USB Test Gizmo

[WaveyDipole]

I recently purchased a couple of these cheap USB testers. I saw them being used on a couple of YouTube videos and thought htey might be useful. For the price I did not expect them to be accurate. Although the voltage reading seem to be correct to within 0.1v or so, the current reading for a plugged in device is very inaccurate. Both behave in pretty much the same way and do not show a current reading for any load much below 200mA. According to my PSU (which is accurate against my 4.5 digit DMM to within one count) the LED array draws 58mA and the SDR radio 98mA, yet both show a reading of 0.00A on both of these testers. The dummy load is rated 1A (reads 5.004ohms), but shows a reading of only 0.88mA on one gizmo and 0.82 on the other. I would have thought is should be within a count or two (I could live with maybe 4 or 5) of 1.00A.

Is this quite usual for these testers?
Why are they so inaccurate on current readings?
Why do they sometimes show no current reading at all?

(Please note that on this charger being used, the top port seems to have a slightly higher voltage reading than the remaining ones).

[frogblender]

Yeah, they suck.

As you've found, the current measurement below about 500mA is horrible.  The exact reason escapes me (dang brain)... but it is a common issue with "low-side" resistor current sensing - basically, at low currents, the opamp's inputs and output are all ground or very near ground.  And the mfg was too stingy to create a negative supply rail, so the opamp's negative power pin is ground too.   And the opamp is pole-axed by it's finite common mode input range.
Once the current rises, the sense resistor voltage rises, and the opamp inputs and output rise away from the ground pin.... and all is good.

[phil from seattle]

I have one of those (around here somewhere) that I bought from AdaFruit a while ago. It only shows 2 decimal places on the current but within those bounds, it appears to correct. Voltage is pretty close to my other DMMs.

[paulca]

I use the little grey one with the oLed for "indicative" readings.  I'm not really interested in accuracy or characterising, just "How much is the device pulling, roughly"