Thermistors adjustments

Hi everyone,

I have just finished assembling my Mendel90 (Sturdy version). I have run the first tests, everything looks fine, except for a little problem of precision concerning the thermistors.

I am using 2 Epcos B57560G104F 100K (1 for the hotend and 1 for the hotbed). When at room temperature, Pronterface reads about 45.0 C each (instead of 25 °C). When I set the temperature of the hotend to "220 (PLA)", my thermocouple reads 185 °C (in the barrel).
And when I set the hotbed temp to "60 (PLA)" in Pronterface, my IR temperature sensor reads around 37-40 °C.

Should I consider these measurments as an issue ? And how could I fix it ?

Oh, and one more question, the 2 alu resistor behind (close to the PSU) get very hot (I can barely touch them for a short time), is it normal ?

Thank you for reading ! and sorry for my english
Jerome

What electronics are you using? Some Melzis have the wrong pullup resistors for the thermistor inputs. They should be 4K7 but some are 10K.

On the kit version the resistors are mounted on the Dibond which acts as a heatsink and they only get warm. Without a heatsink they will get hot. It isn't a problem electrically but might be a hazard to touch.

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[www.hydraraptor.blogspot.com]

Hi Chris,

I'm using a Melzi from Aliexpress :
[mendel90.sitagg.com]

Sorry for poor resolution, I will check tonight at home what is written on the resistors next to the thermistors plugs. In case of 10K resistors (which sounds like a good explanation), is there an easy way to fix it ?

Yes, I didn't think about the heatsink effect of the Dibond. Maybe I will add a other fan to cool the 2 resistors.

Thanks

You can make a table for 10K but it will have less resolution. They are pretty easy to change as they are relatively large surface mount resistors.

The resistors only dissipate 2.5W and they are rated for 150C at that wattage. They don't get anywhere near that hot, so there is no need to cool them. Perhaps just print a plastic grill if you are worried about touching them.

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[www.hydraraptor.blogspot.com]

Hi Chris,

You were right, I have 10K resistors on my Melzi board. I have soldered 2 10K resistors (found 2 at 9.4 K) in parallel as you suggested in an other topic and it works great, I read the same temperature in pronterface and with a thermocouple.

Thanks very much, you saved me a lot of time