SMART-UPS 1000 Front panel overheating temperature 80C

Hello, I have a thermal imager. I have checked the front panel of the SMART-UPS 1000 (1999). The temperature was about 65-70 degrees. It turns out that the heat comes from the resistors. Cheap resistors with a power of 1/4 W.

I decided to change to 1W, to my amazement I had to change some of them to 2W, but the heat is still on!

Here is a photo after the upgrade.

PCB front




PCB back



Front Panel



Change them to 5W? Anyone else with this problem.

Here is schematic attached to this post, the overheating resistors are circled in red.
Full schematic also attached

Resistors can run over 100C easily, even more, with no damage to them. Some are rated for close to 200C actually.
Changing to 5W may not change the thermal image, it will still dissipate the same heat, and eventually become just as hot.
Bottom line, resistors are designed to run hot and as long as the resistor wattage rating is not exceeded, you should be ok. I have one device where I limit the current via a pair of 10W resistors (20W total dissipation) that dissipate about 3W total, and they run up to 102C. I had a pair of 5W as well, (identical ohms) and they also ran at around 102C, maybe a little bit more... So, changing to higher wattage resistors did not make much difference. Final temperature depends on how well the resistors can dissipate the heat. Higher wattage resistors still dissipate the same heat (in general), their rating is only a maximum wattage they can handle, not the maximum heat generated.

I'd say something is failing other than the resistors, to cause things to overheat.

I have a couple of SmartUPS 1500's here with the same panel layout and they run stone cold.

rjenkinsgb said:

I'd say something is failing other than the resistors, to cause things to overheat.

I have a couple of SmartUPS 1500's here with the same panel layout and they run stone cold.

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The UPS works fine, no errors. No errors in managemnt software too. What could be failing, electrolytic caps?

No idea, sorry - I've never had cause to strip one beyond replacing the batteries every few years..
The only thing I can say as a fact is that mine does not run hot; this is with the ambient temperature 23 or so.
It runs my main PC and everything on my desk/bench.

smilem said:

The UPS works fine, no errors. No errors in managemnt software too. What could be failing, electrolytic caps?

Click to expand...

More likely the failing is pointing a thermal imaging camera at it for no reason - unless you have another exactly the same to compare it to then you're not really proving anything.

rjenkinsgb said:

No idea, sorry - I've never had cause to strip one beyond replacing the batteries every few years..
The only thing I can say as a fact is that mine does not run hot; this is with the ambient temperature 23 or so.
It runs my main PC and everything on my desk/bench.

View attachment 132964

Click to expand...

Thank you for photo, can you pop front panel and take reading from PCB?