Hi,
I powered my hexS with its supplied PSU. Now I changed it to PoE from another switch and saw it got +20°C hotter:
Is this a normal behaviour? The hexS is powered by PoE now, it does not power other devices!
It looks like DC-DC regulator working from 48V (or there around, which is likely PoE voltage thrown at hEX S) is much less effective than working from 24V (included power adapter output). And I guess (I don’t have the device to check myself) that temperature sensor is placed close to that DC-DC regulator. I’m pretty sure that the rest of components work with way lower voltages (typically ICs work with something between 2V and 5V).
Yep, but 20 degrees?
At which voltage are you powering the hexS:
- when using the jack
- when using the PoE
?
The hexS power supply is 24V.
It accepts 12-57V as PoE In, if you are using the (normal) 48V, it could be the voltage regulator/converter that creates the heat, but it doesn’t sound right (to me) that the overall temperature or the CPU one increases so much, 20° are a lot.
Maybe it is some kind of false reading or the sensor is over-influenced due to its position.
65° is not “preoccupying”, but it is right on the border of “temperatures I won’t run my electronics at for long periods, if I can avoid it”.
The difference should be detectable by touching the device case (from “mild warm” to “slightly hot”), I would check (if you have one) the outer temperatures with an infrared thermometer.
Another test you can make (again if you can) could be raising the voltage on the jack (it accepts 12-57 V as well) and see if there is the same increase in temperature.
Product page for hEX S sys it’s got
PCB temperature monitor
. So it’s not CPU temperature, it’s somewhere on board and only board inspection can tell which of other elements are around to affect the readings.
Consumer grade equipment can run without any issues at 70C, industry grade goes to 85C or so.
Military grade can surpass 120C.
I would be more worried when it reaches melting point of solder joints 
(that’s 180-190C and I’ve seen it happen with beta versions of telco BSC equipment … gives quite some firework when components start to drop off and create short circuits below).
I had the same issue with my hEX S only powered with POE.
Now i run it with a wallwart,the temperature is a comfortable 42.C
http://forum.mikrotik.com/t/sfp-temperature-increase-while-on-poe-hex-s/168750/1
Found this: https://www.reddit.com/r/mikrotik/comments/mis2gq/rb760igs_powered_from_poe_gets_high_temperature/
Sometimes not normal: http://forum.mikrotik.com/t/mikrotik-hex-s-high-temperature/123851/1
Yep, as said it is not “preoccupying”, but it all depends on where the sensor is and what kind of dissipation happens.
If a temperature sensor is near the whatever is heating so much, the 65° are a good representation, if it is only a few cm away (or there is good enough cooling) a component may be at a well higher temperature.
Since the hexS is passive cooled and there is not - I believe - that much airflow, it is more likely that the reading is accurate, so everything is around 65°, not “dangerously hot” but still “hotter than what I like it to be”.