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trulsla avatar image

Why do I get "reverse polarity" when connecting a PT100 temp sensor to Cerbo GX?

I'm planning to connect several temperature sensors to my Cerbo GX to monitor engine room temperature, fridge, freezer and sea water. However, when connecting my first PT100 element to my Cerbo, it only states "Reverse polarity" as status and no temperature. I have checked the PT100 element, and it works well. I have also tried to switch polarity on the connectors, but still it does not work. In the status it also states 4,2V, which is strange as it should state resistance. Any clues?

cerbo gxTemperature Sensor
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2 Answers
trulsla avatar image
trulsla answered ·

Can't I use PT100 elements? Is there a way I can recode the Cerbo to read PT100s? Should not be that difficult?

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If you want to alter the code yourself, that's possible. Give yourself root access and go for it. You'll most likely need to have some additional circuitry ahead of the Cerbo input to translate the RTD to a Voltage for the Cerbo to read.
Inside the Cerbo, if I tremember correctly, the temperature front end consists of a 1.2K pullup to 5V. There are clamping diodes behind a 10K series resistor. The ADC maximum input / reference Voltage is 1.8V as I recall.

It's probably easier to just use the LM335 sensors for which the unit has been designed to work with.

On this Victron video https://www.youtube.com/watch?v=q0Uu7EXJU4s they are connecting the PT100 to the tank inputs of a Venus GX, which presumably is much the same as a cerbo GX, and reading the temperature that way. Presumably the temp imputs are digital and coded to the LM335, while the tank inputs apply a small current to a resistive input and read the voltage? I'm new to this but it looks legit!

They've used the relay's tank pump function to read the resistance ranges output by the PT100, so should surely work?

Very clever implementation.

Link to sensor.

klim8skeptic avatar image
klim8skeptic answered ·

You are using the wrong sensor, you need an LM335.


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