How do you use a voltage regulator?

               so this question is pretty straightforward, I'm trying to build a standalone ATX type power supply.  I'm using an Xbox 360 power brick, so I will have plenty of power, however the 360's power brick only has 12V out available.  There IS a 5v rail but only the 12V is available because I'm using the 5v to enable the brick.  And if I draw any more current from the 5v rail I overload it and it shuts the brick off.  Plus the 5v rail is only 1 amp so it doesn't even have enough power to run a DVD drive. (actually broke an old one testing it :P)  So, what I want to know is how to use a voltage regulator to scale down the 12V rail so I can have 5v and 12v to use standard molex devices with.  Also If anybody knows of a particularly beefy regulator i might use (5 amps maybe) I'd love to know :)

Skyriam6 years ago
You can always use the old LM7805, you get 5v regulated output BUT only at 1 Amp. If you need more, you need to get the LM338 wich is a 5 Amp Adjustable Regulator, what this means is that you can get anything from 1.2v to 28v or 30v I think, with a current output of 5 Amps, 7 Amps peak (but not constant).

Read the datasheet of the LM338 to see how it works, it's real simple.
jeff-o6 years ago
You can use an LM7805, but you need to use a transistor at the output to boost the current capability. There ought to be dozens of circuits on the web on how to do this, but check the LM7805 datasheet first!

Ah, I was right! See page 24: http://www.fairchildsemi.com/ds/LM/LM7805.pdf
...and a big fat linear regualtor needs a big fat heatsink - it'll be dissipating 35 Watts at 5V 5A