How to build op-amp amplifier for thermocouple that can switch 12v relay using lm358 ?

[don fila]

I have 2200 watt heating element with thermocouple embedded inside the heating element. I used triac and other components to control the low and highness of the temperature. But my problem is how to use thermocouple to detect the temperature of the heating element and turn off automatically when it get overheated and restart when it cool down

 

[mtwieg]

Echoing earlier suggestion that you should use a specialized thermocouple front end amplifier, rather than a generic op amp. Something like the AD8494/5, you can get breakout boards like this for prototyping. Once the thermocouple is properly conditioned into a larger signal, you can use a simple comparator w/hysteresis to create the control behavior you want.