I just want to know that what is the role of centre tapped rectifire?
I have gone through the site but there is no reasonable ans. . .
So plz help. . .

 

One use could be as a full wave rectifier on the secondary side of a transformer that has a center tap.

Have a look at this page: http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
The 3rd schematic down shows two diodes; they could very well be a dual diode that is wired as common cathode.