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Thermistor calibration

Posted by beekeg 
Thermistor calibration
September 26, 2013 07:51PM

I've completed the build of my Tantillus from parts sourced here in the UK. I've run into a problem with the pre-print set up. Basically I can't get the Hotend Mk Vb (from Hotends.com) hot enough to extrude PLA. Even when the set temperature is wound right up past ABS temperatures there is no flow. With the thermistor disconnected for a short time I can get flow, so it is not a power/conduction issue. I am using the thermistor that came with the Mk Vb. When the PLA is flowing the LCD was reading around 350 deg C.

I am guessing that I need to apply a factor somewhere to get the temperature reading correctly, but I can't find information within the build guide on how to do this and how to determine the factor. When the LCD reads 21 deg C the resistance of the thermistor is 101k.

Any assistance would be most appreciated as I'm keen to reach the next cliff face in the learning curve.


Edited 2 time(s). Last edit at 09/27/2013 07:56PM by beekeg.
Re: Thermister calibration
September 26, 2013 08:04PM
It could be your electronics has a different sense resistor.

Near the top of the configuration file for Marlin you will see:

//=============================Thermal Settings ============================
//--NORMAL IS 4.7kohm PULLUP!-- 1kohm pullup can be used on hotend sensor, using correct resistor and table
//// Temperature sensor settings:
// -2 is thermocouple with MAX6675 (only for sensor 0)
// -1 is thermocouple with AD595
// 0 is not used
// 1 is 100k thermistor - best choice for EPCOS 100k (4.7k pullup)
// 2 is 200k thermistor - ATC Semitec 204GT-2 (4.7k pullup)
// 3 is mendel-parts thermistor (4.7k pullup)
// 4 is 10k thermistor !! do not use it for a hotend. It gives bad resolution at high temp. !!
// 5 is 100K thermistor - ATC Semitec 104GT-2 (Used in ParCan) (4.7k pullup)
// 6 is 100k EPCOS - Not as accurate as table 1 (created using a fluke thermocouple) (4.7k pullup)
// 7 is 100k Honeywell thermistor 135-104LAG-J01 (4.7k pullup)
// 1k ohm pullup tables - This is not normal, you would have to have changed out your 4.7k for 1k
// (but gives greater accuracy and more stable PID)
// 51 is 100k thermistor - EPCOS (1k pullup)
// 52 is 200k thermistor - ATC Semitec 204GT-2 (1k pullup)
// 55 is 100k thermistor - ATC Semitec 104GT-2 (Used in ParCan) (1k pullup)

#define TEMP_SENSOR_0 1
#define TEMP_SENSOR_1 0
#define TEMP_SENSOR_2 0

Right now it is set to use 1 and if you have a 1K pull-up rather than the normal 4.7K one you would need to use 51.

Other than that I can not think of any reason why it would read so far off.

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Re: Thermister calibration
September 26, 2013 08:28PM
Thanks for the early response.

I'll check the RAMPS 1.4 board thoroughly and Marlin settings, tomorrow. I'm reading 5k across THERM0 and GND, pins.

Time for bed....
Re: Thermistor calibration
September 27, 2013 07:58PM
It has now sorted itself out with a reload of the Marlin firmware.smiling smiley
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