MAX31865 RTD-to-Digital Converter - Page 1
Forum section: electronics / projects

author: harrysmith
author_rank: Contributor
timestamp: May 31, 2021, 08:06:44 pm
content: Hi everyone, I have a small project to realize. and I want to do it with the MAX31865 and linked it with a temperature sensor. and I want to know if any one already realize it to have more of his experience THANKS

author: DylanZH
author_rank: Newbie
timestamp: June 01, 2021, 04:57:15 am
content: Here is a doc about RTDhttps://pdfserv.maximintegrated.com/en/an/AN6262.pdf

author: jpanhalt
author_rank: Super Contributor
timestamp: June 01, 2021, 06:09:04 am
content: I use the MAX31856 (not 31865). The MAX31856 is for thermocouples. A quick look at the MAX31865 datasheet shows it to be similar, and apparently breakout boards are available for it. If you are just starting, I would suggest getting such a board for a fast start. It will require a microcontroller and some programming.Do you have any specific questions?

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 02, 2021, 05:02:17 pm
content: MAX31865 is forresistance thermometers(RTDs), most commonly of either Pt100 or Pt1000 type. I prefer the three-wire configuration, as they seem to be easier to find and cheaper than four-wire ones.The break-out boards you can find are either clones of theAdafruitone, or just reimplementations of the suggested application circuit inMAX31865 datasheet(PDF). It is a very simple break-out board, requiring only passives to interface to a 3-wire or 4-wire sensor and a microcontroller using 3.3V voltage; the Adafruit one has a 3.3V regulator so it can be used with 5V microcontrollers (like original Arduinos).Essentially, MAX31865 is a resistance-to-digital converter, which yields a 15-bit digital valueVvia SPI (mode 1 or 3); the resistance measured beingR=Rref×V/32768, whereRrefis the reference resistor being compared against. Adafruit uses a 430 Ohm 0.1% reference resistor, but I've seen other values on the BOBs I bought off eBay. For optimum results, the reference resistor should haveaboutfour times the resistance of the sensor at 0°C (which is 100 Ohm for Pt100, and 1000 Ohm for Pt1000 sensors).That resistance is then converted to temperature based on the sensor type. From -100°C to +100°C, Pt100 sensors have a pretty linear response, so if you don't mind an absolute error of less than 2°C, the temperature is just the resistance multiplied by a constant plus/minus another constant. For better accuracy, theCallendar-Van Dusen equationis used, either directly, or via a look-up table. The datasheet describes how, and includes example tables as well.Adafruit provides anArduino libraryfor their break-out board, which is very easy to use. You obviously also need a microcontroller. I recommend getting several break-out boards and sensors (they aren't expensive), so you can rule out device/sensor issues when developing your gadget: comparing the results between different sensors and MAX31865 boards is very informative. Even replacing the reference resistors with some of your own (they're well marked on the BOBs I have, but a bit close to the MAX31865 chip, for hand-soldering) with precisely measured resistance or known 0.1% from a reputable vendor, can be very useful.I haven't done any calibration testing on mine, because I don't have access to calibrated thermometers. I guess I could test at 0°C and 100°C with distilled water (I live close enough to sea level so atmosperic pressure doesn't affect the boiling point much) with water-ice slush and boiling water, respectively, but haven't found a reason to do so yet.Realistically, you should be able to achieve 0.5°C accuracy and 0.05°C precision with these, but that of course depends on the break-out board, sensor, and the resistance-to-temperature model you use.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:45:22 pm
content: Since the Adafruit library was mentioned I recommend to make sure you use the one which has bias control is fixed (=switched off after measurement). There are different versions out there.A