BM235 - current measuring oddness

[Matt-]

Hi!

I've been a happy user of my EEVBlog BM235 for a few years, but have recently run into an odd problem.

Trying to measure current in the mA range does not allow the target board (a TinyPico) to boot.  If I move the lead to the relevant jack for the Amperage range, it works fine, but the resolution is poor (as you'd expect). 

I broke out my backup meter, a UNI-T UT210E, and it's showing 2.4Ω across the probes when on mA, and 0Ω when across the A jack. 

Is this typical, is it likely the cause of my issue, and can anyone suggest how to debug?

Many thanks in advance.

[AVGresponding]

Meters like this measure current by measuring the voltage drop across a known resistance. For lower currents you need a higher resistance, which introduces a higher "burden voltage", effectively lowering the voltage available to the DUT (device under test).

This can result in the voltage being too low for the DUT to work properly. Dave has a video on this (split into two parts):






EDIT: You can try to find yourself a solution if you have access to a precision resistor, say 1R 1% (or better, preferably). Connect the 1R in series between your psu and the DUT, and measure the voltage across it with the mV range on your BM235.
It'll read 1mV per 1mA, approximately. Note that both the tolerance of the resistor and the accuracy of the meter need to be allowed for, and if the current is high enough it'll heat the resistor and skew the reading more.

[Matt-]

Awesome advice.  Thanks so much, @AVGresponding