need help with AC to DC circuit

hi all, im a newbie in engineering but had to do a project on a 2.1 speaker. i need to design a circuit for changing a 240Vac to 14.2Vdc, but after weeks of staring at websites, i still have no idea how to start on it.

i found a circuit online, but i had to modify it to let the output be what it is needed.
output -> Power: 50W; Voltage: 14.2V; Current: 3.5amp

http://www.discovercircuits.com/DJ-Circuits/240vac5vdc.htm this is the link for the circuit i want to modify.

can someone help me please ><

LasVegas10 years ago
If you use a 240v:24v (10:1) transformer with the same bridge circuit you could use a LM350, Adjustable Regulator to calibrate it to 14.2v. It would also give you your 3amps.
LasVegas LasVegas10 years ago
Sorry... Just noticed you need 3.5a. In that case use the LM138 or LM338 for 5 amps output.
westfw10 years ago
That's close enough to a standard laptop supply that you should just buy one from a surplus dealer or eBay. Anything over about 1A gets pretty annoying to design and build yourself, especially if you're going all the way from wall current (commercial power supplies are likely to have custom-built transformers, for intstance.)