12V to 5V

Ok, this may sound like a stupid question, but I need a little help. I'm building a circuit board that uses components for 12v and 5v. The main power is a 12v 8a source so I need to add a step down to my circuit for the 5v devices.

I've seen two diagrams: http://www.eleccircuit.com/wp-content/uploads/2009/09/12v-to-5v-3a-dc-converter-step-down-regulator.jpg http://www.eleccircuit.com/wp-content/uploads/2009/09/dc-power-supply-5v-and-12v-using-2n3055-lm309.jpg

Both look legit except the first that says it take 12v 3a and not 8a. I don't know if it will still work. Next is I'm not sure where to find/buy the transistors. Every time I do a google search, I end up with mixed results. Is it just a matter of getting a hold of NPN transistors or is there another aspect I need to look at. I'm about to make a purchase from sparkfun, so preferably I'd like to be able to order all my parts from there.

Thanks,

How much 5V current do you need? If it's less than 3A, there are many 5V linear regulators to choose from. Mouser lists dozens. Here's a likely one, an ST Microelectronics LD1085V50ST

http://ca.mouser.com/ProductDetail/STMicroelectronics/LD1085V50/?qs=sGAEpiMZZMsGz1a6aV8DcCRy2MVkeVbmq8UhGP29P1g%3d

I don't need much. When I was running off a bread board I was using a 5v 3a step down and everything was working fine. I don't see myself pushing past 2a.

I thought a stepdown would be more complex and require more components. So I can simply plug the 12v+ to the input, and 5v+ from the output? On top of that, I've been experimenting with combining commons. Using this, can I connect the common between the 12v and 5v devices?

Thanks for the quick response

Using this, can I connect the common between the 12v and 5v devices?

Not only can you but you HAVE to connect all the GNDs.

I thought a stepdown would be more complex and require more components.

Stepping down is a lot easier than stepping up.

So I can simply plug the 12v+ to the input, and 5v+ from the output?

Yes, as long as you don't need too much current. Depending on the regulator used you will heed 1-2 capacitors as well. That will be detailed in the chip's data sheet.


Rob

You should be aware that switching supply boards using the LM2596 buck converter with 3A output are available on eBay dirt cheap. I am holding one in my hand that is only 1" x 2". I bought 5 for ~$15. They have adjustable output.

Joe