K Thermocouple+MAX31855+Arduino Mega Problem-PLEASE HELP ME!!!

Hi all!!!, I´m trying to use a K type thermocouple with an arduino Mega...For this I get a MAX31855 microchip for analogic-digital conversion and using SPI (not the module, only the chip). I'm using an Arduino Mega 1280, but I only get bad measurements (I tried different libraries in arduino 1.01 and Arduino 0023)....This is an example what I get (I heat the probe with my hand, and when I get bad measurements I stopped)-I think that it is clear:

And the connection diagram I'm using is:

I think that the error is in my connections, not in code....But if anyone can help me, Ill be pleased-I´m about a month trying to solve this....Thank you for your interest. :slight_smile:


What you've drawn here looks okay. Are you sure all connections are solid with no possibility of an intermittant break?


Thank you for your reply strykeroz....I changed all wires for new ones three times, with the same result....I think that it isn´t the problem, but I will change new for another class of wires....I'm very confused with this.

How are you connecting the wires? Is this on a breadboard or are they soldered?

Thank you for your reply strykeroz. They are connected in a breadboard as you can see inthe following photo
Do you recommend me to solder it or use other kind of wire?

I had a similar problem even when all the connections were soldered to a perf board. I think this chip is very sensitive to noise. I added some extra bypass caps but that did not improve much. I went to much higher temperatures than you did and got larger swings as the temperature increased. Also got some very high and low spikes. As the temperature went up, the readings were pretty much where they should be but drifted high and low in sort of a random way, so the chip was working but picking up noise somewhere.

I was told that the voltage divider might be the problem and maybe a level shifter might do better, but I never tried it.

Hope you have better results than I had.

The datasheet, in "Noise Considerations", states:
It is strongly recommended to add a 10nF (0.01uF) ceramic surface-mount differential cap, placed across the T+ and T- pins, in order to filter noise on the thermocouple lines.

Use any 0.01 ceramic you may have (keep leads as short as possible.)

I believe you ought to keep the thermocouple wires away from the Arduino and interface wires (SO, SI, /CS) and strive to minimise (reduce) their lengths.