MAX6675 vs ££: K-Type Thermocouples with ESP32

The MAX6675 and MAX31855 are two of the most common digital interface ICs used to connect a K-type thermocouple to an ESP32. Both provide cold-junction compensation and a simple read-only SPI interface, but the MAX31855 is the newer and more capable device.

The biggest differences are temperature range, negative-temperature support and fault detection. The MAX6675 measures K-type thermocouples from 0°C to about 1024°C, while the K-type MAX31855 covers -200°C to +1350°C. Both resolve thermocouple temperature in 0.25°C steps.

For a new ESP32 project, the MAX31855 is usually the better choice unless you already own MAX6675 modules or only need simple positive-temperature measurement.

Quick Comparison

Feature	MAX6675	MAX31855K
Thermocouple type	K only	K version; other MAX31855 variants exist for other types
Measured K-type range	0°C to +1023.75°C	-200°C to +1350°C
Resolution	0.25°C	0.25°C
Output	12-bit thermocouple result	Signed 14-bit thermocouple result + reference temperature + faults
Cold-junction compensation	Yes	Yes
Open thermocouple detection	Yes	Yes
Short-to-GND detection	No dedicated diagnostic	Yes
Short-to-VCC detection	No dedicated diagnostic	Yes
SPI-compatible	Yes, read-only	Yes, read-only
Typical supply	3.0–5.5 V IC range	3.0–3.6 V
Native ESPHome	Yes	Yes
Best fit	Low-cost positive-temperature projects	New designs, wider range, better diagnostics

Which One Should You Choose?

Project	Better choice	Reason
General K-type temperature monitor	MAX31855	Wider range and better fault detection
Oven / smoker / kiln below 1000°C	Either	Both cover the range
Freezer / cryogenic / sub-zero test	MAX31855	MAX6675 does not measure below 0°C
Automotive exhaust temperature	MAX31855	Higher range and better diagnostics
Existing cheap MAX6675 board	MAX6675	No reason to replace it if the range is adequate
Need other thermocouple types	MAX31855 family	Versions exist for J, N, T, S, R and E types

What the Converter Actually Does

A thermocouple does not output a useful digital temperature. It generates a tiny voltage related to the temperature difference between the hot junction and the connection point at the electronics.

Hot junction
   │
   │ tiny thermoelectric voltage
   ▼
K-type thermocouple wire
   │
   ▼
MAX6675 / MAX31855
   │
   ├─ thermocouple voltage measurement
   ├─ local cold-junction temperature
   ├─ compensation
   └─ digital conversion
           │
           ▼
         ESP32

The converter therefore has two jobs: measure the very small thermocouple voltage and compensate for the temperature of the terminals where the thermocouple connects to the board.

Cold-Junction Compensation

Cold-junction compensation is the reason board temperature matters. The IC measures the temperature near the thermocouple terminals and uses it as the reference for the thermocouple calculation.

Keep the converter away from ESP32 regulators, power MOSFETs and hot relays.
Avoid mounting the module on a metal plate that is heated by the process being measured.
Do not place one thermocouple terminal beside a hot component and the other beside a cold airflow path.
Let the connector and converter sit at a reasonably uniform local temperature.

A mathematically perfect thermocouple calculation can still be wrong if the cold junction is physically sitting in a strong temperature gradient.

MAX6675 Temperature Range

The MAX6675 is specifically designed for K-type thermocouples and reports hot-junction temperature from 0°C to 1023.75°C with 0.25°C resolution.

That makes it suitable for many ovens, boilers, exhaust tests, furnaces and process-monitoring projects, but it is not useful when the measured temperature can go below freezing.

Its rated thermocouple accuracy is much coarser than the 0.25°C displayed resolution; resolution and absolute accuracy are not the same thing.

MAX31855 K-Type Temperature Range

The K-type MAX31855 covers a much wider -200°C to +1350°C measured range. Analog Devices specifies ±2°C thermocouple accuracy over -200°C to +700°C for the K-type device, with full-range accuracy defined separately in the datasheet.

The signed output also makes sub-zero measurements straightforward, which is impossible with the MAX6675’s positive-only result.

MAX31855 Supports More Than K-Type — but the Chip Must Match

The MAX31855 family includes variants for K, J, N, T, S, R and E thermocouples. That does not mean one MAX31855 board automatically supports every thermocouple type.

A MAX31855K must be paired with a K-type thermocouple. A MAX31855J is calibrated for J-type behaviour, and so on. Check the actual IC marking or module specification.

Fault Detection: MAX31855 Is Much Better

The MAX6675 can report an open thermocouple. The MAX31855 goes further and identifies three useful fault conditions:

Thermocouple open circuit.
Thermocouple shorted to ground.
Thermocouple shorted to VCC.

