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I apologize in advance if this should instead go in the Arduino stack exchange, but I feel it should be here.

I'm using this thermocouple / amplifier pair with my Arduino Uno. I am trying to make my own version of Ben Heck's Solder Reflow Oven by modifying a black & decker toaster oven. I have almost everything set up and working, the one thing I have left to do is get consistent readings from my thermocouple.

My system consists of the following:

-Line, neutral, and gnd go into a 24V power supply. This goes to a 12v regulator and to the arduino. The reason I use 24V is because I was unsure at the time what the largest voltage requirement would be in my subsystem.

-the arduino is controlling a 12V relay and receiving readings from the thermocouple.

When the relay is flipped and the oven coils are receiving current, my thermocouple readings drop by about 15-25 degrees C (this value kind bounces around, but is consistently less). When I turn off the system and power the arduino via usb, my thermocouple reads a nice, consistent value (slowly decreasing as the oven cools).

So it would seem that the power required to heat the oven coils is in some way affecting the thermocouple, but I don't really know much about fixing these types of issues.

What could be causing this?

My only thoughts are either a) it's caused by sucking power away from the arduino because the oven requires so much power or b) since the coils operate at around 120V 10A they may be causing some sort of EMI issue, which I know literally nothing about fixing.

Can I get some insight as to what my problem may be and some suggestions on how to fix it?

edit:

here's a super simple diagrom showing the connections in the entire system

enter image description here

The relay, once flipped, connects the live line together, which is what turns on the oven.

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    \$\begingroup\$ Thermocouple / amplifier? The link says you can read it out via SPI. Do you do that? And does it always get its 3-3.6V? Decoupling caps? How is the relay driven? Maybe, you can add a (simplified) schematic showing how you connect the module and the relay... \$\endgroup\$ – sweber Apr 26 '15 at 6:52
  • \$\begingroup\$ MAX31855 is a cold-junction compensated thermocouple-to-digital (SPI) converter. SainSmart's board looks like a stripped-down version of Maxim's MAX31855PMB1 Peripheral Module, without the filtering/protection passives (D1, L1, L2, C1, C3, C4), and using a cheaper connector; though they did include a K-type thermocouple. \$\endgroup\$ – MarkU Apr 26 '15 at 8:10
  • \$\begingroup\$ What about the MAX31855's internal (cold junction) temperature (bits D15-D04)? That reading should be room temperature regardless of what the oven is doing. The board itself is not being heated by the oven, right? \$\endgroup\$ – MarkU Apr 26 '15 at 8:10
  • \$\begingroup\$ @MarkU So should I just try to take the chip off and make my own board with all the caps, inductors, and diodes that I need? Also, the chip should not be getting heated up as I have a separate housing from the oven. \$\endgroup\$ – Jake Apr 26 '15 at 16:27
  • \$\begingroup\$ @sweber adding a simple connection map for the whole system. \$\endgroup\$ – Jake Apr 26 '15 at 16:38
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I don't see any earths at all in your pictorial sketch. If the output of the 24V supply does not have the output common with earth, the first thing to do is to ground the analog ground of your PCB. If that doesn't make sense to you, ground the (-) output of the 24V supply.

If that does not improve things, and you cannot shield the thermocouple with a braid, try connecting each side of the thermocouple to the ground pin of the Maxim chip through a 100nF capacitor.

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