i am doing a SCR AC to DC rectificer of 220V 50hz sine input

so if i wan to use a UJT relaxation osciliator to fire the SCR, do i need to fixed the frequency of the osciliator to 50 hz as well ? if yes, then f = 1.44/RC , what abt the firing angle ? is it control by varying the Rb1 or Rb2 resistance value while keeping the osciliator frequency at 50hz

 

Is there some reason why a rectifier diode won't work for your application?

You might look at out Ebook article on SCR's. The method for triggering in synch with the line is an RC circuit - http://www.allaboutcircuits.com/vol_3/chpt_7/5.html.