USB tester with high-side sensing? - Page 1
Forum section: electronics / testgear

author: tinkerer42
author_rank: Newbie
timestamp: November 01, 2024, 06:58:31 pm
content: Hi,It seems most cheap and common USB testers that display current and voltage, measure the current on the ground return. This leads to incorrect readings if there is more than one ground path to the device under test - quite a common scenario for me, as other test equipment is often connected and is grounded either directly (scope) or via my PC (USB-based logic analyzer).I'm looking for a USB tester that is known to do high-side sensing (i.e. measure the current on the +5V line) rather than low side. Do you know of any?p.s.if you also know of a USB tester with an output on/off switch feature, that would be great too :-) it would save me physically disconnecting and reconnecting the USB cable every time I want to power-cycle the device under test.Thanks!

author: coromonadalix
author_rank: Super Contributor
timestamp: November 01, 2024, 07:30:16 pm
content: normally they probe the voltage pass the shunt resistor to avoid false readings due to shut / voltage lossi would not be worried, they are not 4 5 6 digits meters with 0.0035% precisionno offence i think you worry too muchi use a few of themtake a mcu like duinos and an Allegro Acs xxx hall effect current sensor and do it on a positive side ??just use a powered usb hub with power on/off switches, i use an 7 ports hub w on / off switches at my job for such measurements

author: tinkerer42
author_rank: Newbie
timestamp: November 02, 2024, 12:07:04 am
content: coromonadalix, it's not an issue of worrying. it's an actual phenomenon that happens on my desk right now, with a cheap USB tester. The moment I attach another grounded cable or instrument to the PCB I'm working on (in addition to the USB that's powering it via the tester), the current reading on the tester drops by anywhere from 20% to 50%. that makes sense too, because the return current now has two paths, some of it goes via the tester and gets measured, and some via the other ground path and isn't. that's why high-side sensing is important.As for a homebrew solution, I can definitely design one, I design such circuits all the time in my professional work. I just want something off-the-shelf that works correctly out of the box in this use-case, without setting something custom up.

author: tinkerer42
author_rank: Newbie
timestamp: November 02, 2024, 12:11:23 am
content: (and by the way, if I did it with an ACSxxx sensor which is hall-effect based, it wouldn't matter which side it was measuring because it is galvanically isolated from the measured circuit. but in my experience these hall effect types are too inaccurate once you get down to 10's of mA level and below, and therefore the USB testers do use shunt-based sensing, and for these it does matter if it's high-side or low-side)

author: nctnico
author_rank: Super Contributor
timestamp: November 02, 2024, 12:58:52 am
content: I find it rather odd people would design a USB device without the ground connected together solidly. Signal integrity and so on.Anyway, I tested the JT-TC66C and Fnirsi FNB58 I have here and both do high-side sensing. I tested with PSU and load. When I make a direct connection from 0V to 0V (PSU and load), the current reading stays the same. When I connect 5V to 5V, the current reading drops to 0. To me this indicates high side current sensing is used. Neither have a power switch though.

author: tinkerer42
author_rank: Newbie
timestamp: November 02, 2024, 01:04:52 am
content: thanks for testing it nctnico, that's exactly the info I was looking for!I'll give one of these a try.