Connecting a thermistor directly to the expansion port (Duet2)

rbjam83

Hello everyone,
I have a Duet 2 Ethernet (without the Duex expansion board) and would like to connect an extra thermistor to monitor chamber temperature. Since I'm already using all 3 of the thermistor ports I'm trying to connect the thermistor to pin exp.39 and exp.40 on the expansion connector, but I get crazy high temperature readings with a lot of noise too.
Do I need to add a resistor in between the thermistor wires? If so, what value?
Thanks!

jay_s_uk

@rbjam83 you can't just add a thermistor like that.
If you look at the schematic, you need a connection to VSSA (3.3v) as a reference voltage and then a voltage divider.
It may be easier to add an SPI daughter board for the max31865 and convert your hotend to a PT1000, then you'll have a spare port

T3P3Tony

@rbjam83 as @jay_s_uk says adding a spi expansion board may be easier. If you do want to use a thermistor the follow the circuit in the duex5 :

https://github.com/Duet3D/Duet-2-Hardware/blob/master/Duet_Expansion/Duex5v0.11/Duex_Schematic_v0.11.pdf

Specifically you need a 2K2 0.1% pullup to ADVREF, and connect the thermistor between the thermistor pin and VSSA.

rbjam83

@T3P3Tony Thanks.
After a quick search online, looks like it will indeed be difficult to do this. Can't find such precise resistors near me.

dc42

@rbjam83 you can use a 1% resistor, it may just reduce the accuracy a little.

rbjam83

@dc42 I'll give it a try.
Thanks!