MAX31855K Thermocouple Breakout Hookup Guide
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Brief Theory of Operation
Roughly a couple hundred years ago, a man named Thomas Seebeck discovered the principal that thermocouples use. He noticed that if you take two wires made of dissimilar metals, connect them at the two ends, and make a temperature gradient between one end and the other, a voltage potential formed, and current flowed. One junction is held in the environment where the temperature of interest exists. This is known as the hot junction. The other junction is referred to as the cold junction.
There are many types of thermocouples, which mainly differ by the types of metals used in the two wires. The most common general purpose thermocouple is type K. They are made out of chromel and alumel. These two alloys produce a potential of approximately 41.276 µV/°C. The MAX31855K uses this linear approximation to calculate the temperature.
The thermocouple’s hot junction can be read from -200°C to +700°C with an accuracy of ±2°C. The cold junction is inside the MAX31855K and can only range from -20°C to +85°C while maintaining ±2°C accuracy. The MAX31855K constantly measures the temperature of the cold junction using an internal temperature-sensing diode. The internal 14-bit ADC uses the above equation, the voltage across the internal diode, and the amplified voltage of the thermocouple to solve for the hot junction temperature.
The calculated temperature is clocked out the SO pin in a SPI compatible format (half-duplex). When not feeding out data, this pin is tri-stated and will ignore any inputs from the master. A reading of 0b 0000 0000 0000 corresponds to 0°C. Whereas 0b 01 1001 0000 0000 corresponds to a measured temperature of +1600.00°C, and 0b 11 1100 0001 1000 corresponds to a measured temperature of -250.00°C.