what happens if we dont use centre tapped transformer.

what happens if we dont use centre tapped transformer in a full wave rectifier .... Just use a normal transformer as used in half wave.plz explain.

It depends on the rectifier configuration. If you want to use half wave rectification, use one end of the transformer secondary as ground and rectify the other side with a diode. If you want full-wave rectification with a single winding, use 4 diodes in a bridge configuration.

Brian.

You reduce the ability of the transformer to deliver as much current as the current flowing through the secondary is DC. If it was a centre tapped transformer the currents in both halves of the secondary have their DC component opposed so the core does not saturate.
Frank