Thermistors not working correctly

Good day.
I made Ramps-FD V2.B from github and have a problem with the thermistors. All other works- endstops, stepper drives, heaters. Use Arduino Due with Repetier firmware. And have such a problem: thermistors not work as should. In Pronterface it shows about 99C in room temperature. When I heated it up to 200C, Pronterface shows about 140C. Thermistors from China, model NTC 3950 specified and 100% working. Checked soldering for few times. Try to specified all types of thermistors in Repetier firmware configurator and in all cases in room temperature shows about 90C and about 120-140C heated up to 200C.
What can be the problem and how can I fix it?


  • Upd: 100kOhm NTC 3950 thermistors
  • NTC3950 is not really a thermistor type. It is just a resistance/temperature curve specified and you can only hope it follows really that curve. This is for 25°C and the farther away you go the more they tend to deviate from the formula.

    Anyway, at room temperature it should not be that much off. Result is influenced by 2 things.
    1. Thermistor resistance. At 25°C it should have 100kOhm, you should be able to verify that,
    2. On board is a 4,7K resistor that reduces current through thermistor and influences voltage measured. Make sure it is 4.7k of the build in curves do not work. If not you need your own curve with different R1.

    Make sure you did not use a thermocouple input, that would not work as well.

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