Overview

The MAX31855 is a cold-junction compensated thermocouple-to-digital converter manufactured by Maxim Integrated (Analog Devices). Designed to interface with K-Type (and J, N, R, S, T, E variant) thermocouples, it measures extreme temperatures ranging from −270∘C-270^\circ\text{C} to +1372∘C+1372^\circ\text{C} in kilns, 3D printer hotends, ovens, and automotive exhaust systems.

Integrating a 14-bit Signed ADC, internal cold-junction temperature compensation sensor, open-thermocouple / short-circuit fault detection, and a read-only SPI interface (up to 5 MHz), the MAX31855 outputs temperature data in 0.25∘C0.25^\circ\text{C} resolution steps over a 32-bit SPI telemetry frame.

Quick reference

Operating voltage (VIN)3.3 V to 5.0 V DC (breakout includes 3.3V LDO regulator)
IC supply voltage (VDD)3.0 V to 3.6 V DC (3.3 V nominal)
InterfaceSPI Read-Only (3-wire interface: CS, SCK, SO / MISO)
Supported ThermocoupleK-Type (Chromel-Alumel) standard variant (MAX31855K)
Probe Temperature Range−270∘C to +1372∘C-270^\circ\text{C}\text{ to }+1372^\circ\text{C}
ADC Resolution14-bit Signed (0.25∘C0.25^\circ\text{C} LSB resolution)
Cold-Junction Range & Acc−40∘C to +125∘C-40^\circ\text{C}\text{ to }+125^\circ\text{C} (±2.0∘C\pm 2.0^\circ\text{C} accuracy)
Fault DetectionDetects open thermocouple, short to GND, and short to VCC

Pinout

Breakout module 5-pin header & 2-pin screw terminal block:

PinNameTypeDescription
1VINPowerSupply power input (+3.3 V to +5.0 V DC)
2GNDPowerGround reference (0 V)
3DO / SODigital OutputSPI Serial Data Output (MISO)
4CSDigital InputActive-Low SPI Chip Select
5CLK / SCKDigital InputSPI Serial Clock
+ / -Screw TerminalsAnalog InputThermocouple lead wire terminals (Yellow = +, Red = -)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply Voltage (Module)VVINV_{VIN}3.33.3 / 5.05.5VPower rail with LDO
Active Supply CurrentICCI_{CC}—1.52.5mASPI clock active
Thermocouple AccuracyTerrT_{err}-2.0±2.0\pm 2.0+2.0°CProbe temp 0∘C…700∘C0^\circ\text{C} \dots 700^\circ\text{C}
ADC ResolutionResADCRes_{ADC}—14—bitsSigned 14-bit output (0.25∘C0.25^\circ\text{C} step)
SPI Clock FrequencyfCLKf_{CLK}0—5.0MHzRead-only SPI transfer
Conversion Timetconvt_{conv}—100140msConversion interval

32-Bit SPI Telemetry Frame Structure

When CS goes Low, 32 bits of data are clocked out on SO on the falling edges of SCK:

  • Bits 31–18 (14 bits): Thermocouple 14-bit signed temperature data (0.25∘C0.25^\circ\text{C} resolution).
  • Bit 16: Fault flag (1=Fault present1 = \text{Fault present}).
  • Bits 15–4 (12 bits): Internal cold-junction reference temperature data (0.0625∘C0.0625^\circ\text{C} resolution).
  • Bit 2: Short to VCCV_{CC} fault flag.
  • Bit 1: Short to GND fault flag.
  • Bit 0: Open Circuit (Thermocouple Disconnected) fault flag.

Temperature Calculation (∘C^\circ\text{C})

Thermocouple Temp (∘C)=Bits 31..18 Signed 14-Bit Integer4.0\text{Thermocouple Temp } (^\circ\text{C}) = \frac{\text{Bits 31..18 Signed 14-Bit Integer}}{4.0}

Wiring

MAX31855 Pin→Arduino / MCUESP32Notes
VIN5V / 3.3V3.3VPower rail
GNDGNDGNDSystem ground
CSDigital D10GPIO 5SPI Chip Select
DO (MISO)Digital D12GPIO 19SPI MISO / Data Out
CLK (SCK)Digital D13GPIO 18SPI Clock

Example (Adafruit_MAX31855 Library)

cpp
#include <SPI.h>
#include "Adafruit_MAX31855.h"

#define MAXDO   19
#define MAXCS   5
#define MAXCLK  18

// Software SPI initialization
Adafruit_MAX31855 thermocouple(MAXCLK, MAXCS, MAXDO);

void setup() {
  Serial.begin(115200);
  Serial.println("Adafruit MAX31855 Thermocouple Test");
  delay(500);
}

void loop() {
  double c = thermocouple.readCelsius();

  if (isnan(c)) {
    uint8_t e = thermocouple.readError();
    if (e & MAX31855_FAULT_OPEN) Serial.println("FAULT: Thermocouple is OPEN (Disconnected)!");
    if (e & MAX31855_FAULT_SHORT_GND) Serial.println("FAULT: Thermocouple shorted to GND!");
    if (e & MAX31855_FAULT_SHORT_VCC) Serial.println("FAULT: Thermocouple shorted to VCC!");
  } else {
    Serial.print("Probe Temperature: "); Serial.print(c); Serial.println(" °C");
  }

  delay(1000);
}

Common mistakes

  • Inverting K-type wire polarity: K-Type thermocouple leads use ANSI color codes (Yellow = Positive +, Red = Negative -). Connecting Red to + causes temperature readings to drop when the probe is heated.
  • Grounded thermocouple probe shorts: Standard non-insulated metal braid thermocouples can short to grounded metal frames, triggering the SHORT TO GND fault bit. Use insulated probe sheaths.

Notes

  • MAX31855 vs MAX6675 vs MAX31856: MAX6675 is older (0°C to 1024°C only); MAX31855 supports negative temperatures (-270°C to +1372°C); MAX31856 supports all thermocouple types (K, J, N, R, S, T, E, B) via SPI write commands.

Assets & downloads