For permanent installations, this diagnostic information is valuable because a broken probe can otherwise look like a strange temperature event rather than an obvious wiring fault.

Important Grounding Difference

The two chips do not treat the thermocouple negative lead in exactly the same way.

The MAX6675 datasheet says T− should be connected to ground externally, and that connection is required for its open-thermocouple detector to operate correctly.

The MAX31855 datasheet instead says do not connect T− directly to ground in the normal configuration. It is designed around a floating/ungrounded thermocouple input so it can detect shorts to ground and VCC.

This matters with metal-sheathed probes. If the thermocouple junction is electrically bonded to a grounded exhaust, chassis, pipe or heater body, the MAX31855 can report a ground fault or suffer ground-reference errors.

Grounded vs Ungrounded Thermocouple Probes

Probe type	Junction electrical connection	Practical effect
Ungrounded	Junction insulated from metal sheath	Best fit for MAX31855 and electrically noisy/grounded machinery
Grounded	Junction bonded to metal sheath	Faster response but can create ground-loop/fault issues
Exposed	Bare junction exposed to process	Fast response, electrically exposed and less mechanically protected

For a MAX31855 project on machinery, an ungrounded thermocouple probe is normally the lowest-risk choice.

SPI Wiring to ESP32

Both devices use a simple read-only SPI-style interface. You need clock, MISO/data-out and one chip-select pin. MOSI is not required.

ESP32             MAX6675 / MAX31855

3.3V  ------------- VCC
GND   ------------- GND
GPIO18 ------------ SCK / CLK
GPIO19 <----------- SO / MISO
GPIO5  ------------ CS

Thermocouple:
T+ ---------------- positive lead
T- ---------------- negative lead

ESPHome explicitly recommends 3.3 V supply for both common modules, which keeps SPI output levels compatible with the ESP32. The bare MAX6675 IC can operate from a wider supply range, but the complete breakout-board design still needs to be checked.

ESPHome MAX6675 Configuration

spi:
  clk_pin: GPIO18
  miso_pin: GPIO19

sensor:
  - platform: max6675
    name: "Exhaust Temperature"
    cs_pin: GPIO5
    update_interval: 5s

ESPHome’s MAX6675 component is intentionally simple. The sensor performs the conversion internally and ESPHome periodically reads the result over SPI.

ESPHome MAX31855 Configuration

spi:
  clk_pin: GPIO18
  miso_pin: GPIO19

sensor:
  - platform: max31855
    name: "Exhaust Temperature"
    cs_pin: GPIO5
    update_interval: 5s

    reference_temperature:
      name: "Thermocouple Board Temperature"

The MAX31855 component can also expose the internal reference temperature used for cold-junction compensation. That is useful when debugging strange offsets caused by the converter board being heated by nearby electronics.

You Can Share the SPI Bus

Several thermocouple converters can share the same clock and MISO lines as long as each one has its own chip-select pin.

ESP32 SCK  ───── MAX31855 #1 SCK
          └─────── MAX31855 #2 SCK
          └─────── MAX31855 #3 SCK

ESP32 MISO <────── shared SO bus

GPIO5  ─────────── CS #1
GPIO16 ─────────── CS #2
GPIO17 ─────────── CS #3

This is useful for multi-cylinder exhaust gas temperature, multi-zone ovens or several heater elements.

Conversion Speed

Neither device is intended to be a high-speed waveform sensor. ESPHome’s current drivers allow roughly a couple of hundred milliseconds for a conversion before reading the result.

For process temperature this is normally fine. Even a 1-second update interval is fast compared with the thermal time constant of many metal-sheathed probes.

Home Assistant also does not benefit from storing 20 temperature samples per second from a slowly changing oven.

Thermocouple Polarity Matters

If T+ and T− are reversed, the measured thermocouple voltage changes sign. At room temperature the error may appear confusing because the cold-junction compensation is still active.

Follow the connector/module T+ and T− markings.
Do not rely only on wire colour unless you know which thermocouple colour standard is being used.
Use proper K-type extension wire if the cable must be lengthened.
Avoid inserting ordinary copper wire into a temperature-gradient region because that creates additional thermoelectric junctions.

Why Cheap K-Type Probes Often Limit the System

The converter may be rated above 1000°C, but the probe cable, insulation and sheath may not be.

A low-cost bead probe with PVC or fibreglass insulation can have a much lower continuous temperature rating than the MAX31855 or MAX6675.

For exhaust gas, kiln or furnace work, choose the probe sheath and cable insulation for the real temperature and chemical environment—not just the interface chip.

Noise: Thermocouple Signals Are Tiny

K-type thermocouples generate only tens of microvolts per degree Celsius. Long cables running beside ignition coils, motors, contactors, inverters or heater wiring can pick up substantial interference.

