MAX31865 RTD Platinum Resistance Temperature Detector PT100 to PT1000

Description and specifications

This module allows direct reading of the measured temperature with PT100 or PT1000 sensors, the sensitivity of the sensor is adjusted by external resistance, and the data is read directly. The analog-to-digital conversion is done using the high-precision delta-sigma method.

The module also has overvoltage protection (up to +/- 45V) and short circuit detection.

• It works at a voltage of 3.3 V;
• SPI interface.

The Datasheet for MAX31865 is available in the Downloads section below.

Integrated circuit features MAX31865

• Recommended for industrial use;
• Designed to convert "data" values ​​of thermocouples into digital values;
• The integrated circuit supports thermocouples with values ​​between 100Ω and 1kΩ (2, 3 or 4 wires);
• 15-bit internal ADC resolution;
• Resolution of 0.03125 degrees Kelvin (may vary depending on thermocouple nonlinearities);
• Integral Circuit Accuracy: 0.5 Kelvin;
• The reading of input data is differentiated;
• Maximum conversion time of 21 ms under ideal conditions;
• Fault detection system for certain causes;
• Supply voltage: 3V - 3.6 V;
• Maximum Communication Frequency SPI: 5 MHz.

The recommended calculation method for temperature finding is Callendar-Van Dusen. It is based on the extraction of the temperature from the following equation:

R (T) = R (0) * (1 + aT + bT² + c (T - 100) T³ )

a, b and c are constants and have the values:

a = 3.90830 x 10^-3

b = -5.77500 x 10^-7

c = -4.18301 x 10^-12 for the interval -200 degrees Celsius - 0 degrees Celsius, 0 for the interval 0 degrees Celsius - +850 degrees Celsius.

Instructions for use

1. Make the necessary hardware connections.

The communication interface is SPI. Thus, there are two ways to use this module:

• Through SPI hardware;
• Through SPI software;

To ensure compatibility with multiple development boards, it is recommended that you use SPI software. The connection scheme is the following (for the 3-wire sensor):

To set up the module to work with PT100 or PT1000 temperature sensors, you must unify the green jumper jumper in the attached picture below as follows:

• 4-wire sensor: You do not have to connect the jumper. The threads are a pair if the resistance between them is about 2 Ω (usually they have the same color). One pair connects to either "F +" and "RTD +" pins or "F-" and "RTD -" pins. The other pair connects to the remaining free terminals.
• 3-wire sensor: To use a 3-wire sensor, you must unite jumper jumper J1's. The 2 wires with a 2 Ω resistance between them represent a pair and must be connected to "RTD" and "F - ", and the remaining thread must be connected to "RTD + " or "F +".
• 2-wire sensor: Jumper J1 and J2 must be connected . The wires must be connected to the "F - " and "RTD +" pins.

2. Configure the module according to the temperature sensor.

In the attached picture below is a condenser (green border) that must have:

• 10nF for PT1000;
• 100nF for PT100.

3. Download the required library.

To use this module, you need to download a library developed by Adafruit. It has the name ADAFRUIT MAX31865 library and can be downloaded either from Manage Libraries or directly from GitHub and add it to your development environment.

Also, in the Downloads section, an Adafruit guide is available for a similar module that can be of use to you.

Here's a sample code for Arduino.

Note: The C1 capacitor on the first sheet of the datasheet must be changed depending on the use with 100 Ω or 1 kΩ thermocouples.

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