MAX31855K reported temp dropping when AC current applied

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MAX31855K reported temp dropping when AC current applied

Post by zedix » Tue Sep 12, 2017 5:55 pm

I'm trying to use a Raspberry Pi, MAX31855K thermocouple amplifier, and solid state relay to control an espresso machine's boiler using a PID.

However I'm hitting a frustrating snag. I have everything hooked up and temps are reported properly (I'm reading temps using python libraries off the Pi). Heating up the probe reports a higher temperature, and vice versa, so I know the leads are hooked up properly.

The problem happens when I plug the espresso machine in the wall. The MAX31855K immediately starts reporting a temperature ~20F too low (instant temperature read drop). As soon as I unplug the machine, the temp returns to its accurate value.

I was using this thermocouple ( ... cts_id=307) which has a stainless steel sleeve, and appears to be a "grounded" thermocouple, which apparently the MAX31855K doesn't support - even though I'm following plans made by someone who used this thermocouple succesfully. I suspect its still supposed to work since the espresso machine's ground is not the same as the breakout board's ground (the 'floating' raspberry pi ground).

Fair enough, I bought a new k type thermocouple that is ungrounded - and same issue!!

At this point, I don't know what to do. Could the MAX31855K breakout board be defective? Anything else I could check? Here's a circuit diagram:


Thanks in advance!

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