What resistance is a 0m30 shunt resistor? - Page 1
Forum section: electronics / projects

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 03, 2023, 07:01:03 pm
content: Hello, quick question here!I am designing a current sense circuit and referencing other designs. I keep seeing shunts marked 0m30, what resistance is this? I have had trouble finding resources to figure out what code this uses. My guess is that this is 3 miliohms, but I am not 100% sure.Thanks!

author: Dave
author_rank: Super Contributor
timestamp: June 03, 2023, 07:55:53 pm
content: 0.3 milliohms or 300 microohms.You will often find the unit prefix used in place of the decimal point in electronics, be it on schematic diagrams, PCB silkscreens, or on the components themselves.Supposedly it originates from the time when schematic sheets were hand drawn and copied back and forth. Degradation of image quality meant that decimal points had a tendency of disappearing, so the simple solution was to avoid using them entirely.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 03, 2023, 08:53:06 pm
content: Cool, Thanks!

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 03, 2023, 09:28:28 pm
content: Can you really handle 300uOhm accurately?I mean, even the solder will affect the value at such low value.

author: langwadt
author_rank: Super Contributor
timestamp: June 03, 2023, 09:33:26 pm
content: Quote from: DavidAlfa on June 03, 2023, 09:28:28 pmat such low values, hopefully, the resistor and footprint has a kelvin connection for the sense

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2023, 09:39:47 pm
content: Quote from: DavidAlfa on June 03, 2023, 09:28:28 pmMost of the applications for such low value resistors are as current sensing shunts. The accuracy of the resistance is frequently not that important, as there will be a calibration performed after assembly. Many times such low resistances will be 4 terminal, but 2 terminal ones are very common, as they are cheaper. The key issue is not solder, but copper in the sensed path. Its awful temp coeff (0.4%/C) ruins any calibrated accuracy over even a modest temperature range.

author: Benta
author_rank: Super Contributor
timestamp: June 03, 2023, 09:44:32 pm
content: Is this for a 1000 A current shunt?

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2023, 09:48:31 pm
content: Quote from: Benta on June 03, 2023, 09:44:32 pmYou would generally use a much lower resistance for 1000A. 300 micro-ohms will dissipate 300W.

author: langwadt
author_rank: Super Contributor
timestamp: June 03, 2023, 10:09:12 pm
content: Quote from: coppice on June 03, 2023, 09:39:47 pmmost metals are roughly 0.4%/C

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2023, 10:25:29 pm
content: Quote from: coppice on June 03, 2023, 09:48:31 pmI would use a Hall effect sensor for 1000A.300µOhm is fine for up to 100A or so. The source impedance is low, so it doesn't matter if the amplifier has a low input impedance and relatively high bias currents, making low offset easier to design, or use a chopper amplifier.

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2023, 11:08:33 pm
content: Quote from: langwadt on June 03, 2023, 10:09:12 pmTrue, but follow the guidelines inhttps://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.htmland the metals with a horrible coefficient can have a minimal effect. The contacts on the resistor itself should be made of a material with a good coefficient.