J Type Thermocouple Signal Conditioning

[BHARGAV SHANKHALPARA]

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......

 

[dorke]

Sounds like Homework to me.
You should sweat it out yourself,
then come with specific questions.

 

[BHARGAV SHANKHALPARA]

Hi Dorke...

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.

 

[duke37]

Perfection can not be obtained.
My experience with thermocouples is that they are nearly linear. If you are using a computer then non linear correction could be invoked.
The diode voltage drop will change by 2mV/degC - silicon or germanium.
Why are you stuffing 5V across the thermocouple?