What is so special about current-shunt resistors (in non-4-terminal packages) - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 04, 2021, 05:44:46 pm
content: So for a board I'm making I need to measure current. In order to do so I want to use a series 1 ohm resistor across which I measure the voltage.I was looking on farnell/mouser, and under 0603 current sense resistors, the cheapest 0.1% resistors are 10 bucks a piece (in a regular 0603 package, so not special 4-terminal package). If I just look at the 'regular' selection, I can get 0.1% resistors for 40 cents a piece. Both are thin film, both have tempco's <25ppm/K. What is the difference? I would just take the regular chip-smd-resistor, but am kinda worried I'm missing something obvious here.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 04, 2021, 06:00:38 pm
content: Don't know exactly what kind of resistor you are talking about but current sense resistors are made of special material with low tempco over a current range that is not normally offered for thin film. It's all about the metal.Can you post link to the DS?

author: ajb
author_rank: Super Contributor
timestamp: June 04, 2021, 09:18:24 pm
content: I just took a look and don't see "current sense" resistors offered in 0603 and 0.1%, so hard to say what you're looking at. Presumably the categories at the distributor are just based on which parts the manufacturers choose to sell as "current sense" resistors versus regular generic chip resistors. Even given the same resistance, tolerance, and tempco there may be more subtle differences. Maybe they use materials that perform better when operated at higher temperatures over long periods of time or something, or they use a different trimming pattern. Even if those things don't increase the marginal unit cost very much, the number of current sense resistors that are out there is a very small fraction of the total number of chip resistors, so just economies of scale and NRE recovery and having desirable features on a specialty part will mean a higher price for specialized parts. It's also possible that manufacturers make all of their current sense resistors to a tighter spec or to meet specific qualifications, because the number sold doesn't support multiple variants like it does with the commodity chip resistors, so they just make everything in that specialty line to the higher spec.Maybe looking at the brochure for a current sense resistor line will provide some hits about what's different.

author: Analog4
author_rank: Contributor
timestamp: June 05, 2021, 05:59:25 am
content: There are a lot reasonable priced SMD current sense resistors in 0603 with 0.5% tolerance. Calibration to a finer tolerance would be difficult, since solder termination might be dominant. Automated testing is cheaper than hand testing to tighter tolerance.

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 09:46:33 am
content: Read all the specs carefully. It may be that they are just milking it. For me a current sense resistor is usually something that will do over 1A, is low resistance and will be quite accurate and stable. For larger currents this is a specific resistor but at low currents I am not sure it makes any difference. I have noticed that typically low value resisters end up in the current sense category as what else do you use such a low value for? It could be that achieving the low value resistance is difficult and this makes them expensive but some manufacturers have found a way to make them cheaper at low values and are passing the savings on while others keep milking it or are still using an older more expensive process as changing is not worth it for what the cost of the new process is given the lower volumes.