Please educate me on these "thermal fuses". - Page 1
Forum section: electronics / repair

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: May 10, 2025, 05:27:35 pm
content: I'm working on a commercial power electronics board. I have 3 of them, supposedly idential. 2 boards work, 1 doesn't.The boards have a section marked "Thermal Fuses". The boards that work have a jumper across J1. The board that doesn't work is missing that jumper. R 26, slightly below J1 is different too. Just sloppy soldering or?U6 is a LM 2903 Dual CompatorIs the J1 jumper made of solder ? Did it melt ? How do I replace it ?What are the things that look like vias about ? Are they not plated through and if they get hot the solder melts out of them ? Or how do they work ?This board doesn't work. Note the lack of jumper J1.This board works.This board works.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2025, 05:58:59 pm
content: I don't see any thermal fuses at all. P10 looks like a connector footprint, that isn't used on any of these boards.J1 will be a copper wire link, never just solder. The pads on the non-working board look as though they've never been touched since they left the factory, and there's certainly nothing there which looks as though it could get hot enough to melt solder anyway. My guess is they're perhaps slightly different revisions, one of which doesn't require J1 to be fitted and the others do.Are they all otherwise identical? Did the non-working one ever work, or has it been dead from day one? If so, it might be as simple as J1 was never fitted, but should have been. However, more than likely it's something else - maybe seeing the whole board would help, as there's really not much to go on from just a few unused connector pads.: Q3, C17, R29 have gone walkies too. Maybe that part of the board really did get hot enough to melt solder and have components fall off? Are they stuck elsewhere on the PCB, or rattling about in the bottom of the box?

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: May 10, 2025, 06:59:26 pm
content: Quote from: AndyC_772 on May 10, 2025, 05:58:59 pmThanks for the reply !I agree.QuoteI agree with this too, thus it is a head scratcher.QuoteI believe so, but with high density SMD components, it is hard to tell.QuoteThe non working board worked properly for years.QuoteI see that. Seems unlikely but who knows. Maybe they weren't needed, just like J1. There is a voltage across J1.QuoteI got the board, nothing else.There is supposedly an SMD thermistor on the board. It isn't reading properly. There are over 200 resistors on the board. I can't find the thermistor. Fun times.I suspect the comparitor U6 has something to do with the issue. I'll test the inputs and outputs of it on a good board versus a bad board.

author: thephil
author_rank: Regular Contributor
timestamp: May 10, 2025, 07:29:32 pm
content: As LINUXGuy said: that looks like an unpopulated connector footprint. Probably meant to connect to a cable that goes to the thermal fuse located somewhere else in the device if populated. It seems that pin 1 and 3 have actual traces going to them (on this side of the board) and J1 is meant to bridge them in case no thermal fuse(s) is/are used.

author: MathWizard
author_rank: Super Contributor
timestamp: May 11, 2025, 03:46:41 am
content: Well what does the non-working board, not do ?? What powers it? Are there op-amp circuits not working, or some digital stuff not working ??

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: May 12, 2025, 04:11:28 pm
content: The microcontroller on the board sends a hardware error detected message. It is also displaying an abnormally low or out of bounds ambient temperature.The micro seems to run fine and send messages to the display interface fine. I suspect that one of the sensors on the board is reading incorrectly. Specifically, there is an ambient temperature sensor someone on the board that I cannot find. It appears to be reading the ambient temperature incorrectly. I suspect the temp sensor is a SM thermistor but there are ~ 200 resistors, all SMD, on the board so finding which one is the thermistor is difficult.I have no schematics. The manufacturer recommends to replace the board, at considerable cost.

author: Kean
author_rank: Unknown
timestamp: May 12, 2025, 05:31:38 pm
content: When you say "The non working board worked properly for years", what changed? Did it just suddenly stop working, or is it possible that someone did something like changing a setting, upgrading firmware, or swapping these PCBs between units to try solve another issue?It seems like that one PCB was built for a different use case, thus missing that jumper and at least a couple of other components. More comparison of the PCB and software/settings may be in order.