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

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

• Sensor Compatibility: Handles 2-, 3-, and 4-wire PT100 to PT1000 platinum RTDs.
• High Accuracy: Features a 15-bit ADC resolution, providing a nominal temperature resolution of 0.03125°C, with a total accuracy of 0.5°C max over all operating conditions.
• Interface: Communicates with microcontrollers using a 3 or 4-wire SPI-compatible interface.
• Integrated Fault Detection: Includes programmable detection for common errors like RTD open circuits, short circuits to voltage (out of range), or shorts across the RTD element, which increases system reliability.
• Voltage Protection: Inputs are protected against overvoltage faults up to ±45V.
• Supply Voltage Range: The main chip operates from 3.0V to 3.6V. Breakout boards often include voltage regulators and level shifters to support 3.3V to 5V microcontrollers such as Arduino or Raspberry Pi.
• Conversion Time: A maximum conversion time of 21ms for a 50Hz notch frequency filter.

• Vin: This is the power pin. Since the chip operates at 3 VDC, we have included an on-board voltage regulator that accepts 3-5V DC and safely steps it down. To power the board, supply it with the same logic-level voltage as your microcontroller — for example, 5V for a 5V microcontroller, such as an Arduino.
• 3Vo: This is the 3.3V output from the voltage regulator; you can draw up to 100mA from this if needed.
• GND: Common ground for power and logic.
• SCK: This is the SPI Clock pin, an input to the chip.
• SDO: This is the Serial Data Out / Microcontroller In Sensor Out pin, used for data sent from the MAX31865 to your processor.
• SDI: This is the Serial Data In / Microcontroller Out Sensor In pin, used for data sent from your processor to the MAX31865.
• CS: This is the Chip Select pin; drop it low to start an SPI transaction. It is an input to the chip.
• RDY (Ready): This is a data-ready indicator pin. You can use this pin to speed up your reads if you are writing your own driver. Our Arduino driver doesn't use it to save a pin.

• By default, the sensor is configured for 4-wire RTD operation, but can be set for 2 or 3-wire. For a 4-wire setup, leave the jumpers as they are!

• For 3-wire usage: Solder the jumper labeled 2/3 Wire closed and cut the wire connecting the left side of the 2-way jumper just above Rref. Then, solder the right side labeled 3 closed.

• For 2-wire use: solder the two triangular jumpers below the terminal blocks closed, or connect short wire jumpers between the two terminal blocks on each side (essentially jumpering the two right-side terminal holes together, and the same for the left side).

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:

• Connect the Vin to the power supply; 3.3V or 5V is fine. Use the same voltage as the microcontroller's logic. For most Arduinos, that is 5V.
• Connect GND to common power/data ground.
• Connect the CLK pin to Digital #13
• Connect the SDO pin to Digital #12
• Connect the SDI pin to Digital #11
• Connect the CS pin to Digital #10

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);
}

Adafruit MAX31865 RTD PT100 or PT1000 Amplifier

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