# Electronics question - current sensing resistors **URL:** \ **Category:** Factual Questions\ **Created:** [August 4, 2021, 6:44am UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012 "2021-08-04T06:44:03Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 5, 2021, 12:51am UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/21 "2021-08-05T00:51:20Z")
> [@Melbourne](#): > > What we needed was / is a small low-cost mass-produced 100A surface-mount current sensor at 1%. The difficult parts were heat dissipation and thermal offset. We use [these guys](https://www.mouser.com/datasheet/2/54/css4j-4026-1862184.pdf) where I work. Cheap, can easily do 100s of amps, 1% accuracy, no heat problems, has 4 terminals built in. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [August 5, 2021, 1:54am UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/22 "2021-08-05T01:54:53Z")
> [@Dr.Strangelove](#): > > We use [these guys](https://www.mouser.com/datasheet/2/54/css4j-4026-1862184.pdf) where I work. Cheap, can easily do 100s of amps, 1% accuracy, no heat problems, has 4 terminals built in. We used to use that kind of material, but the voltage offset (‘thermal EMF’) of that material is too great. From having the two ends at different temperatures. From track heating … ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 5, 2021, 2:32am UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/23 "2021-08-05T02:32:16Z")
> [@Crafter\_Man](#): > > So if you use a 0.1 Ω shunt, and the voltmeter measures 0.11 V, then all it has to do is shift the decimal point over to the right one place (1.1 V) and then display an A instead of a V (1.1 A). This is how a lot of those cheap current displays work: the decimal place will shift based on the location of a jumper. There’s no separate voltmeter as such. The CSR’s feed straight back to the dedicated chip that runs the display, through a multiplexer. But I bet you are right about the reason they’ve used a CSR that is a multiple of ten. And they probably figured 0R01 was too low. > [@beowulff](#): > > If you can tolerate a 240mV drop, then you can get away with an 8 bit ADC with 1.25v reference, and be able to measure to about 5mV, or 50mA of USB current. The integrated ADC and reference in a micro-controller is sufficient. But as I’ve said the display only reads to the nearest 100mA anyway. So with only (say) a 120mV drop as you would get from a 0R05 CSR you would still have way more resolution from an 8 bit ADC than required. Heck, even if the display could cope with from 0-10A (which is more than the unit can output) that would only be 100 values. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 5, 2021, 10:27pm UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/24 "2021-08-05T22:27:40Z")
> [@Melbourne](#): > > From having the two ends at different temperatures. From track heating … Curious. Well, you know your application better than I do. I’m a little surprised that the difference in temperature could be high enough to be relevant. Aren’t thermal EMF coefficients in the realm of microvolts /K (whereas the voltage drop across the shunt should be in millivolts)? Seems like not a big deal unless you’ve got like a 100 C difference. You can’t use a ground plane or something to smooth out the temperature difference? ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [August 5, 2021, 10:30pm UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/25 "2021-08-05T22:30:07Z")
> [@Dr.Strangelove](#): > > You can’t use a ground plane or something to smooth out the temperature difference? I’ve always wondered about that. I’d like to test the same device with a 4 layer board, but that’s not my decision. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 5, 2021, 10:37pm UTC](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012/26 "2021-08-05T22:37:33Z")
Ahh, yeah, you’ve got fewer options on a 2-layer board. Still… I can imagine some solutions, like tightly interwoven “combs” of traces. They can be at a bare minimum spacing, since the voltage difference is tiny. Might even be possible to glue a heatspreader on top if you’re confident in the soldermask layer. [Previous page](https://boards.straightdope.com/t/electronics-question-current-sensing-resistors/948012.md?page=1)