Figuring out a power supply

Hi

I'd like to add a 3.5" hard disk to a device that currently has a 2.5"
drive.

The 2.5" drive is powered by 5V drawn from the motherboard, and I
think I'd prefer to avoid powering the 3.5" from here in case the
current draw is higher than the mobo was designed to deliver.

So what I think I want to do is add a new power regulator board to the
device to supply 12V and 5V at about 1A each to the new disk drive.

I know there is an unused regulated 13.8V pin on the current power
PCB. This seems ideal for providing the 12V supply via a suitable
regulator, but from what little I know about power supply circuits
going from 13.8V to 5V rings warning bells. Would I be better poking
around on the PSU PCB to see if there is a lower voltage tapping from
the transformer that I should rectify and regulate, or would this drop
be OK anyway? If the drop is a bit high, are there any tips/tricks I
can use to get this down to something lower whilst disappating less
current.

Finally, I've looked around for circuits that might be suitable, but
have failed to find a simple '14V to 12V/5V' one. Can anyone point me
in the right direction if this is what I need?

Thanks
IanC

 

Going from 13.8V down to 5V can easily be done using a conventional linear
regulator such as the 7812, but will waste (13.8V - 5V)*1A = 8.8W in heat
(and that's a lot of heat). Chances are there is a 5V supply somewhere in
the device's PSU. The question is whether the 5V rail can supply the
additional 1A without overloading. If it can't be done that way, then you
could use a switching regulator on the 13.8V rail. For suitable regulators
check this link:

http://www.national.com/appinfo/power/

Just key in your specs in the text boxes and the site will recommend a
suitable switcher for your application.

cheers,
Costas