Archived from groups: alt.comp.hardware.homebuilt (
More info?)
On Mon, 12 Apr 2004 15:11:26 GMT, Joseph Davis <george1879@charter.net>
wrote:
>Hey everyone.
>
>I have an old P3, RAID card, and a bunch of HDs laying around. I'm
>worried about power requirements, as the biggest spare PS I have is
>250W... is it possibly to use multiple PS in a custom built case?
>Would I have to wire the power supply outputs in parallel so that all
>power + ground are common, or can I use one to power the motherboard
>and a HD or two, and the other to power the drives on the RAID?
>
>Just curious... I have a lot of spare parts and a lot of time on my
>hands.
>
>Joseph
There may be a vast difference between a generic 250W and a good
name-brand. Which you have may determine it's true output capability, and
further, how it's output is distributed... with a bunch of old HDDs you'd
be better off with a power supply having a lot of 12V amperage. If the
system's bios has a setting to delay for "n" number of seconds, that might
help, give the power supply more time to spin-up the multiple hard
drives so they're properly detected.
You don't necessarily need a custom built case for two power supplies,
most any full tower will do if you stack the 2nd PSU above the first
(and of course cut out the needed hole(s) for fan, power, screws).
You cannot simply wire the two unit's output in parallel. They regulate
themselves based on voltage monitoring, at a minimum on the 5V rail.
There may be a way to reverse engineer them and provide a good current
sharing method beyond hooking them together with isolation diodes as that
would be rather lossy. What you could do is use the 5V output from one
and the 3V from another, and have the drives distributed amongst both
units. Easier (and probably plenty good enough) would be to simply use
the 2nd power supply to power only the drives and any misc offboard parts
(like case fans).
Assuming both power supplies are ATX, you could connect both unit's PS_ON
leads (pin 14, usually a green wire) together then they both come on
together, but that's based on the assumption that the motherboard can
'sink the combined current... I have done this and had no problems but it
is still a consideration. An alternative might be to use a power rail
from one of the unit's to flip a relay which connects the other unit's
PS_ON line to ground. Then there is the manual method, a simple switch to
flip every time you power on the PC (by pressing the "regular" case
switch) and then also having to turn off the 2nd power supply when the
system is powered down, which to me seems like an undesirable situation.