There's a circuit board on top of the UMO. That converts thermocouple to a voltage from 0V to 5V to represent 0C to 500C. If you insist on using a thermistor first realize that most can't take temperatures above about 230C. Secondly you will have to bypass that board on top of the printer and instead run a cable ALL the way from the thermistor - along the bowden - under the machine - and plug it into where the current temp cable connets. The current cable is 3 wires (power/ground/signal from head). The new method is just two wires (don't connect to power pin - but use the other 2). Also like amedee says you need to add a 4.7K resistor. google "Ultimaker 4.7K" and I think you'll find photos of where that goes for bed sensor or head sensor. Don't get the 2 confused.
Ah a pull up, yes thats the answer im looking for, thanks.
- 3 weeks later...
for documentation purposes:
we just had the problem, that our UMO showed 50°C on the first temp input and around 220°C on the second, but roomtemp (as it should) on the temp input for our heating bed.
we also used a 100k thermistor, had all inputs fitted with 4.7k pullups and tried several 100k options in marlin firmware.
we dont know what the problem was, but for us exchanging the arduino mega board with a fresh one, fixed the problems!
If you use a thermistor in place of the original thermocouple you need to solder a resistor on the board as well...
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