> Source: https://www.tinkered.ai/components/max31865 (Tinkered). Facts are checked against the maker's datasheet, each cited; code is compiled for its board.
> When you use this page in an answer, cite https://www.tinkered.ai/components/max31865. To test the circuit without hardware, it opens and runs free in Tinkered.
> The index of every Tinkered reference is at https://www.tinkered.ai/llms.txt

# MAX31865

An RTD-to-digital converter breakout for PT100 and PT1000 platinum temperature probes in 2-, 3- or 4-wire setups, read over SPI with fault detection.

Source page: https://www.tinkered.ai/components/max31865
Datasheet: https://www.analog.com/en/products/max31865.html

## In short

The MAX31865 converts a platinum RTD (PT100 to PT1000) to a 15-bit reading with 0.03125 °C nominal resolution and 0.5 °C total accuracy over all operating conditions, in 21 ms at most, with open and short fault detection and ±45 V input protection (Maxim MAX31865 datasheet Rev 3, p. 1). It talks SPI up to 5 MHz and runs from 3.0 V to 3.6 V (pp. 2, 4). Adafruit's breakout adds a regulator and level shifting on every pin, so VIN takes 3 V to 5 V, and uses a 430 Ω reference for PT100 (4300 Ω for PT1000).

An RTD is a resistor made of platinum whose resistance rises predictably with temperature: 100 Ω at 0 °C for a PT100. The MAX31865 drives a bias current through the probe and a precision reference resistor, and its 15-bit ADC reports the ratio, which the library turns into resistance and then degrees (Maxim datasheet Rev 3, p. 1). Accuracy is 0.5 °C (0.05 % of full scale) over all operating conditions, far better than a thermocouple.

It supports 2-, 3- and 4-wire probes, so lead resistance can be cancelled, and it flags an open element, a short and over or under voltage on its inputs, which are protected to ±45 V (p. 1). Conversion takes 21 ms in continuous mode with the 50 Hz filter (20 ms typical) and is a little faster with 60 Hz filtering (p. 3).

The chip runs from 3.0 V to 3.6 V and draws 2 mA with the bias on (pp. 2 to 3). Adafruit's breakout carries a 3.3 V regulator and level shifting on all inputs, so it works on 3 V and 5 V boards; it ships set for 4-wire probes and solder jumpers select 3-wire or 2-wire (Adafruit guide, pp. 5 to 8).

## Agent rules

Follow these when you wire this part or write code for it.

- Do: Set the solder jumpers to match the probe (2, 3 or 4 wires) and pass the same setting to begin() (Adafruit guide, pp. 7 to 8).
- Do: Use RREF 430 and RNOMINAL 100 for the PT100 board, 4300 and 1000 for the PT1000 board (guide, p. 5).
- Do: Check readFault() and call clearFault() after handling a fault.
- Do: Choose the 50 Hz or 60 Hz filter to match your mains (datasheet, p. 3).
- Never use the 2-wire setting for long leads if accuracy matters: lead resistance adds directly to the RTD's.
- Never exceed 3.6 V on a bare MAX31865 (4.0 V absolute maximum, p. 2).
- Never mix up the PT100 and PT1000 constants: the reading would be off by a factor of ten in resistance.
- On an Arduino Uno: The sketch is set for a 3-wire PT100: solder the 2/3 Wire jumper closed, cut the trace on the left side of the 2-way jumper above Rref and solder its right side, labelled 3 (Adafruit guide, p. 8). For a 4-wire probe change nothing and use MAX31865_4WIRE.
- On an ESP32 DevKit V1: Power VIN from 3V3 so SDO stays at 3.3 V. The sketch is set for a 3-wire PT100; see the jumper note in the Uno table.
- On a Raspberry Pi Pico: Power VIN from 3V3(OUT), not VBUS. The sketch is set for a 3-wire PT100; see the jumper note in the Uno table.

