[Malvern] PC PSUs

[Chris Eilbeck]

On Wed, Feb 07, 2007 at 09:41:00PM -0000, Ian Pascoe wrote:
> Hi All
> 
> Why are PC PSUs so highly rated ie 400+ watts?
> 
> I know that the CPU and Graphics card gets rather hot so must take some of
> the power, and of course you have the power for the hard and optical
> drives, but even so this still seems a high wattage compared to what there
> is inside a normal PC these days.
> 
> My only thought on this is that they have to spec up what the power drain
> would be if all PCI slots were used, all USB devices were powered from the
> PC etc etc - am I on the right track here?

Graphics cards are the big killer these days but the main processor sucks a
fair whack too.  A modern hard drive will take 30-40W on startup, 15W
operating.  A USB device cannot suck more than 2.5W from the bus (500mA at
5V).

It's worth noting too that switching power supplies are inefficient and the
voltage regulation is poor if they are run much less than their rating i.e.
you'll only achieve the spec if they're 90-100% or rated load, at 10-15%
say, they'll be out of spec and there's a good chance of blowing components
because of the voltage rails being high compared to what they should be. 
Sucking 100W from a 500-600W supply is bad, mmkay?

Chris
-- 
Chris Eilbeck
MARS Flight Crew                              http://www.mars.org.uk/
UKRA #1108 Level 2                                                UYB
Tripoli UK Member #9527                                          LSMR