Thermistor/LCD Display shows opposite change in temperature

Has anyone ever experience an opposite temperature change readout from a thermistor connected to Ramps 1.4 running current Marlin?

Marlin 1.1.8
Arduino Mega 2560 R3
Ramps 1.4
12864 LCD w/SD Card Support
100K Thermistor - generic thermistor included with the Ramps/Driver/Heated Plate/LCD combo package I purchased from ebay.

I verified the thermistor is working properly by checking the resistance with my DMM - at 25ºC the resistance is around 101kohm. I also checked and verified that the resistance measured decreases as the temperature of the thermistor increases.

The readout on the LCD with the thermistor connected to the ramps T0 slot shows 25ºC at the room temp of 25ºC, but when I place the sensor between my fingers(increasing the temperature), the temp readout on the LCD decreases gradually down to around 20ºC). Both the LCD readout and sensor are changing values in response to a temperature change, but the direction of temperature change is backwards.

I’m pretty sure there is no polarity on a thermistor, but have tried reversing the connection on the board anyway with no change. I have also tried various options for #define TEMP_SENSOR_0 (1,11,13) with a re-upload to the board after each change, as well as tried the different physical slots on the ramps board(T0,T1,T2) and ended up with the same results.

I’m guessing the solution is something simple like commenting out a line in the firmware or selecting a specific sensor/table. I’ve searched the web for a solution from others having this problem, but I couldn't find any mention of a similar experience.

Does this sound like something obvious to anyone more familiar with 100k thermistors or Marlin?

I can perform further tests and report the results if you need more information.

Any help is greatly appreciated!

~Walter~

Edited 1 time(s). Last edit at 12/30/2017 09:00PM by WalterP23.

Not much can go wrong with the temperature sensor lookup code, What temperature sensor do you have, and what sensor number did you choose.

Well, I thank you for responding to the post, but upon re-uploading the code today the thermistor/LCD are now working as they should.

While I'm glad the issue has somehow resolved itself, I'm hesitant to fully trust that the issue won't re-manifest at a more critical time.

I really wish I had recorded a video of the issue when it was occurring...

As of now, I guess the problem is solved. But I will be sure to report back and provide a video of the issue if it happens to reoccur.

Happy New Year!

~Walter~

I have had problems with Marlin after being flashed. And need to do a M502 then save using a M500.

The reason is because the Flashing process does not erase the save data. Reloading Firmware defaults and saving helps My Marlin Flashes to operate correctly.

M500: Store parameters in EEPROM
M501: Read parameters from EEPROM
M502: Revert to the default "factory settings."