K thermocouple conditioner with lm358

http://shin-ajaran.blogspot.com.co/2013/07/quick-and-dirty-way-wiring-k-type.html

The quick and dirty way of wiring of LM358 is at the self explanatory figure below, drawn by hand from yours truly.

http://electronics.stackexchange.com/questions/202648/problems-with-op-amp-thermocouple-circuit

Next, I looked for a schematic, even though I don't understand it. It doesn't have two resistors in the voltage divider that is familiar to me. I tried to duplicate it but didn't have a 100k potentiometer or a 270k resistor, so I tried 120k ohm resistor that was handy instead. I didn't bother with the source follower on the output since it's going to my DMM which is high impedance.



http://hobbyist86.rssing.com/chan-9523472/all_p2.html

Thermocouple 

Thermocouple

I got a type K thermocouple and tested that directly with a voltmeter.  It only show a couple of mV with a flame on the tip.  I added a simple 100X amplification circuit and connected that to the analog input.  Now I see a range around 300-400mV and see a clear increase when the flame is on it.  One problem I saw is that the sensor has a fair amount of mass, mostly stainless steel.  Once it heats up, it takes a while for it to cool off.  As long as there is a distinct increase when the flame comes on, this could also be used for the flame sensor.

100X Amplification Circuit for Thermocouple

The item that I struggled with the most was how to make a reliable flame sensor. This is covered in the Flame Sensor Update post. Note - the drawing for the amplifier circuit in that post has the resistor values reversed. Here is the corrected drawing.

The gain formula for a LM358 non-inverting circuit is

               gain = 1 + R2/R1

In this case, R1 = 1K and R2 = 200K, resulting in a gain of 201. This means that a thermocouple output of 5mv will produce an output of just over 1V. Using a gain of 500 would allow for better resolution of the temperature, but 200 is adequate for this purpose. I don't care about the actual temperature. I just need to see if the value is increasing to verify that the flame actually lighted.