While writing the software for my wife’s new kiln I encountered a blocker. The MAX31855 module I got off Ebay wasn’t cooperating.
The SPI coms worked fine but the values received as cold (internal) temperature and probe temperature looked like they were random. Ordered 2 and they were both duds. Thought it might be a 5v vs 3.3v issue on the data lines so used a resistor divider network on he Scl and Sd lines on the second module just in case I damaged the first. The culprit unit:
Then ordered another 2 from a different supplier. This time the chip kept on replying with a short to ground. After some investigation the probe negative line was shorted to ground by an intentional track to ground on the PCB. Cut that track to break the connection and voila, error gone.
Last challenge was to get a stable reading. Turns out a 10nF ceramic cap across the thermocouple at the connection to the PCB was good enough. The values were alternating from 15 to 32 degrees before the modification. After the change it was a stable 24 degrees.
Lessons learned:
- Test components received from Ebay before giving feedback.
- The optional cap recommended to reduce noise, is not optional.
- Only trust the OEM parts for fancy features like dual voltage support?