[AVR] J Type Thermocouple Signal Amplifier

[BHARGAVSHANKHALPARA]

Dear friends ,

I need to make a j type temperature controller .
Please suggest me how to proceed , never used one before .

1. I need circuit used for j type thermocouple signal amplification using opamp op07 and diode for cold junction compensation , I have seen temperature controllers using thermocouples in mkt and they use a diode in the back .PLease suggest me design using this diode for cold junction compensation .Please let me know what exactly this cold junction compensation is ? in easy language and what is the purpose of the diode in this design .What is isothermal block? and how does this isotherm block and diode for compensation are used physically ....

2. I am using avr ATMEGA32A controller.

3.Please give me some details regarding software also . Do we implement any lookup table for thermocouples or the output from amplifier is linear enough to display on the Display(Four 7 segment display).


Also it should have provision for the k type thermocouple in future.
Any practical experience and self made design experience is welcome thx......



Thanks and regards.

 

[pic.programmer]

Have you seen these links.

https://www.edaboard.com/threads/294824/

https://www.edaboard.com/threads/208046/

 

[Easyrider83]

I used this schematics with 5V power supply. Works fine in my soldering station.

 

[pic.programmer]

What is the max temperature you want to measure ? Can you use OPA340 or OPA350 OpAmps ? Do you want to use Common Cathode 7 Segment Displays or Common Anode type Displays ?

 

[KlausST]

Hi,

There are various thermocouple signal conditioning ICs.

TI, AD, LT..

Klaus

 

[BHARGAVSHANKHALPARA]

hello..

Max temperature i want to measure is 150 degree Celsius. OP07 is easily available at our location. we are using 16X2 LCD Display.

- - - Updated - - -

Sorry for not posting work,i done...!!

I have one temperature meter. and it gives perfect temperature readings using J type Thermocouple. So i decided to do signal conditioning circuit same as that temperature meter. for that i configured internal adc of ATMEGA32A with 2.56 Volt Internal Reference. In that temperature meter they use two ADC channel (one for hot junction reading and another for cold junction compensation).

Schematic Diagram of signal conditioning circuit is attached.

for cold junction compensation they use one resistor and 1N4148 Diode between +5v and ground. This cold junction compensation gives 0.595 voltage at 34 degree Celsius temperature (when i give heat to diode temp. start increasing and value of voltage start decreasing) and op-amp output (hot junction output) gives 0 voltage when temperature below 34 degree. for more than 34 degree its start giving non linear reading of voltage.

Problem is that after reaching voltage at adc i cant understand that how to obtain temp. from voltage (non-linear) and where i should put cold junction reading so i obtain perfect temperature.

 

[pic.programmer]

Yes, I can provide you circuit using OP07 OpAmp. I can also provide you C Code but OP07 has two pins for offset null settings. Do you know how to null the offset using a 5k preset ? If you can do it then I will provide you circuit using OP07 ekse use OPA350 OpAmp. OPA350 OpAmp is a very good 5V Rail to Rail OpAmp. I will design the circuit such that for 150 degree C it will give 5V output.

Use AD594. They are dedicated ICs for Thermocouple measurement. There output will be in mV and hence you need OpAmp but the output will be linear.

Please mention the range of temperature you want to measure. You want to measure 0 to 150 degree C or -25 to 150 degree C ?