# Thermocouple amplifier and cold junction compensation

I am trying to design a thermocouple amplifier with cold junction compensation. I already designed a simple amplifier for the thermocouple using LM358. Note that I can built this entire circuit using only cheap and easy to find components, such as low power transistors, LM358, LM324, resistors, diodes, pots, trimmers. I know that I could buy from the internet modules, but I can't at this moment . I also need a cold junction compensation, which should be connected to Arduino, to an analog input.

Schematic: https://ibb.co/Sfs9dPX

• This is your learning challenge. The answers are everywhere May 1, 2020 at 12:33
• There are loads of solutions to CJC compenasation out there. From pre-made ICs to measuring the cold junction temperature and using firmware to compensate. Googling cold junction compensation techniques should give you some starting points
– MCG
May 1, 2020 at 12:36

I already designed a simple amplifier for the thermocouple using LM358.

Unfortunately, the LM358 makes a very poor amplifier for thermocouples given that it has an input offset voltage that may be as high as +/- 3mV. Given that a thermocouple (K-type for example) produces 41 μV per degree celcius, it means that your temperature accuracy is only to within +/- 3000/41 = +/-73 degrees celcius.

Here is your proposed design and some figures from the LM358 data sheet and, at the bottom the graphs for various thermocouples: -

This is not the intelligent way to design thermocouple amplifiers - use a precision InAmp (like the AD8221) if you want anything like decent accuracy. Use a negative rail so that if someone connects the thermocouple backwards you can still retrieve measurements (albeit them being negative). The cold junction compensation is trivial so, get your basic amplifier design right first.

• Do you know any part number for these InAmp's ? I prefer them in THT versions. I found INA114 and INA115. Or should any InAmps do the job ? May 1, 2020 at 13:35
• Also it is possible to use a standard OpAmp as an instrumentation amplifier ? If I will use it in some configuration? May 1, 2020 at 13:48
• I always use AD8221 InAmps because they are so good and accurate but there will be others. I wouldn't bother with standard op-amps because their input offset voltage is usually poor (standard varieties). May 1, 2020 at 13:55
• Using an LM324 in InAmp config instead of a simple LM358, will result in better accuracy ? ibb.co/BcmzQ0x May 1, 2020 at 14:03
• What does the data sheet tell you about input offset voltage? And NO "average" op-amps configured as an InAmp is wasting your time. May 1, 2020 at 14:09