I want to convert 220v ac supply(50 Hz) to 5-20v DC with out

using stepdown Transformer ckt. can anybody suggest some ideas..

please dont. the transformer will provide isolation.

You can use a autotransformer (ie an inductive voltage divider) to step the voltage down to a suitable level and then rectifiy it as normal. The use of an autotransformer arrangement is prefered over a plane resistive divider since you do not have as much resistive power loss/heating, you just get reative loss in the inductors. you just need to do some maths on the reactive component at main frequency.

The company Vircor www.vicr.com do high voltage -> low voltage DCC converters, just rectify the mains and pass it into the vircor. These are isolated so are safe - but cost a bit.

As Styx says, it's not a good idea. If you really want to do this, you need to supply a lot more information about it, like:

What is it intended to feed?.

What is the maximum current requirement? - very small is advisable!.

What is the minimum current requirement?.

What voltage is required? - variable voltage done like this, isn't really a good idea. And a general purpose lab power supply would be suicidal!.