Electrical – Thermocouple Op-Amp – Arduino

arduinooperational-amplifierthermocouple

First of all, I know that this is not the best way to do this, even more so because I am using the LM358. I don t want extraordinary results.

Now, on the matter of the problem. I have attached the circuit. The only difference is that I mounted the 100nF Capacitor right at the input+ and GND of the op-amp.

The problem is that I get nothing as a result and if I remove the CAP I get a reading, but only for a few miliseconds, then it goes back to zero. What can I do?

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The code is basically this, where the show() function is a function to display the number on an three digit led segment display. 

 Input = analogRead(0);
 Input = map(Input, 0, 450, 25, 350);
 show(Input);

Best Answer

You have a 50:50 chance of having an op-amp with a negative Vos, so it might have to get fairly hot before you see a change, but a soldering iron is hotter than that.

I suspect you may have connected the thermocouple backwards (red is typically minus in north America), in which case the op-amp would remain saturated at the lower rail.

To avoid the first problem, connect the minus side of the thermocouple to a voltage divider that gives you a couple of mV. Something like 10 ohms/20K to +5.

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