MAX31855 | GUILCOR SENSORS

MAX31855
Temperature sensors

Precision thermocouple-to-digital converter with SPI output, supporting multiple thermocouple types for accurate temperature measurement.

Maximum precision
+/- 1,5°K

Minimum temperature
-270°C

Maximum temperature
+1372°C

Minimum dimensions
32 x 15 x 5

Response time

Fast

Drift

Low

Self-warming
Low

Price
Medium

What is a MAX31855 sensor ?

The MAX31855 is a next-generation SPI digital thermocouple converter designed to measure the ultra-low voltage generated by a type K thermocouple (or J, N, T depending on the version) and convert it directly into a readable temperature.

It replaces the MAX6675 by offering:

better accuracy,
the ability to read negative temperatures,
and a 32-bit SPI output with integrated error codes.

Operating principle

The MAX31855 measures the voltage of the thermocouple and the temperature of the internal cold junction (integrated sensor).

The conversion equation is based on an internal polynomial model derived from the ITS-90 tables for type K.

The SPI reading returns a 32-bit word containing:

the temperature of the thermocouple,
the reference temperature (cold junction),
and an automatic error code (open thermocouple, short-circuited, etc.).

Technical Specifications

Parameter	Typical value
Compatible thermocouple type	Type K
Thermocouple measurement range	−270 → +1372 °C
Internal temperature range (cold junction)	−55 → +125 °C
Resolution	14 bits (0,25 °C / LSB)
Precision	±1,5 K (−200 → +700 °C)
Interface	SPI (32 bits, read-only)
Power supply voltage	3,0 → 3,6 V
Typical current	1,5 mA
Cold welding compensation	Yes (internal, built-in sensor)
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	3.3 V Power Supply
5	SO	SPI Output (MISO)
6	CS	Chip Select
7	SCK	SPI Clock
8	-	NC

Self-warming

The internal current is less than 2 mA: the internal temperature varies by less than 0.1 °C per conversion cycle.

Ideal for high-precision thermal measurement systems.

Application areas

⚙️ Industrial process control (extrusion, molding, welding)

🔥 High-temperature monitoring (oven, engine, plastic injection)

🧠 3D printers and IoT thermal measurement systems

🧪 Scientific or thermal calibration instrumentation

💡 Mixed cold/hot applications (−200 → +1300 °C)

Should I choose a MAX31855 sensor ?

Strengths points

🌡️ Ultra-wide measurement range (−270 → +1372 °C)
→ Compatible with type K thermocouples, it covers both cryogenic applications and very high-temperature processes.
🎯 Improved precision and 14-bit reading
→ Thanks to a resolution of 0.25 °C and finer cold junction compensation, the MAX31855 offers superior stability compared to the MAX6675, even over long periods.
⚙️ Complete 32-bit SPI Interface
→ The SPI output provides the thermocouple temperature, the cold junction temperature, and a built-in error code, facilitating diagnostics and remote monitoring.

Weaknesses points

💧 Power supply only 3.3 V
→ Unlike the MAX6675, the MAX31855 does not accept 5 V, which sometimes requires a level shifter with 5 V microcontrollers (such as the Arduino Uno).v
📉 Read-only mode
→ The SPI communication is unidirectional: the sensor sends its data but does not receive any commands — which limits advanced interactions.
💰 Slightly higher cost
→ Its precision and thermal protections make it a slightly more expensive sensor, but largely justified for demanding applications.

Useful information

Here is some useful information regarding the MAX31855 sensors.

Resolution and precision

The MAX31855 performs a signed 14-bit conversion of the voltage generated by the type K thermocouple, with internal cold junction compensation.

The temperature of the thermocouple and that of the cold junction are transmitted together on 32 bits.

Parameter	Valeur typique
Thermocouple resolution	14 bits (0,25 °C / LSB)
Cold junction resolution	12 bits (0,0625 °C / LSB)
Thermocouple beach	−270 → +1372 °C
Cold junction beach	−55 → +125 °C
Precision	±1,5 K (−200 → +700 °C)
Annual drift	±0,2 K
Conversion time	220 ms typical
Max. SPI Frequency	5 MHz
Alimentation	3,0 → 3,6 V

🔹 Remarks:

Negative temperatures are natively managed (signed values).
Cold solder compensation is automatic and factory calibrated.
Each SPI reading corresponds to a complete conversion.

Example of application – Digital reading SPI

The MAX31855 returns a 32-bit word with each SPI read.

This data contains the thermocouple temperature, the cold junction, and error codes.

🔹 Structure of the 32-bit word

Bits	Name	Description
[31 → 18]	Thermocouple temperature (14 signed bits)	Value × 0.25 °C
[17 → 4]	Cold junction temperature (12 signed bits)	Value × 0.0625 °C
[3]	Reserved	—
[2]	SCV Error	Short circuit to VCC
[1]	SCG Error	Short circuit to GND
[0]	OC Error	Open thermocouple

🔹 Conversion Equation

Thermocouple :

T_TC(°C) = Value_14b x 0,25

Cold junction:

T_TC(°C) = Value_12b x 0,625

Total temperature:

TTOTAL = TTC + TCJ

🔹 Practical example

Lecture SPI :

0x0190_0C50

Cutting :

Thermocouple : 0x0190 = 400₁₀
Cold junction : 0x0C5 = 197₁₀

T_TC = 400 × 0.25 = 100 °C

T_CJ = 197 × 0.0625 = 12.3 °C

T_TOTAL = 100 + 12.3 = 112.3 °C

✅ Measured temperature: 112.3 °C

Integration into an electronic system

The MAX31855 communicates via SPI (read-only).

It can be interfaced with any microcontroller that has an SPI interface or a compatible software library.

🔹 Typical components

Composant	Function
MAX31855	K thermocouple converter → SPI digital
Thermocouple type K	High temperature probe
Microcontroller (3.3 V)	Lecture SPI (ESP32, STM32, Raspberry Pi)
Logic level converter (5 V ↔ 3.3 V)	Optional for Arduino compatibility
Stabilized feed	3,3 V DC
Decoupling capacitor (100 nF)	Noise reduction on VCC

🔹 Functional diagram (ASCII)

+3.3V │ [Thermocouple K] T+ ───┐ │ [MAX31855] │ T− ───┘ │ GND ─────┴───────────┐ │ ┌─────────────┴─────────────┐ │ Microcontroller │ │───────────────────────────│ │ CS → D5 (GPIO OUT) │ │ SCK → D6 (SPI CLK) │ │ SO → D7 (MISO) │ │ VCC → 3.3V │ │ GND → GND │ └───────────────────────────┘

💡 Plan for a short connection between the thermocouple and the module to minimize noise and µV losses.

🔹 Operating principle

1️⃣ The microcontroller sets CS = LOW.

2️⃣ The MAX31855 sends 32 bits: thermocouple temperature, cold junction, error codes.

3️⃣ The microcontroller reads the data and converts it according to the equations above.

4️⃣ The sensor then returns to sleep mode until the next reading.

Practical advantage

Advantage	Description
🧠 Negative and positive lecture (−270 → +1372 °C)	Compatible with all thermal scenarios
⚡ Fast and stable SPI	Up to 5 MHz, ideal for real-time applications
🔥 Integrated cold welding compensation	No external calibration required
🧩 Automatic error code	Immediate diagnosis (open, short-circuited, etc.)
💡 Improved accuracy (±1.5 K)	Ideal for industrial processes and laboratories
⚙️ Direct successor of the MAX6675	Pin-to-pin compatible and software equivalent