No temperature reading from either thermistor

Hi,

For some reason Pronterface and Repetier Host have stoped reporting correct temperatures from my reprap. Both the bed and extruder are reported as (aprrox) 50 degrees centigrade while it should be around 20. To me it looks like the programs are not getting any input from the board.

They were working correct this morning. This afternoon I had to remove the nozzle as it's blocked and ever since the temperature readings have been off. The sensors do report changes in temperatur but incorrectly. When I turn on the heating the sensors do show an increase but it's far less that what it is in reality (8 degrees rise in Extruder temperature was reported as about 1.5 degrees increase).

I measured the resistance of both sensors (also at the connectors for the board) and they seems to have the right resistance for the actual temperatur. So the sensors themselves seem to be ok. I checked the settings in Marlin and uploaded it into my Sanguinololu to check if this was causing a problem or not. This did not help.

Does anybody have a suggestion as to what the problem could be?

Bye,
Ming-Hua

Check your wire shielding. If a hole burned through it may be grounded to the heater block. The thermistor could have also been damaged somehow. They do resist heat but they have a limit. Too much exposure at too high of heat is bad. You may also have luck changing the type of thermistor in your firmware.

Thanks for the reaction. I have replaced the wire shielding as a precaution and checked to make sure there is no contact with the heater block. As to the thermistors, both are showing about 135kOhm which I believe is right for the current temperature. I can do some more tests using a putty oven later to check if they are working fine.

The odd thing is that I've only handled one of them and right now the programs are not getting the correct information from either of them. Can this happen if a single thermistor is damaged?

Edit: additional information: if I detach the thermistors the reported value stays the same (about 50 degrees Centigrade) as if that is the starting value instead of 0.

Bye,
Ming-Hua

Edited 4 time(s). Last edit at 03/15/2015 04:18PM by Minimaker.

For everybody who runs into this problem, it looks like something got shorted in the ATmega644P-20PU. I replaced it with another copy I had around and the temperature reading is correct now.