Ultimaker original Temperature sensor error

[bob 0]

I made a custom heated bead for my Ultimaker Original, it has worked for about a year with out an issue. Unfortunately the bed Thermistor shorted with the bed power cable today, neither the Thermistor nor the arduino mega was damaged, though I replaced both anyway.

 

Now the issue that I have, is that the bed temperature shows approximately 800°c and thus causes a max temperature error, this is the same temperature that is displayed when no Thermistor is connected as well as if I connect 100k ohm resistor (its a 100k thermistor, so should show 25°c). This leaves the ultimaker controller as the most likely error source. I don't have one lying around, and the next possible replacement is in china, which would take a month to get shipped (I also don't want to change to a different controller since the system is made for a 19v power supply, and no one carries them anymore). Does anyone know what possible components on the board could have fried? or could I just switch to the sensor 2 on the board, and how would I set that up in the firmware?

 

Thanks

[gr5 2,271]

I would put the old thermistor back in there.  There are probably a dozen differen types of "100K thermister".  They all have very different tables that convert resistance to temperature.

 

Do you still have the 4.7K resistor installed?  Maybe that got burned out.  When everything is hooked up, what is the actual voltage on either side of the 4.7K resistor?  There should be a voltage divider with the 4.7K resistor and the 100K resistor.  So if one is 4.7K and the other is 100K then the 5V should get divided (well multiplied really) by ratio R1/(R1+R2) (not sure which is which and too lazy to check schematic, sorry).

 

That means either 0.22V or 4.77V assuming the thermistor is 100K ohms when you take the measurement.

 

[bob 0]

Its the exact same 100k Thermistor (I purchased a few of them when I upgraded to a heated bed) , I just replaced it with a different one in case the first one had fried.

[bob 0]

I checked the 4.7K resistor, that was what fried, thanks, would never have checked that...

 

Unfortunatelly I seen to have another issue now, the printer pauses between each straight line for about 2 seconds... Any idea what could cause this? I already tried re-replaceing the arduino, that did not help...

Edited November 3, 2018 by bob