any hope of achieving the claimed accuracy.Edit: The exact shape of the PCB footprint is not a factor in the precision. All that matters is that you have source and sense leadsseparatedand connected to the proper component legs and your sense voltage measurement is very high impedance and/or very low resistance traces from the sense connections.

author: georges80
author_rank: Frequent Contributor
timestamp: March 28, 2014, 03:46:04 pm
content: As most have written above, the isolated pads will allow sensing of the voltage directly at the resistor tabs without error introduced by current flow and solder resistance (assuming high impedance voltage sensing). With dedicated pads, the high current path will ONLY go through the large tabs of the resistor to their own dedicated pads. The manufacturer can then design the sense tabs and how they connect to the rest of the resistor structure and characterise them based on the knowledge that only (extremely low) sense current will flow through the sense tabs. PRESUMABLY the manufacturer of that resistor provides a recommended footprint, I highly recommend you follow that recommendation if you wish to obtain the documented accuracy of the part.There's an art to kelvin sensing resistors, especially ones that don't have isolated tabs. Many papers have been written. Here is just one found via google:http://www.analog.com/library/analogdialogue/archives/46-06/shunt_resistors.htmlI use current sense resistors (2 and sometimes 3 depending on topology) on my LED drivers. Typically at 3 to 10A so resistors in the 0.015 range down to 0.004. Where the sense lead comes off the pad and whether the pad is 'designed' to support kelvin sensing makes a large difference in sensing accuracy.cheers,george.

author: nctnico
author_rank: Super Contributor
timestamp: March 28, 2014, 08:30:57 pm
content: I like that Analog devices appnote.I have been investigating power sensing resistors for a potential upcoming project. The manufacturer's datasheets suggest to use 2 sense traces from the middle of the pads. That would include the resistance from the solder joints. It is not easy to sense large currents with better than 1% accuracy. A larger value resistor will get hot so the temperature coefficient will bite you.

author: AlfBaz
author_rank: Super Contributor
timestamp: March 30, 2014, 12:54:40 am
content: Thanks for the explanations and links guysQuote from: nctnico on March 28, 2014, 08:30:57 pmI don't have any links on hand but LT have some current sensing amplifiers and related appnotes about utilising low offset, low drift amplifies to either increase the "dynamic range" (ie 1mA to 200A) or limiting the range in favour of low loss sense resistors