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Author Topic: Thermocouple temperature sensor/controller  (Read 1636 times)
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Hello,

I am relatively new to the Arduino world.  Currently, have a type K thermocouple attached to a 3D printer extruder I purchased (Mk7 Extruder) and I wish to create a temperature controller for it (without purchasing one).  I have read that the voltage output of the thermocouple will be too small for the Arduino to read, thus an amp is needed.  I was looking into an op amp, however I am not sure exactly how I would implement it and set up the circuit on my breadboard.  After getting a temperature reading I would then like to implement a PID controller. 

I did see this: 

https://www.sparkfun.com/products/306

However, I would like to keep it simpler using an op amp as below if possible:

https://www.sparkfun.com/products/9456

If anyone could point me in the right direction I would greatly appreciate it!

Thanks,
Nick
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The last thing you did is where you should start looking.
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http://www.adafruit.com/index.php?main_page=adasearch&q=Thermocouple
http://learn.adafruit.com/thermocouple
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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I would go for a MAX31855-based solution, such as the Adafruit board linked to in the previous post.
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Thank you for your input guys.  I did see that amp, however is there a way to create my own?
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Thank you for your input guys.  I did see that amp, however is there a way to create my own?
You could use an opamp or inamp and the NIST thermocouple tables which show the values and formulas to translate your voltage reading to temperature. If you're just looking at a small range of temperatures or just trying to hold a fixed temperature then you should be able to build something decent.
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Unless mediocre performance and accuracy are OK with you.. I wouldn't recommend an Op-Amp to amplify the output of a thermocouple. The two links to the maxim part are the best advice you will get as to accurate and repeatable temperature measurements using an Arduino and a thermocouple. An op-amp 'might' be within a hundred degree's or so but not repeatable and certainly not easy. The gain isn't an issue whatever it takes to amplify a 10 to 200 mV signal but the big issue is the cold junction compensation and while there are some old National Semiconductor App notes that describe cold junction compensation I doubt that they are repeatable today because many of those old op-amps are not made any more and I'd NOT trust Ebay to sell originals.
The Maxim package is available on a breakout board and is a one package solution, the op-amp solution is a 3 or 4 package high performance set of op-amps and it still will need real calibration over temperature both the measured temperature and the ambient temperature while the calibration of the Maxim device is a well known set of procedures. I'm an old retired engineer who now has the money and time to do as I please and I love to built things but not that one. IF and when I make my reflow oven I will be buying the Adafruit thermocouple amplifier. Adafruit makes well built and well thought out products that are thoroughly tested before they are sold.

Doc
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Great Doc, thank you for your help!  I will purchase from Adafruit, as I can see it is much simpler. From my understanding, if I were to use an op amp I would need to a way to reference the cold-junction temperature, which in itself requires another method of temperature sensing?  I am curious how I would do it if I were to try.

Thanks
Nick
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