Overview

The MAX6675 is a classic cold-junction-compensated K-type thermocouple-to-digital converter IC manufactured by Maxim Integrated (Analog Devices). Bundled in low-cost 3D printer hotend kits (Marlin firmware), ceramics kilns, and Arduino electronics kits, it measures high temperatures from 0∘C0^\circ\text{C} to +1024∘C+1024^\circ\text{C} (32∘F32^\circ\text{F} to 1875∘F1875^\circ\text{F}).

Integrating a 12-bit Analog-to-Digital Converter (ADC), cold-junction compensation diode, open-thermocouple detection circuitry, and a simple read-only 3-wire SPI interface (CS, SCK, SO), the MAX6675 converts thermocouple microvolt signals into 0.25∘C0.25^\circ\text{C} digital temperature steps.

Quick reference

Operating voltage (VCC)3.0 V to 5.5 V DC (5.0 V nominal)
InterfaceSPI Read-Only (3-wire: CS, SCK, SO / MISO)
Supported ThermocoupleK-Type (Chromel-Alumel)
Probe Temperature Range0∘C to +1024∘C0^\circ\text{C}\text{ to }+1024^\circ\text{C}
ADC Resolution12-bit (0.25∘C0.25^\circ\text{C} resolution per LSB)
Conversion Time220 ms maximum per sample
Cold-Junction Compensation−20∘C to +85∘C-20^\circ\text{C}\text{ to }+85^\circ\text{C} (±3.0∘C\pm 3.0^\circ\text{C} accuracy)
Open Thermocouple DetectionBit 2 of 16-bit SPI frame sets to 1 if probe disconnected

Pinout

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

PinNameTypeDescription
1VCCPowerSupply power input (+3.0 V to +5.5 V DC)
2GNDPowerGround reference (0 V)
3SO / MISODigital OutputSPI Serial Data Output
4CSDigital InputActive-Low SPI Chip Select
5SCKDigital InputSPI Serial Clock
+ / -Screw TerminalsAnalog InputThermocouple lead wire terminals (Yellow = +, Red = -)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply VoltageVCCV_{CC}3.05.05.5VDC
Active Supply CurrentICCI_{CC}—0.71.5mAContinuous conversion
Cold-Junction Temp ErrorTCJ_errT_{CJ\_err}-3.0±3.0\pm 3.0+3.0°CTambient=25∘CT_{ambient} = 25^\circ\text{C}
Thermocouple ConversionTconvT_{conv}0—1024°CFull scale reading
LSB ResolutionLSBtempLSB_{temp}—0.25—°C1/4∘C1/4^\circ\text{C}
Conversion Timetconvt_{conv}—170220msInterval between conversions

16-Bit SPI Telemetry Protocol

To read data, drive CS Low and supply 16 clock pulses on SCK:

  • Bit 15: Dummy sign bit (Always 0).
  • Bits 14–3 (12 bits): Thermocouple 12-bit unsigned temperature data (0.25∘C0.25^\circ\text{C} per LSB).
  • Bit 2: Open-Thermocouple Detection Flag (1=Probe Disconnected1 = \text{Probe Disconnected}, 0=Normal0 = \text{Normal}).
  • Bit 1: Device ID bit (Always 0).
  • Bit 0: Three-state buffer flag.

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

Temperature (∘C)=Raw 16-bit Word ≫34.0\text{Temperature } (^\circ\text{C}) = \frac{\text{Raw 16-bit Word } \gg 3}{4.0}

Wiring

MAX6675 Pin→Arduino UnoESP32Notes
VCC5V / 3.3V3.3VPower rail
GNDGNDGNDSystem ground
CSDigital D10GPIO 5SPI Chip Select
SO (MISO)Digital D12GPIO 19SPI Data Out
SCK (CLK)Digital D13GPIO 18SPI Clock

Example (MAX6675 Arduino Library)

cpp
#include "max6675.h"

int thermoSO  = 12; // MISO
int thermoCS  = 10; // CS
int thermoCLK = 13; // SCK

MAX6675 thermocouple(thermoCLK, thermoCS, thermoSO);

void setup() {
  Serial.begin(9600);
  Serial.println("MAX6675 Thermocouple Test");
  delay(500); // Wait for MAX6675 chip stabilization
}

void loop() {
  float tempC = thermocouple.readCelsius();
  float tempF = thermocouple.readFahrenheit();

  if (isnan(tempC)) {
    Serial.println("MAX6675 Error: Thermocouple probe disconnected!");
  } else {
    Serial.print("Probe Temp: "); Serial.print(tempC); Serial.print(" °C | ");
    Serial.print(tempF); Serial.println(" °F");
  }

  // MAX6675 conversion time requires 250ms delay between reads
  delay(300);
}

Common mistakes

  • Polling faster than 220 ms: The MAX6675 ADC requires 220 ms220\text{ ms} to perform a single conversion. Polling the SPI interface faster than 4 Hz causes the IC to return duplicate old temperature readings.
  • Negative temperature measurement failure: The MAX6675 cannot measure temperatures below 0∘C0^\circ\text{C}. For sub-zero temperature monitoring (e.g. freezer telemetry), use the MAX31855 or MAX31856.

Notes

  • MAX6675 vs MAX31855: MAX6675 measures 0∘C0^\circ\text{C} to +1024∘C+1024^\circ\text{C}; MAX31855 measures −270∘C-270^\circ\text{C} to +1372∘C+1372^\circ\text{C}.

Assets & downloads