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lamaPLC: Max31865 RTD to Digital Converter - PT100/PT1000 Platine

Max31865 RTD to Digital Converter The MAX31865 is an easy-to-use resistance-to-digital converter designed for platinum resistance temperature detectors (RTDs). An external resistor determines the sensitivity for the RTD used, and a precision delta-sigma ADC converts the RTD resistance ratio to the reference resistance into a digital output. The MAX31865’s inputs are protected against overvoltage faults up to 45V. It also includes programmable detection for RTD open or short circuits and cable open or short circuits.

Key Features and Specifications

Pinout

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2026/02/14 22:38

Configuration

MAX31865 configuration



PT100 / PT1000 sensors

Check the PT100 sensor RTDs (Resistance Temperature Detectors) are straightforward devices: simply a small strip of platinum that measures precisely 100 Ω or 1000 Ω at 0°C. Bonded to the PT100/PT1000 are two, three, or four wires.

Thus, the 4-wire RTD has two wires attached to each side of the sensor. Each wire has about 1Ω of resistance. When connected to the amplifier, the innovative amp measures the voltage across the RTD and across the wire pairs.

For example, the approximate resistances of a 4-wire PT100 RTD at 0 °C are as follows. (For a PT1000, the middle resistance would be about 1002Ω rather than 102Ω).

When the amp measures this sensor, it first assesses the resistance between one set of red and blue wires. It then measures the resistance between the red wires and the blue wires. To get the resistance of a single wire, divide each resistance value by two. The final calculation is 102 - 1 - 1 100Ω.These are very similar to the 4-wire type, but only have one 'pair' of connected wires. This is because the wires for the RTD are all the same gauge and length; therefore, instead of two pairs, the amplifier reads one pair and uses that resistance for both wires. It is as simple as it gets, with only one wire on each side. You might need to calibrate the sensor by placing it in an ice bath to measure the resistance at 0°C (around 102 Ω), then subtract 100 Ω to find the total resistance of the connection wires!

Connect the two ends of the PT100/PT1000 resistor to the RTD+ and RTD- terminals on the sensor module. For example, a resistance of 102 Ohms can be measured. In a 3-wire or 4-wire setup, the wire connections go to the F+ and F- terminals. These connections might differ from the resistance values of the respective sides by only a few Ohms, meaning the resistance between F+ and RTD+ or F- and RTD- may vary slightly, just a few Ohms.

PT100 IEC 751 Basic resistance values in Ohm PT100 sensors according to DIN/IEC 751

2025/12/10 17:50
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SPI Wiring

Since this is an SPI-capable sensor, we can use either hardware or software SPI. To ensure consistent wiring across all Arduinos, we'll start with 'software' SPI. The following pins should be used:

Arduino code

To start reading sensor data, install the Adafruit MAX31865 library from the Arduino library manager.

https://www.adafruit.com/products/3328

This basic example uses the Adafruit library to read temperature from a PT100 sensor using a 3-wire configuration. 

#include <Adafruit_MAX31865.h>
 
// Use hardware SPI: pass only the CS pin
Adafruit_MAX31865 thermo = Adafruit_MAX31865(10);
 
// Use 430.0 for PT100 and 4300.0 for PT1000
#define RREF      430.0
// The 'nominal' 0-degrees-C resistance of the sensor (100.0 for PT100)
#define RNOMINAL  100.0
 
void setup() {
  Serial.begin(115200);
  Serial.println("MAX31865 PT100 Test!");
 
  // Change to MAX31865_2WIRE or MAX31865_4WIRE as needed
  thermo.begin(MAX31865_3WIRE); 
}
 
void loop() {
  uint16_t rtd = thermo.readRTD();
  float ratio = rtd;
  ratio /= 32768;
 
  Serial.print("RTD value: "); Serial.println(rtd);
  Serial.print("Ratio = "); Serial.println(ratio, 8);
  Serial.print("Resistance = "); Serial.println(RREF * ratio, 8);
  Serial.print("Temperature = "); Serial.println(thermo.temperature(RNOMINAL, RREF));
 
  // Check for faults
  uint8_t fault = thermo.readFault();
  if (fault) {
    Serial.print("Fault 0x"); Serial.println(fault, HEX);
    thermo.clearFault();
  }
  Serial.println();
  delay(1000);
}

Sources

Adafruit MAX31865 RTD PT100 or PT1000 Amplifier

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