Question

plse help me to understand working of the sillicon controlled rectifier, and why it is called "controlled"?

Answer 1

Light activated silicon controlled rectifies(LASCR)

An LASCR is a four layer semiconductor device that conducts current in one direction when activated by a sufficient amount of light and continues to conduct untill light falls below a specified value.
as it is four layer device so it is also resemble with thyristor

Answer 2

http://en.wikipedia.org/wiki/Silicon-controlled_rectifier
http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

Answer 3

scr is used to conduct in one direction and the conduction is controlled by the gate that's why it is called as SControlledR but triac is same as scr but that operates in both direction or conducts in the both cycles of ac but scr in one cycle ,,, so scr is called as rectifier...