Use twisted thermocouple extension cable.
Keep thermocouple wiring away from mains, ignition and motor conductors.
Do not run the thermocouple in the same loom as injector or ignition-coil wiring if avoidable.
Place the converter physically close to the thermocouple connector where practical.
Decouple the converter supply close to the IC/module.
Use a grounded shield carefully; avoid creating a ground loop through the probe.

Automotive Exhaust Gas Temperature

For EGT measurement on an engine, the MAX31855 is normally the better choice because its K-type range extends to 1350°C and it provides better diagnostics.

Use a probe specifically rated for exhaust gas temperature and vibration. An ungrounded junction is often easier to interface cleanly to the MAX31855 when the exhaust manifold and vehicle chassis are already grounded.

Do not place the thermocouple converter next to the exhaust. Keep the electronics in a cooler protected location and run proper thermocouple extension cable to the probe.

Ovens, Smokers and Kilns

For a smoker or domestic oven below a few hundred degrees Celsius, either sensor works.

For kilns or higher-temperature ovens, the MAX31855’s additional range is useful, but the probe remains the critical component. Many inexpensive K-type probes fail long before the converter’s upper limit.

MAX6675 Open-Thermocouple Detection

The MAX6675 reports an open probe through its status bit, but the datasheet requires T− to be grounded for that detector to work.

If a MAX6675 module produces full-scale or fault readings after rewiring a probe, check both thermocouple continuity and the module’s T−/ground arrangement.

MAX31855 Faults in ESPHome

ESPHome’s MAX31855 driver checks the chip’s fault flags. When it detects an open circuit, short to ground or short to VCC, it publishes an invalid temperature rather than quietly treating the fault as a real process temperature.

That makes Home Assistant automations safer because a broken thermocouple is less likely to masquerade as a valid number.

Common Problem: Temperature Is Much Too Low or Moves Backwards

Thermocouple polarity is reversed.
Wrong thermocouple type for the converter.
Ordinary copper cable has been inserted in the wrong part of the thermocouple circuit.
Grounded probe is causing a MAX31855 fault/reference problem.
Cold-junction area is being heated or cooled artificially.

Common Problem: Reading Jumps When a Motor or Relay Switches

Thermocouple cable is picking up EMI.
SPI wiring is too long or poorly grounded.
Converter power supply is noisy.
Thermocouple shield is creating a ground loop.
Probe is grounded to machinery at a different electrical reference.

Fix the physical noise path before hiding the problem with heavy software filtering.

Common Problem: MAX31855 Reports Short to Ground

The first thing to check is whether the probe is actually a grounded-junction thermocouple. A metal sheath can be electrically bonded to the thermocouple junction even when the two lead wires look normal.

Measure resistance from each thermocouple lead to the probe sheath with power disconnected. If there is continuity, the probe is grounded or damaged.

MAX6675 vs MAX31855 Accuracy

Both output temperatures in 0.25°C increments, but that does not mean both are accurate to 0.25°C.

The MAX31855 is the better choice when accuracy and diagnostics matter, but total system error still includes the thermocouple tolerance, cold-junction error, thermal gradients and installation effects.

If you need genuinely high-accuracy temperature metrology rather than rugged high-temperature monitoring, an RTD such as PT100/PT1000 with a suitable interface may be more appropriate.

When MAX6675 Still Makes Sense

You already have working MAX6675 breakout boards.
The process stays above 0°C and below 1000°C.
You only need one basic K-type channel.
Open-circuit detection is sufficient.
Cost and module availability matter more than advanced diagnostics.

When MAX31855 Is Clearly Better

Sub-zero temperature measurement.
Automotive EGT.
Higher-temperature K-type measurement above 1000°C.
You want explicit open/short fault detection.
You want the board’s internal reference temperature exposed in ESPHome.
You are designing a new system rather than reusing old hardware.

Final Recommendation

For a new ESP32 K-type thermocouple project, choose the MAX31855. It covers a much wider K-type range, supports negative temperatures, provides stronger wiring diagnostics and integrates cleanly with ESPHome.

Use the MAX6675 when you already have the hardware or when a simple 0–1000°C monitor is all you need. It remains easy to use and fully supported, but it is the older and more limited device.

Whichever converter you choose, the probe and installation usually determine the quality of the final result. Use the correct thermocouple type, preserve polarity, keep the cold junction thermally stable and pay serious attention to grounding and electrical noise.

Related ESP32 Guides

Datasheets and External Resources

Analog Devices MAX6675 product page — official 0°C to +1024°C K-type specifications.
MAX6675 datasheet — cold-junction compensation, grounding, open-circuit detection and noise guidance.
Analog Devices MAX31855 product page — current device family and thermocouple options.
MAX31855 datasheet — K-type -200°C to +1350°C range and fault diagnostics.
ESPHome MAX6675 component — native SPI configuration.
ESPHome MAX31855 component — native SPI configuration and reference-temperature sensor.