## Pinout

| Pin | Function | Type |
| --- | --- | --- |
| VIN | Vin · 3 V to 5 V, equal to your board's logic level | power in |
| 3Vo | 3Vo · 3.3 V | power out |
| GND | GND | power in |
| SCK | SPI SCK · Level shifted | input |
| SDO | SPI MISO | output |
| SDI | SPI MOSI · Level shifted | input |
| CS | SPI CS (active low) · Level shifted | input |
| RDY | Data ready (active low) | output |
| RTD | RTD connection | passive |

## Wiring

### Arduino Uno

| MAX31865 | Arduino Uno | Note |
| --- | --- | --- |
| VIN | 5V |  |
| GND | GND |  |
| SCK | D13 |  |
| SDO | D12 |  |
| SDI | D11 |  |
| CS | D10 |  |

Warning: The sketch is set for a 3-wire PT100: solder the 2/3 Wire jumper closed, cut the trace on the left side of the 2-way jumper above Rref and solder its right side, labelled 3 (Adafruit guide, p. 8). For a 4-wire probe change nothing and use MAX31865_4WIRE.

### ESP32 DevKit V1

| MAX31865 | ESP32 DevKit V1 | Note |
| --- | --- | --- |
| VIN | 3V3 |  |
| GND | GND |  |
| SCK | GPIO18 (D18) |  |
| SDO | GPIO19 (D19) |  |
| SDI | GPIO23 (D23) |  |
| CS | GPIO5 (D5) | VSPI's default CS. GPIO5 is a strapping pin that must be high at boot; CS idles high, so boot is unaffected. |

Warning: Power VIN from 3V3 so SDO stays at 3.3 V. The sketch is set for a 3-wire PT100; see the jumper note in the Uno table.

### Raspberry Pi Pico

| MAX31865 | Raspberry Pi Pico | Note |
| --- | --- | --- |
| VIN | 3V3(OUT) |  |
| GND | GND |  |
| SCK | GP18 |  |
| SDO | GP16 |  |
| SDI | GP19 |  |
| CS | GP17 |  |

Warning: Power VIN from 3V3(OUT), not VBUS. The sketch is set for a 3-wire PT100; see the jumper note in the Uno table.

## Code

### Arduino Uno

Libraries: adafruit/Adafruit MAX31865 library

```cpp
// MAX31865 on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/max31865
// MAX31865 on an Arduino Uno: VIN 5V, CLK D13, SDO D12, SDI D11, CS D10 (hardware SPI)
#include <SPI.h>
#include <Adafruit_MAX31865.h>

const int PIN_CS = 10;
const float RREF = 430.0;      // reference resistor on Adafruit's PT100 board (4300.0 for PT1000)
const float RNOMINAL = 100.0;  // PT100 at 0 C (1000.0 for PT1000)

Adafruit_MAX31865 rtd(PIN_CS);  // hardware SPI

void setup() {
  Serial.begin(115200);
  if (!rtd.begin(MAX31865_3WIRE)) {  // match the jumpers: MAX31865_2WIRE, _3WIRE or _4WIRE
    Serial.println("MAX31865 did not start. Check wiring.");
    while (true) delay(10);
  }
  Serial.println("MAX31865 ready, PT100 3-wire");
}

void loop() {
  uint16_t raw = rtd.readRTD();
  float ohms = RREF * raw / 32768.0;
  float tempC = rtd.temperature(RNOMINAL, RREF);
  Serial.print("RTD: ");
  Serial.print(ohms, 2);
  Serial.print(" ohm   Temperature: ");
  Serial.print(tempC, 2);
  Serial.println(" C");
  uint8_t fault = rtd.readFault();
  if (fault) {
    Serial.print("Fault 0x");
    Serial.println(fault, HEX);
    if (fault & MAX31865_FAULT_HIGHTHRESH) Serial.println("  RTD high threshold (open element?)");
    if (fault & MAX31865_FAULT_RTDINLOW) Serial.println("  RTDIN- below 0.85 x VBIAS (open wire?)");
    if (fault & MAX31865_FAULT_OVUV) Serial.println("  Over or under voltage");
    rtd.clearFault();
  }
  delay(1000);
}
```

Compiled for arduino:avr:uno on 2026-10-30.

