Different readings when testing a diode on-board and off-board

Thread Starter

motaha

Joined Jan 24, 2024
4
Hello,

I have a two identical PCBs (one faulty and one non-faulty). The PCB has a buck converter (AOZ1212) to convert 24V to 4.5V. This converter uses a Schottky diode as flyback/freewheeling diode. When I was debugging the faulty board, this diode read 0.2V in forward bias, and 2.3V in reverse instead of OL, using and digital multimeter. On the working board the diode read 0.2V in forward bias, and OL in reverse (these measurements were taken when the power was off and the diode was soldered to the PCB). I then unsoldered the diode from the faulty PCB to test it off-board, and it read 0.2V in forward bias, and OL in reverse. The question now is why am I getting different readings from on-board and off-board, maybe something is skewing the readings when testing on-board, but why isn't that same something skewing the on-board results of the non-faulty board?

Note: the output of both buck converters (from both boards) is the same, 4.6V. I want to know what might be causing this difference in readings, maybe that will help me identify what is going on with the faulty PCB.

Any ideas?

Thanks
 

MrChips

Joined Oct 2, 2009
30,984
Your observations could possibly suggest that maybe there is nothing wrong with the diode and maybe something else on the board is showing this anomaly.
 

Thread Starter

motaha

Joined Jan 24, 2024
4
Yes, I am having some trouble figuring out what might be the root cause. Any ideas on where to start? Did you see something like this before, maybe that might give me some leads?
 

Thao Vu

Joined Apr 27, 2017
22
It might be caused by the leakage current capability of the diode. I guess the diode on the faulty board may be defective and have higher leakage current when you measure reversely. Let's try to install new diode and see the result.
 

MisterBill2

Joined Jan 23, 2018
19,042
When any component on any circuit board is attached to the rest of the board, the resistance across that component is the parallel resistance that includes the rest of the components. That is rather inescapable.
So the difference in readings on the two circuit boards points toward a fault of some other component connected to the diode.
 
Top