Weird problem with Mornsun power module - Page 1
Forum section: electronics / general

author: walter.martin3@btinternet
author_rank: Newbie
timestamp: September 26, 2025, 06:52:29 pm
content: I was wondering if anyone has enountered anything like this - I have been getting field returns of a product that contains a 12V mornsun power module (before they were on the naughty step). The really weird thing is the resistors that form part of the feedback circuit on the module although marked correctly as 8.6k are reading between 5 and 7k when removed from the module and checked on the bench ( they have also been cleaned aand baked in case of contamination but still read incorrectly) . This is causing the output voltage to be much higher than spec and causing product failures. I have never seen anything like this before were a surface mount resistor is either incorrect or had degraded and changed value. Usually they fail either open or short. There is no evidence of overstress and there is a lot of protection so I am left thinking that these resistors were either the wrong value when we received the modules or somehow degraded later. We only functionally test the product and it looks like it would still pass final test . Any suggestions or advide would be greatfully received

author: thm_w
author_rank: Super Contributor
timestamp: September 26, 2025, 08:52:49 pm
content: Have you tried to contact Mornsun? Maybe post some photos of the modules, is there a chance they were counterfeit?And what part are you talking about, the potted modules? You are sure it wasn't damaged while de-potting, if that is what you did.I've seen degraded SMDs but as you say its either clearly corroded, or burn marks are visible, etc.

author: wraper
author_rank: Unknown
timestamp: September 26, 2025, 09:00:02 pm
content: Resistors with decreased resistance are very rare but it does happen. Usually when they work under high voltage, but also possible from ESD. Basically shorter conductive paths are formed between conductive particles suspended in non-conductive medium. As this appears to be low voltage circuit with no stress, probably they've got some bad resistors.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 27, 2025, 02:21:47 pm
content: In ~15 years I've had two first-hand confirmed cases of resistors changing value on a board (as in, I know they were correct when first assembled on the board, and were then later confirmed to have changed significantly. And not due to an obvious reason like overheating to 500C). Never found a root cause, but it's an extremely freak occurrence. If you're seeing this happen suddenly in multiple modules, in the same component, then it's likely a quality issue on Mornsun's side.Might as well try opening a case with your distributor, or Mornsun if you bought direct. In the short term, sounds like you should revise your production tests to catch this problem.

author: walter.martin3@btinternet
author_rank: Newbie
timestamp: October 01, 2025, 05:18:44 pm
content: thanks everyone , I have attached an image of the module. In use it is encapsulated in polyeurathane potting. The markings on th FB resistors are 91B which should be 8.6k but some are measuing as low as 5k when removed from the module. As the feedback esistance drops, the output voltage rises until eventually teh module shuts down. I have tried contacting Mornsun in China but just getting standard response that they dont have any issues. It could be ESD so going to check that at a lab tomorrow. I improvised with a BBQ lighter but didnt see any changes. Another possibility was moisture trapped by the potting but even when teh resistors are removed and cleaned they still measure incorrectly. This has to be the weirdest problem I have encountered

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 01, 2025, 06:45:43 pm
content: I wonder if something like X-ray/CT would be able to see defects in the resistor element. Probably yes if it's a metal element, not sure about thin/thick film...