hai any body give me the circuit for ac-dc power supply without transformer....
the output dc is for 6 white leds i.e., the output dc voltage should be 20v.... use bridge circuit

 

You have partly described the output - what current were you going to run the leds at?

You have not described the input.

Ciao, Tim

 

Whats wrong with a transformer? Are you suicidal?

You can simply use a wall-wart style AC adapter. Like the type used to charge your cell phone. Many laptop and printer versions have higher voltage levels, and can supply high enough current to light many LEDs.

I use a Lexmark printer supply. It is 30v and .8amp. Nice and compact, as the plug end is just a plug, not a wart.

 

This is one of those subjects that get threads closed here at AAC for safety reasons. There is no way a power supply can be made safe without a transformer, the AC mains have the potential to come through an kill someone, not necessarily the person who built the project.

Wall warts use transformers, or you can get one separately, but it really is a must.

 

Who said it was mains input?

 

That is what a transformerless power supply is, almost always. Right now we're waiting for the OP's feedback.

Of course, the OP's own words also provide a clue...

hai any body give me the circuit for ac-dc power supply without transformer....

Where do you think the AC is coming from?

 

Hello,

We ask again. Where does the AC come from.
What level has the AC?
If it is mains we can not continue.

Bertus

 

Maybe he has a bicycle and needs a front light. It could be from a 28V alternator cos he lives remotely. Maybe he has a 9VAC switchmode plugpak for his guitar effect, but doesn't like mains hum from transformers to read the music at night. Maybe he has a low AC voltage source of some kind, and wants to step it up up and rectify it. Anybody else got some good suggestions, cos I'm about out. ;-)

 

I hope the reason he hasn't replied isn't because he suddenly discovered why transformerless supplies are dangerous...

 

I hope the reason he hasn't replied isn't because he suddenly discovered why transformerless supplies are dangerous...

That is the precise reason why we do not support discussions of line attached transformerless supplies.

 

here leds current is 3.5A

 

OK, but we still need clarification of what you mean by transformerless. We can help you with your problem, but the transformer is a must (or some other type of power supply).

 

If you are using mains power for your project, then you absolutely must use some form of transformer with electrically isolated primary and secondary windings.

If your power source is low-voltage AC (at or below 35 VAC RMS) then you can use a buck-type supply; however it will still need an inductor of some type. Toroids, pot-cores or E-cores are preferred for low losses and low emissions.

 

why do you think it is impossible without transformer ................

here is the circuit for transformerless power supply

http://3.bp.blogspot.com/_TXBMxL7E6...1600/Transformerless+Power+Supply+Circuit.gif

 

Hello,

We asked several times if the circuit is mains powered.
You will never manage the 3.5 A that way.
You MUST use a transformer.

Bertus