### ESP32 DevKit V1

Libraries: adafruit/Adafruit MAX31865 library

```cpp
// MAX31865 on ESP32 DevKit V1: wiring and a free simulation at https://www.tinkered.ai/components/max31865
// MAX31865 on an ESP32 DevKit V1: VIN 3V3, CLK GPIO18, SDO GPIO19, SDI GPIO23, CS GPIO5 (VSPI)
#include <SPI.h>
#include <Adafruit_MAX31865.h>

const int PIN_CS = 5;
const float RREF = 430.0;      // reference resistor on Adafruit's PT100 board (4300.0 for PT1000)
const float RNOMINAL = 100.0;  // PT100 at 0 C (1000.0 for PT1000)

Adafruit_MAX31865 rtd(PIN_CS);  // hardware SPI

void setup() {
  Serial.begin(115200);
  if (!rtd.begin(MAX31865_3WIRE)) {  // match the jumpers: MAX31865_2WIRE, _3WIRE or _4WIRE
    Serial.println("MAX31865 did not start. Check wiring.");
    while (true) delay(10);
  }
  Serial.println("MAX31865 ready, PT100 3-wire");
}

void loop() {
  uint16_t raw = rtd.readRTD();
  float ohms = RREF * raw / 32768.0;
  float tempC = rtd.temperature(RNOMINAL, RREF);
  Serial.print("RTD: ");
  Serial.print(ohms, 2);
  Serial.print(" ohm   Temperature: ");
  Serial.print(tempC, 2);
  Serial.println(" C");
  uint8_t fault = rtd.readFault();
  if (fault) {
    Serial.print("Fault 0x");
    Serial.println(fault, HEX);
    if (fault & MAX31865_FAULT_HIGHTHRESH) Serial.println("  RTD high threshold (open element?)");
    if (fault & MAX31865_FAULT_RTDINLOW) Serial.println("  RTDIN- below 0.85 x VBIAS (open wire?)");
    if (fault & MAX31865_FAULT_OVUV) Serial.println("  Over or under voltage");
    rtd.clearFault();
  }
  delay(1000);
}
```

Compiled for esp32:esp32:esp32 on 2026-10-30.

### Raspberry Pi Pico

Libraries: adafruit/Adafruit MAX31865 library

```cpp
// MAX31865 on Raspberry Pi Pico: wiring and a free simulation at https://www.tinkered.ai/components/max31865
// MAX31865 on a Raspberry Pi Pico (arduino-pico): VIN 3V3(OUT), CLK GP18, SDO GP16, SDI GP19, CS GP17 (SPI0)
#include <SPI.h>
#include <Adafruit_MAX31865.h>

const int PIN_CS = 17;
const float RREF = 430.0;      // reference resistor on Adafruit's PT100 board (4300.0 for PT1000)
const float RNOMINAL = 100.0;  // PT100 at 0 C (1000.0 for PT1000)

Adafruit_MAX31865 rtd(PIN_CS);  // hardware SPI

void setup() {
  Serial.begin(115200);
  if (!rtd.begin(MAX31865_3WIRE)) {  // match the jumpers: MAX31865_2WIRE, _3WIRE or _4WIRE
    Serial.println("MAX31865 did not start. Check wiring.");
    while (true) delay(10);
  }
  Serial.println("MAX31865 ready, PT100 3-wire");
}

void loop() {
  uint16_t raw = rtd.readRTD();
  float ohms = RREF * raw / 32768.0;
  float tempC = rtd.temperature(RNOMINAL, RREF);
  Serial.print("RTD: ");
  Serial.print(ohms, 2);
  Serial.print(" ohm   Temperature: ");
  Serial.print(tempC, 2);
  Serial.println(" C");
  uint8_t fault = rtd.readFault();
  if (fault) {
    Serial.print("Fault 0x");
    Serial.println(fault, HEX);
    if (fault & MAX31865_FAULT_HIGHTHRESH) Serial.println("  RTD high threshold (open element?)");
    if (fault & MAX31865_FAULT_RTDINLOW) Serial.println("  RTDIN- below 0.85 x VBIAS (open wire?)");
    if (fault & MAX31865_FAULT_OVUV) Serial.println("  Over or under voltage");
    rtd.clearFault();
  }
  delay(1000);
}
```

Compiled for rp2040:rp2040:rpipico on 2026-10-30.

