Interfacing TC directly with arduino without any extra circuitary involved.

I want to interface thermocouple to arduino board directly. I have an anlog pin which will read the analog input directly. Then why I need to extra circuitary to read the voltage sent from TC.

I will be using either J or K and termperature range from 0-500 Deg C. For corresponding voltages will be from minimum 0 to something.

What are the difficulties.

Thermocouples produce a very small output, for example 41 microvolts per degC for the K-type. Even if you reduce the analog reference voltage on the Arduino to its minimum (1V), the 10-bit ADC gives you a resolution of about 1 millivolt. Therefore, reading a K-type thermocouple directly via an analog input, the best resolution you can get is about 25 deg C.

You also need to consider compensating for the cold junction temperature. Thermocouple interface chips such as MAX31855 do this too.

To expand on what DC42 said.

If you do not incorporate cold junction compensation (the junctions where the thermocouple wires connect to the circuit) there will be reading errors introduced due to the thermo-electric effect created at these terminals by the dis-similar metals (these connections are referred to as the cold junctions). In effect a thermocouple sitting in the ambient air will read 0degrees irrespective of what the ambient temperature actually is. The thermocouple (hot) junction produces a small voltage but the connection (cold) junctions also produce voltages which are in opposition to the hot junction voltages