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 :


And the output is:


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 ?

  • 1
    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 '19 at 20:06
  • I've the same problem with my shields; Where is the componant to remove on the shield to solve the problem? a picture of the modification is welcome thanks a lot fabien
    – fabien
    Sep 27 '19 at 8:27

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)

EDIT: Fixed it with a workaround!Like shown in the picture below, simply remove the wrong component (SMD Capacitor) and create a solder bridge . See comments for more informations!

Here is a picture of my modification of one chinese board as requested by the comments:

Modification of the chinese MAX31865 driver board

(It's not the best solution, but it works. For final using you should get the right filter ferrite and solder it on the board!)

  • 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 '19 at 11:35
  • If wanted, I can provide some pictures of my modification
    – fshero
    Jun 3 '19 at 11:41
  • Can you provide a picture of the modification please..it will help me a lot.
    – java bee
    Dec 27 '19 at 20:46
  • Sure :) Look at my updated post. Hope this helps.
    – fshero
    Dec 30 '19 at 10:30
  • You have the eye of a vulture! Great find! Had the exact same issue, couldn't make the boards work until I removed that darn capacitor. These Chinese components are cheap, but come with some surprise headaches. May 25 '20 at 17:01

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