How does this thermocouple amplifier work? (SOLVED)

[AMyatt]

Hi everyone,

I got this very simple circuit from a website (I can't find the website to link to it), but the website didn't come with any description of how the circuit worked simply that it would take a 2-wire thermocouple on the input and provide roughly a 0-5v output (after trimming the pot).

I don't have much knowledge of Op-Amps and I am having trouble understanding how this works. I don't see how the thermocouple's output would vary when its input is only connected to ground.

Can someone please explain how this circuit works?

Thanks

[ejeffrey]

That is just how thermocouples work.  There are two wires made of different metals welded together at a point.  It generates a voltage that depends on the temperature of the weld point.  They don't require any power (other than heat) and in fact can be used to generate power (not much with standard thermocouples)  Technically it is the temperature difference between the weld point (called the hot junction) and where the thermocouple leads meet your circuit (called the cold junction).  In order to have acceptable performance you probably need cold junction compensation: a secondary thermometer on the PCB that measures temperature of your circuit and compensates the reading.

[AMyatt]

Thanks ejeffrey.

I didn't realise that thermocouples actually output a voltage, this circuit makes a lot more sense now.

[mariush]

You should be aware that there are various types of thermocouples, depending on the two metals that are combined in the tip.  Some are called J type, the most common I think are the K type and there's other.  See this page which gives more info about thermocouples: http://www.omega.com/prodinfo/thermocouples.html

but basicallym each type outputs a slightly different voltage at a specific temperature, and the output voltage most often isn't linear, so for example if you get 10mV at 1c and 20 mV at 2c you can't say that you'll have 100mV at 10c
The opamp will just amplify the small voltage coming from the thermocouple but a basic opamp won't adjust the output voltage to keep in consideration the deviations in the thermocouple voltage.

There are specialized chips which contain an opamp and the brains to do those conversions automatically and give you a voltage that scales linearly with the temperature.

For example, AD594 or AD595: http://www.analog.com/static/imported-files/data_sheets/AD594_595.pdf
These output about 10mV per C, 0mV at 0C, 100mV at 10c and so on, so it makes it easy to output the temperature directly on something with minimal operations. Unfortunately these chips are a bit expensive, but there are similar such chips.

[AMyatt]

Thanks mariush.

I have a K type that I'm going to use with this project.
I'm using it with an AVR MCU and plan on using a lookup table for the relation between voltage and temperature.

[SPRX]

Dave done a Thermocouple Tutorial, which may help you: eevblog-419-thermocouple-tutorial
 
http://www.eevblog.com/2013/02/03/eevblog-419-thermocouple-tutorial/