Strange Thermistor Readings

Hi All,

Another thermistor related problem for all of you. I recently installed a PCB Heated Bed Mk2 and a 100k thermistor onto my Prusa with RAMPS 1.4 and Marlin.

Attaching the thermistor to T1 or T2 retrieves a temperature reading of 110 Degrees Celsius. switching it with the hot end thermistor input (T0), the heated bed thermistor retrieves the correct room temperature value (22C). To confirm I plugged the working hot end thermistor into the T1 and T2 and got the same 110C reading.

Measuring the inputs on RAMPS with the arduino powered reveals 5.5k ohm resistance on T0, and 6.2k ohm resistance on T1 and T2. I had an extra Arduino Mega on hand and tried switching in out in case those inputs were damaged at some point during my fiddling but there was no change.

Excuse my ignorance if the answer is obvious and staring me in the face, but at this point I can't tell what's going on.

Thanks in advance.

EDIT: I seem to have also placed this thread in the wrong category. Sorry, and can a moderator move it for me?

Edited 1 time(s). Last edit at 07/23/2012 11:14PM by Mondihs.

Double check in the Marlin config that the proper thermistor table is set for T1, it may be set different from T0.

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TEMP_SENSOR_0, and TEMP_SENSOR_BED are both defined for thermistor table 1 (100k thermistor). And to my knowledge both are NTC 100k Thermistors. I did also try defining TEMP_SENSOR_1 and TEMP_SENSOR_2 to 1 with no effect.

Also in Pins.h for motherboard 33 I have:

TEMP_0_PIN 13
TEMP_1_PIN 15
TEMP_2_PIN -1
HEATER_BED_PIN 8
TEMP_BED_PIN 14

Which should make my heater thermistor input at T1 on RAMPS.

Thanks again

Any ideas?

The heated bed works and warms up if I set the temperature above the false reading, and the heated bed thermistor reading rises with it. I still don't know what would be causing this problem. Both the hot end and heated bed thermistors are using the same temperature table.

Is there something in Marlin that would change the properties of pins 14 and 15 from pin 13? Or is there something wrong with my RAMPS?

I'll try changing to sprinter later tonight to see if its a problem with Marlin, but a look at the Github Issue Tracker doesn't seem to reveal anyone else having a similar problem.

I have a slightly different problem with bed temperature on Marlin, but only when FAST_PWM_FAN is set. See [github.com]

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