MAX6675 | GUILCOR SENSORS

MAX6675
Temperature sensors

Cold-junction compensated thermocouple-to-digital converter (Type K) with SPI interface, ideal for high-temperature industrial monitoring.

Maximum precision
+/- 2,0°K

Minimum temperature
0°C

Maximum temperature
+1024°C

Minimum dimensions
32 x 15 x 5

Response time

Fast

Drift

Low

Self-warming
Low

Price
Low

What is a MAX6675 sensor ?

The MAX6675 is a digital thermocouple converter that reads the voltage generated by a type K thermocouple and converts it into a 12-bit coded temperature value, transmitted via the SPI interface.

It is an indirect temperature sensor: it does not measure the temperature itself but interprets the thermoelectric voltage of the thermocouple.

This design allows for measurements of up to +1024 °C, with excellent stability and simple communication.

Operating principle

The MAX6675 converts the voltage generated by the K thermocouple (a few µV/°C) into temperature.

It performs:

1️⃣ a 12-bit analog-to-digital conversion,

2️⃣ automatic cold-junction compensation,

3️⃣ and then SPI transmission (MSB first).

T(°C) : Brut value x 0,25

where:

VIR = voltage measured by the thermopile
S = sensitivity factor
Tamb = internal temperature of the sensor

The reading is done in 16 bits, with the upper 12 bits containing the useful temperature.

Technical Specifications

Parameter	Typical value
Compatible thermocouple type	Type K
Compatible thermocouple type	0 → +1024 °C
Resolution	12 bits (0.25 °C per step)
Precision	±2 K (0 → +700 °C), ±3 K max beyond
Interface	SPI (3 fils : CS, SCK, SO)
Power supply voltage	3,0 → 5,5 V
Typical current	1,5 mA
Conversion time	220 ms
Cold welding compensation	Yes (internal)
Case	SO-8 or breakout module
Annual drift	Low (±0.2 K/year)

Wiring Configuration

Brooch	Name	Function
1	T-	Negative thermocouple input
2	T+	Positive thermocouple input
3	GND	Mass
4	VCC	Power Supply 3.3–5 V
5	CS	Chip Select (active low)
6	SO	SPI Output (MISO)
7	SCK	SPI Clock
8	-	NC

Self-warming

Very low (conversion current < 2 mA).

The MAX6675 is designed to minimize thermal noise and cold junction measurement errors.

Application areas

⚙️ 3D Printers and Heating Systems

🔥 Industrial Ovens and Cooking Instruments

🧠 Arduino, ESP32, STM32 Boards (Thermocouple Reading)

🧱 High-Temperature Measurement (Metallurgy, Welding, Extrusion)

💡 Automated Thermal Process Monitoring

Should I choose a MAX6675 sensor ?

Strengths points

🔥 Large temperature range up to +1024 °C
→ Compatible with K-type thermocouples, the MAX6675 provides reliable and stable measurements over a very wide range, ideal for high-temperature environments.
⚙️ Automatic compensation for cold welding
→ The sensor internally corrects errors related to the connection point temperature, eliminating the need for an additional sensor.
💡 Fast and Universal SPI Interface
→ Simple digital transmission over 3 wires (CS, SCK, SO), compatible with Arduino, ESP32, Raspberry Pi, STM32, and most microcontrollers.

Weaknesses points

🌡️ Beach limited above 0 °C
→ The MAX6675 does not support negative temperatures, unlike its successor (MAX31855).
📉 Average precision
→ An error of ±2 K remains acceptable for thermal control, but insufficient for fine metrology applications.
⏱️ Significant conversion time
→ Each measurement takes about 220 ms, which limits the sampling frequency in fast-loop systems.

Useful information

Here is some useful information regarding the MAX6675 sensors.

Resolution and precision

The MAX6675 converts the voltage generated by a type K thermocouple into a 12-bit digital signal transmitted via SPI.

Each least significant bit corresponds to 0.25 °C.

Parameter	Typical value
Resolution	12 bits (0,25 °C / LSB)
Precision	±2 K (0 → +700 °C) / ±3 K beyond
Repeatability	±0,5 K
Annual drift	±0,2 K
Conversion time	220 ms
Max. SPI Frequency	4,3 MHz
Cold welding compensation	Integrated
Entry Beach	0 → 1024 °C (Type K only)

🔹 Remarks:

The sensor does not measure negative temperatures.
The internal noise is very low (< 0.1 °C RMS).
Stable results even in high electromagnetic noise environments.

Example of application – Digital Reading SPI

The MAX6675 communicates over 16 bits via the SPI interface (3 wires).

Each reading provides a 16-bit binary word, where the 12 most significant bits contain the temperature, and the remaining bits are status indicators.

🔹 SPI word structure

Bit(s)	Name	Description
15 → 3	Temperature (12 bits)	Temperature value × 0.25 °C
2	Always 0	Reserved
1	Thermocouple error (D2)	1 if sensor disconnected
0	Sorted bit (D0)	Not used

🔹 Conversion equation

T(°C) = Brut Value x 0,25

Example:

SPI Reading → 0x07D0

0x07D0 = 2000_10

T = 2000 x 0,25 = 500°C

✅ Measured temperature = 500 °C

🔹 Thermocouple error

If bit D2 = 1, it means the thermocouple is disconnected or improperly inserted:

Check the continuity of the cable.
Check the polarities of the thermocouple (T+ and T−).

Integration into an electronic system

The MAX6675 is designed for easy integration with any microcontroller supporting SPI (Arduino, STM32, ESP32, Raspberry Pi, etc.).

No external calibration is required.

🔹 Typical components

Component	Function
MAX6675	K thermocouple converter → digital
Thermocouple type K	Temperature probe (0 to 1024 °C)
Microcontroller (MCU)	SPI Lecture and Display
Pull-up resistance (optional)	10 kΩ on CS for stability
Alimentation	3.3 V or 5 V (depending on module)

🔹 Functional diagram (ASCII)

+3.3V / +5V │ [Thermocouple K] T+ ───┐ │ [MAX6675] │ T− ───┘ │ GND ─────┴──────────┐ │ ┌───────────┴────────────┐ │ Microcontroller │ │────────────────────────│ │ CS → D5 │ │ SCK → D6 │ │ SO → D7 (MISO) │ │ VCC → 3.3V / 5V │ │ GND → GND │ └────────────────────────┘

🔹 Operating Principle

1️⃣ The microcontroller pulls CS to LOW.

2️⃣ The MAX6675 sends 16 bits on the SO line.

3️⃣ The microcontroller reads the 12 bits of temperature and calculates T=value×0.25T = value \times 0.25T=value×0.25.

4️⃣ The sensor automatically goes into sleep mode until the next reading (energy saving).

Practical advantage

Advantage	Description
🧠 Direct conversion Type K → digital	No external calibration or amplification required
⚡ Universal SPI Interface	Stable and fast communication
🔥 High temperature measurement (up to +1024 °C)	Adapted to industrial environments
💡 Automatic cold weld compensation	No external sensor required
🧩 Low cost and reduced footprint	Compact format (SO-8 or breakout module)
🔧 Easy to interface with Arduino, ESP32, Raspberry Pi	Ready-to-use libraries