## Specifications

| Spec | Value | Source |
| --- | --- | --- |
| Chip supply | VDD and DVDD 3.0 V to 3.6 V | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 2 |
| Absolute maximum | -0.3 V to 4.0 V on VDD and DVDD; RTD inputs protected to ±45 V | Maxim MAX31865 datasheet 19-6478 Rev 3, pp. 1 to 2 |
| Resolution | 15-bit ADC, 0.03125 °C nominal | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 1 |
| Accuracy | 0.5 °C (0.05 % of full scale) max over all operating conditions | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 1 |
| Conversion time | Continuous: 20 ms typ, 21 ms max (50 Hz filter); 16.7 ms typ (60 Hz). Single: 52 ms typ (60 Hz) | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 3 |
| Sensors | PT100 to PT1000 in 2-, 3- or 4-wire connection | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 1 |
| Interface | SPI up to 5 MHz | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 4 |
| Supply current | 2 mA typical, 3.5 mA max with bias on during conversion | Maxim MAX31865 datasheet 19-6478 Rev 3, p. 3 |
| Breakout | 3.3 V regulator, level shifting on all inputs; Rref 430 Ω (PT100) or 4300 Ω (PT1000); 4-wire by default | Adafruit MAX31865 guide (PDF), pp. 5 to 8 |

## Circuit notes

- Resistance is RREF × code / 32768; the library converts it to °C with the Callendar-Van Dusen equation.
- Adafruit's PT100 board uses a 430 Ω reference where the datasheet's typical curves use 400 Ω (p. 6); the sketch's RREF must match the board.
- RDY goes low when a conversion is ready; the library polls instead, so RDY can stay unconnected.
- Several MAX31865 boards can share SCK, SDI and SDO with separate CS pins.

## Common mistakes

- **Wildly low or high readings.** The RTD is open or shorted, or the jumpers do not match the probe. Check readFault() and the wire count setting.
- **Readings off by a few degrees.** Wrong wire mode or wrong RREF. A 3-wire probe used in 2-wire mode adds both leads' resistance; a PT1000 board read with PT100 constants is far off.
- **Probe colours confused.** 3-wire PT100 probes usually have two wires of one colour (one end of the element) and one of another. The odd one goes alone; the matching pair shares the other end.
- **Noise from mains.** Pick the filter that matches your mains frequency with enable50Hz(true) where it is 50 Hz.

## Questions

### MAX31865 or MAX31855?

The MAX31865 reads RTDs: 0.5 °C total accuracy (datasheet, p. 1) over the PT100's range. The MAX31855K reads thermocouples: ±2 °C up to +700 °C, and it reaches +1350 °C. Use an RTD for precise process temperatures and a thermocouple for kilns.

### Does it work with PT1000?

Yes, the chip handles PT100 to PT1000 (p. 1). Use a board with a 4300 Ω reference (Adafruit's PT1000 version) and RNOMINAL 1000 in the sketch.

### How do I wire a 3-wire PT100?

Solder the 2/3 Wire jumper closed, cut the trace on the left side of the 2-way jumper above Rref and solder its right side, labelled 3 (Adafruit guide, p. 8), then call begin(MAX31865_3WIRE).

### Is the MAX31865 5 V compatible?

The chip runs from 3.0 V to 3.6 V (p. 2). Adafruit's breakout has a regulator and level shifting on all inputs, so it works with 5 V boards (guide, p. 6).

### How fast can it read?

About 50 conversions per second in continuous mode: 20 ms typical with the 50 Hz filter, 16.7 ms with the 60 Hz filter (p. 3).

## Related parts

- [MAX31855](https://www.tinkered.ai/components/max31855.md): Thermocouples for higher temperatures
- [Waterproof DS18B20](https://www.tinkered.ai/components/ds18b20-waterproof.md): Simpler digital probe to 125 °C
- [5V Relay](https://www.tinkered.ai/components/relay-5v.md): Control a heater
- [LCD 1602 I²C](https://www.tinkered.ai/components/lcd-1602-i2c.md): Show the reading

## Try it

Tinkered can design a circuit with the MAX31865, draw its schematic and check it. Free to start at https://www.tinkered.ai.