Dear Ian,

Thanks for your reply,
I think you really need a better control algorithm. What have now is a fairly simple first order system which is bound to have a static error the size of which depends primarily on the loop gain. To ensure the measured current more clasely equals the desired current you probably need to move to a second oder (PI) or third oder (PID) system. I think you will find info on coding these on other threads on this site.

My Controller system has input -> Thyristor Firing Angle and Output The Welding current.
The input is to be adjusted to get the desired output. And for this purpose the feedback coming from Rogogwski coil is used.

I am not convinced with idea of using a higher order system. Do I really need a PI or PID control?

I am not an expert in this matter but as a general understanding

I believe that 'I' control is needed when you have something cummulating in the system for example a water tank where in water coming in goes on cummulating or a boiler where heat coming in goes on cummulating. Or a motor where the shaft speed goes on increasing in response to applied current. But In my system once a firing cycle is over nothing from previous cycle remain for next cycle. Do I really need a 'PI' control isnt P alone enough?

Kindly correct me If I said something wrong. As I stated earlier I am not an expert in control engineering.

Regards,
Prahlad Purohit