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I am using PT100 (3 Wire) to measure the temperature with Arduino Uno and MAX31865. I have connected everything perfectly. Configuration in the code i am using is :

max1.begin(MAX31865_3WIRE);

And the output is:

output

RTD value: 0 
Ratio = 0.00000000
Resistance = 0.00000000
Temperature = -242.02

I am testing it in a room temperature.

My question is , is there any settings or calibration that i need to do, or any other way to test the PT100 sensor, because the temperature is showing correct readings ?

  • I had the same problem and was solved due to the comment of fshero (Jun 3 at 11:35) about a capacitor wich drops the voltage on PIN 20 of the MAX31865. I have logged in just to highlight the solution. Thanks fshero!! – LeandroIP Aug 19 at 20:06
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I nearly have the same problem. I use five of the MAX31865-Boards with 2-Wire Termocouples on an Arduino Nano. Two of the five boards are working perfectly. The other 3 boards show exactly the same wrong behaviour you described. My temperature readings are exactly like yours (-242.02).

But I think I found the reason for the not working ones: The two working boards I bought on Amazon. The other three not working boards I bought on AliExpress. Maybe I (and maybe you too) got really bad fakes from China and they are simply dead :(

(BTW: If I remove one working board from the SPI bus and read out the temperature with the arduino, I get also the value of -242.02. This value seems to be an error value in the Adafruit MAX31865 library)

  • I figured out the source of the error in my case: After some "debug measuring" with a multimeter I recognized, that the voltage of the VDD Pin of the Sensor is too low (1.6V but 3.3V are required)! It seems the chinese version has a capacitor at the place, where a originally a filter ferrite should be (see part FB1 at adafruit_products_schem.png). This causes the voltage drop. I removed the capacitor and replaced it with a solder bridge. Et voilá. It's working perfectly! :D – fshero Jun 3 at 11:35
  • If wanted, I can provide some pictures of my modification – fshero Jun 3 at 11:41

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