FW: [Malvern] PC PSUs

[Steve Cashmore]

Ian Pascoe wrote:
> Steve and chris - thanks!
> 
> When a PC starts up does everything get powered up at the same time, or is
> it done sequentially ?  If the latter what governs the start up regieme?

Some servers can have sequenced startup for the multiple disk drives, 
but not in the general case.  I've seen drives that can have their motor 
spin up delay pre-programmed.  I've seen intelligent PSUs that can 
sequence their 12v motor outputs.  Some servers have multiple redundant 
hot-pluggable PSUs.
> 
> However, from what you both have said, I presume it is the former, and that
> PC designers expect the PSU to be able to cope with any peak demands on
> startup.
> 
> Apart from cost, is there any reason why each component couldn't have it's
> own tailored PSU?  (I'mn thinking here of the benefits given to high end
> audio systems where each individual component in something like a CD player
> has it's own regulated power supply)

The PSU does have multiple rails however such as +12v, +5v, maintained 
+5v and some offer -12v & -5v & +3.3v.  The motherboard commonly has 
on-board regulators for the CPU supply derived from the 5v rail.  The 
maintained +5v supply is for the 'instant' (Hah!) startup from various 
sleep states feature or Wake-on-LAN.  Often subcomponents such as disk 
drives and plug-in cards also have on board regulators for local supply 
provision.
Individual mains powered PSUs for each component would be a price, 
packaging and safety nightmare.  At least at the moment there's only one 
safety insulated device and wiring to care about.

-- 
Steve

> Ian
> 
> -----Original Message-----
> From: malvern-bounces at mailman.lug.org.uk
> [mailto:malvern-bounces at mailman.lug.org.uk]On Behalf Of Chris Eilbeck
> Sent: 07 February 2007 22:45
> To: malvern at mailman.lug.org.uk
> Subject: Re: [Malvern] PC PSUs
> 
> 
> 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
> --