Regualtion means "changing load should not change the output whether it be current or voltage"
Regulate any voltage less than 400...we know our domestic supplies rms value is 220 v...but remind you there is no short circuit protection i.e; if load (lamp here) gets shorted so gets and the A.C supply gets shorted and BOOM...........
........so i leave it up to you to go step by step analysis and come up with a better design with short circuit protection and so on
