Current sensing (ref Chargerlab POWER-Z KM001) with incredible specs :-) - Page 1
Forum section: electronics / projects

author: dav888
author_rank: Regular Contributor
timestamp: March 24, 2021, 09:02:42 pm
content: Ran across the spec for this usb-c power-meter (POWER-Z KM001)I can't see that specification to be even close to true, anyone that happen to own one that can help verify it?Quoteit also says 28mOhms series-resistance, which would be a reasonable shunt for 1A+but you wont get 0.0001A precision with that.found this picture, 10milliohm sense + what looks like a TI INA226https://lygte-info.dk/pic/USB%20meter/ChargerLab%20PowerZ%20KM001C/DSC_5618a.jpgI'd expect to see 10%+ errors in anything under 5mA unless the bottom-side of that board is filled with analog instrumentation amplifiers

author: thm_w
author_rank: Super Contributor
timestamp: March 24, 2021, 09:21:03 pm
content: Its just resolution not accuracy, the INA226 has a 16-bit ADC so its going to give a lot of digits.If the max current is 5A then that give about 100uA per bit of resolution.https://www.ti.com/lit/ds/symlink/ina226.pdfThey are pulling the 0.1% straight from the INA226 gain error, but there are other errors to be accounted for as well as you note. Given the expected use of this device, I wouldn't be too concerned with its actual accuracy down past 10mA or so.

author: dav888
author_rank: Regular Contributor
timestamp: March 24, 2021, 10:03:00 pm
content: exactly! I'm on a hunt for the most compact (least components) way of doing somewhat accurate +/-5uA measurements on a rail with a potential current of up to 1.5A.there are a bunch of implementations out there, like zs1100a, Rocketlogger, battlab, nordics PPK2 kit. some uses a more software driven approach, while some rely on analog wizardry. in my case I could have a hard switch and choose the range, i.e. 0-10mA (or 100mA) vs 100mA to 1.5A. One idea would be to use the INA-series alert output, configured to assert when there is an Overflow (on I guess the busvoltage) to control a fet to engage another shunt resistor. Problem with just using one FET to bypass the higher-ohm res is that you have to deal with RDson that is not linear to current over it.anyone with expertise in this field?

author: thm_w
author_rank: Super Contributor
timestamp: March 24, 2021, 11:09:38 pm
content: You can find some relevant threads here, eg:https://www.eevblog.com/forum/beginners/switching-between-current-shunts-multiplexer/https://www.eevblog.com/forum/testgear/designing-a-simpler-current-ranger/Is it possible to measure across the shunt resistor ignoring RDSon? Or just get a really low resistance FET. Or the usual, calibrate it like crazy option.

author: dav888
author_rank: Regular Contributor
timestamp: March 25, 2021, 12:01:08 am
content: thx! will check those links.really prefer something that can self-calibrate

author: shaling
author_rank: Contributor
timestamp: March 28, 2021, 10:28:00 am
content: Maybe of interest,https://www.eevblog.com/forum/projects/multi-decade-dynamic-auto-switching-digital-ammeter/uses MOSFETs to switch shunts.

author: dav888
author_rank: Regular Contributor
timestamp: March 28, 2021, 04:27:41 pm
content: checking!