I need a variable power supply without transformer (5V-25V,) current variable upto 2A for my hobby project.. Can it is possible? if it is then post the ideas pls.... Thanking u

 

You can use a wall wart.

Don't even think about trying to run a power supply without a transformer off line voltage. This is a idea that will get this thread closed very quickly, as it is a major safety issue. There is no safe way to get low voltage DC feeding directly from the mains.

This keeps coming up on the forums, and the result is always the same. Only a transformer or a wall wart (same thing) can provide safe isolation from line current.

 

thank u very much... Can u post a circuit capable of producing The same variable power supply(5V-25V) ampere variable upto 2A with transformer pls

 

Look for the LM317 30v 5a CV/CC circuit in the National datasheet. I built it in protoboard and seems to work, but i wouldn't make it at 5A output, as the worse case dissipation will be like (35v-1.2v)*5a = 170w. I'm planning to build it for 3a, using two paralleled transistors with 0,1 ohm emmiter resistors, so the heatsink doesn't needs to be absolutely huge (still will going to be pretty large, however).

My only problem seems to be that the <snip> 15v+15v 3a transformer i got around is like a 11v+11v transformer at 3a. Note: i'm planning to make it positive and negative supply. Don't use a 30v transformer for this, as full-bridge rectified it will go to 41vcc, and that's more than the LM317 can handle.

Anyways, you probably would want to check LM723 circuits. Pretty good IC that is ready to be used with a bypass transistor and custom current limiting. You probably will need a rheostat for the current limiting, however. Also, it starts at 7v output.