MAX6675 Thermocouple Amplifier

The MAX6675 cold-junction compensated thermocouple amplifier converts a K-type thermocouple reading to a 12-bit SPI word. It's the classic solution for measuring temperatures up to 1024°C.

Wiring is straightforward: T+ and T- to the thermocouple, SPI to the microcontroller. The CS pin is active-low — pull it low, read 16 bits, pull it high. The top bit is always zero; the next 12 bits are the temperature in units of 0.25°C; bit 2 is an open-circuit fault flag.

Accuracy is ±3°C from 0-700°C and ±6°C beyond. For a reflow oven controller, this is marginal — the solder joint temperatures during reflow vary by ±5°C anyway, so ±3°C sensor accuracy is acceptable. For any precision application, the MAX31856 (±0.7°C) or a calibrated PT100 RTD is better.