Fritzing

A thermocouple amplifier made of 3 Op-Amps for arduino, with slight modification it can be turned into a generic in-amp.

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This is my very first project, consider my zero knowledge in electronics.

I haven't tested in real life yet so I don't know if it works or it has a good gain, in theory the gain should be 91 but it can be adjusted changing the resistor Rg. I tested under spice and at least it works there.

I made it following many basic In-amps schematics and reading Analog Devices documents about thermocouples.

It uses 2 LM358 dual Op-amps and a LM35 temperature sensor near the cold junction for Cold Junction Compensation. It hasn't a cold junction compensation circuitry, the compensation must be done with software.

It should works with any thermocouple but I guess types S ans R are not the best choices due their low voltage and low resolution of the arduino (10bit), it should work with grounded or ungrounded TC and it has noise filter.

IMPORTANT: thermocouples are not linear, the output must be corrected with polynomials equations to obtain the adequate temperature, this equations and coefficients can be found in nist.org or with lookup tables.

The two outputs, temperature sensor LM35 and the In-Amp are connected to the pins A0 and A1 of the arduino board.

The V+ for the amplifier is the Vin of the arduino board, that means a DC power supply must be connected to the arduino.

It doesn't has any arduino code, my intention is to use it with labview for temperature PID control of a reactor oven between ambient temperature and 900ºC. It should be nice use it as stand alone but it will require buttons and LCD.

Please if you make an useful change or if you encounter errors in it, notify me so I can make the modification too.

PROBLEMS:

Arduino analog input is from 0 to 5V, at 0C the TC sends 0V but below 0C the signal is negative, so beware. for that reason don't try common open thermocouple detection because it sends negative voltage to the arduino.