Prusa i3 Temperature Issue

I have the following initial configurations:

• Device: Prusa i3
• Board: Melzi 2.0 Ardetntissimo with ATMEGA1284P
• Extruder Thermistor: 100K 1% high-precision NTC
• Bed Thermistor: 100K NTC 3950

I am using Marlin Firmware from www.github.com. The problem that I have faced is about incorrect thermistor temperature readings from both, Extruder and Heat Bed. The first one showed room temperature around +45 and the second one was around -7. After an extensive search, I figured out that I should replace two 103 resistors (10KOhm; R4 and R5) with 4701 resistors (4.7KOhm, 1%) on the Melzi Ardentissimo board. After the resistor replacement I got totally different readings:

• Extruder is +23
• Bed is -20

I still do not understand why the bed temperature is so low. When I heat it up with a soldering iron the readings go up. Looks like it is something related to firmware. I tried changing thermistor settings in Marlin but no luck. In addition, this version of Marlin does not have thermistor tables.

Any suggestions?

Hi,

Check you have the good thermistor settings in Marlin.

++JM

It must have a lookup table, I doubt they have a arithmatic algorythm in place. Maybe they've merged the tables with other header files. Do a text search in the folder. Do you have a link to the fw code?

Edited 1 time(s). Last edit at 09/17/2015 11:12AM by imqqmi.

Yesterday I figured out that they do use arithmetic algorithm for calculation temperature. Here is a code source link: https://github.com/reprappro/Marlin/tree/master

You were right. The thermistor settings was wrong. I got a technical support answer from RepRapPro. They said that with their Marlin version I am able to change the thermistor settings without compiling and uploading new version by using the following G-Code:

1. M304 H0 B 3980 //Sets BETA value
2. M304 H0 T 100000 // Sets thermistor value
3. M500 // Saves information to EEPROM

I stand corrected! Thanks for sharing the solution!