Strange BM235 behavior with diode reading - Page 1
Forum section: electronics / testgear

author: electrozap
author_rank: Contributor
timestamp: June 29, 2021, 04:54:03 pm
content: Hello!I'm having a strange behavior with my BM235, and I'm wondering if anyone would have insight on why it would be happening.I have a circuit with mosfets and diodes in parallel, and they're all facing the same direction. On the BM235, when I test the circuit, I'm getting 0.424V in one direction and 2.552V in the other direction. This lead me to try to test the board with a thermal camera and current going through the board thinking that it was a busted diode (or body diode), and I wanted to see which one it was, but it didn't allow any current through.After testing the board and getting nothing, I tried different multimeters to see if the same thing happened on other meters. On the Fluke 87V from work, it showed an open circuit. On my Sanwa PM300, it's showing open circuit, and on my AMECaL ST-9927T, it's also showing an open circuit.I'm seeing that the Fluke 87V seems to test at 1mA at 3V (I could be wrong about this one), the BM235 seems to test at .3mA at 3V, and the AMECaL is at 2.7V (maybe 2V) based on the FS9922-DMM4 datasheet. It didn't list a current - just that it should have a 1K resistor. The Sanwa advertises a 3.2V open circuit voltage.For fun, I also tested the voltage that the AMECaL was testing at to see if it wasn't as high. It was measuring at 2.6V, which is above the 2.552V, but not by much. The Sanwa was measuring at 2.987V. I'm not at work at the moment, so I can't measure the Fluke 87V.Is there something different about the way that the BM235 measures diodes that it's picking up on something? Is it a faulty device? I tried testing a new board that didn't have the same potential for failure, and the BM235 and other meters had the same behavior. edit for clarity: the BM235 had the same weird behavior. They didn't all do the same thing on the other board.Thanks!

author: AVGresponding
author_rank: Super Contributor
timestamp: June 30, 2021, 09:13:54 am
content: Have you tried comparing the meters with other measurements such as high resistances? I'd be leaning towards some kind of contamination on the meter PCB/lead sockets that's allowing a tiny current to flow and screwing with the readings.

author: electrozap
author_rank: Contributor
timestamp: September 08, 2021, 01:38:31 pm
content: Sorry for not getting back to you in a while. On a 1Meg resistor, I'm seeing 1.004Meg on the Fluke 87V at work and 1.004Meg on the BM235. For some reason, it seems that the BM235 starts at 1.008 and works its way down, but the Fluke goes straight to 1.004.If it was a leakage current, is there anything that I could work on with it to fix the issue?Thank you!Edit: for more detail, the meter doesn't do this on all diodes. It only seems to do it on this one circuit for some reason. It doesn't even do the same thing when testing the opposing fets.

author: AVGresponding
author_rank: Super Contributor
timestamp: September 08, 2021, 05:49:25 pm
content: Doesn't look like contamination around the lead sockets or the resistance reading would likely be quite a bit off at 1MΩ.Certain types of semiconductor junctions can be quite leaky when reverse biased, this thread might give you some helpful pointers, as it seems a similar issue, potentially:https://www.eevblog.com/forum/testgear/eevblog-bm786-multimeter-bug/