ultimaker original temp sensor fails when touching heater block

[3DSVP 0]

So i've been trying to get a e3D hotend to work on my ultimaker original, using the same thermistor as before.
It gives me correct reading when it is not in the heater block yet, but once i insert it, the temperature slowly drops, untill i get the mintemp error.
I have tried inserting a new thermistor, but this doesn't change anything.
Again, it only fails once it is in the heater block.

 

EDIT: After some testing i found, when not fully inserted yet, it stays stable. But once i start the preheat program, (using a infrared thermometer i can see the actual temp) i notice that once the temperature rises, the temperature read from the thermistor goes down, so it seems to have to do with the heat

Edited March 28, 2018 by 3DSVP

[gr5 2,271]

Do you have

1) the older UMO with the thermocouple and the circuit board on the head that converts the thermocouple voltage to a voltage readable by the arduino

or

2) the newer UMO+ with the white circuit board and a PT100 temp sensor and no circuit board on the print head that converts the voltage

 

What is the reading of the temperature when the "thermistor" when it is at room temperature.

 

There are thermistors, thermocouples, and PT100 (which is technically a thermistor but different enough that it deserves it's own category.  Thermocouples are 2 pieces of different metal touching and this creates a battery like voltage that changes with temp.

 

Thermistors change resistance with temperature change.  Most can't handle temps above 250C.  Actually most can't handle temps above 100C.  Thermistors vary widely.  every manufacturer of thermistors has a different table that translates resistance to temperature.  PT100s are consistent across all manufacturers because they are simply made of platinum.  also pt100 can handle much higher temps.

[3DSVP 0]

I would be using the most original Original electronics (aka Green circuit board + thermocouple with circuitboard on the head)

 

[gr5 2,271]

I can't imagine why the temp would go *down* with temperature.  

 

However, there is a common problem with electrical interference.  The signal on the thermocouple is very sensitive.  make sure those wires are at least 2cm away from other wires to the heater and/or fan wherever possible.  Once they get past the little circuitboard the signals are not sensitive to electrical noise.

 

5 hours ago, gr5 said:

What is the reading of the temperature when the "thermistor" when it is at room temperature.

You didn't answer the above question as the answer could explain everything.