Heat Bed Thermistor

[Enough!Participant @enough]

The typical heat bed thermistor …. does its resistance increase or decrease with rising temperature?

I'm assuming it decreases but I would like to get it confirmed.

[Enough!Participant @enough]

[Neil WyattModerator @neilwyatt]

Yes, I'm pretty sure the standard is an ntc thermistor. Also used for the extruder.

They are surprisingly accurate.

Neil

[Enough!Participant @enough]

Thanks, Neil (not Niel as I'm sure you know ).

So a temperature display that flickers from 60 °C to 7000-odd °C is indicating an intermittent short rather than open circuit.

[duncan webster 1Participant @duncanwebster1]

depends how it is wired into the circuit and what is measured. If it is connected Vcc-thermistor-fixed resistor-ground then as the tem goes up, the thermistor resitance goes down so the voltage at the thermistor-resistor joint goes up, and if te thermistor goes open circuit the voltage goes to zero.

However if it is the other way up, the voltage at the joint goes down with increasing temperature and goes to Vcc if the thermistor is open circuit

[Enough!Participant @enough]

Certainly ohm-meter measurements, with the thermistor plug disconnected at the PCB, indicate an intermittent open-circuit. Moreover that's the failure mode I'd expect from a constantly flexing cable. There's no visual sign of any short.

So I think your explanation has it, Duncan – thanks.

 

Edited By Bandersnatch on 26/05/2020 22:31:52

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