I am trying to read the temperature of a shrink machine with three Max6675 using its library, displays it in a LCD and save the info into a USB memory, but there is a problem, randomly one or two sensors displays high temperatures, as 800 C o 900 C, i thought first that was the position of the sensor (touching any metal) but when i restart the Arduino, even if i dont touch the sensor it reads again the real temperature.

Horno_Rev1.ino (17 KB)

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I had a similar problem using a thermocouple embedded in a heater element - the AC mains capacitavely coupled to the thermocouple and upset the stabilty of the reading...

solution was to add a 100uF capacitor between the -ve thermocouple input and ground