Date   : Tue, 01 Sep 2009 09:00:46 +0100
From   : philb@... (Phil Blundell)
Subject: Leccy @ Acorn World '09

On Tue, 2009-09-01 at 00:16 +0100, Rob wrote:
> On 01/09/2009, Rick Murray <rick@...> wrote:
> > A question I have - without actually measuring, how can one tell the
> > power rating? For example, I believe my RiscPC power supply is something
> > like 83W (it's the smaller of the two). My bigger PC is something like
> > 350W (gee, spot the difference!). Now these will apply to OUTPUT power,
> > yes? The amount of ooomph given on the DC side. How does one translate
> > that into input? If the PC thinks it is taking 350W then I'd crap myself
> > as turning my room on would be akin to running an immersion heater!
> 
> Basically, Watts is Volts x Amps.  So 83W is amost 7 amps at 12 volts,
> but only 0.35 Amps at 240V.  However, yes that's it's output rating.
> There is usually a fair degree of lossage in PSUs.  This manifests
> itself in the form of heat.  E.g. my laptop PSU proudly states "input
> 100-240V, 2.3A" (i.e. 230-550W) and "output  19V 3.95A" (i.e. 75W)
> The difference of at least 150W probably explains why it runs
> blisteringly hot most of the time!

Lots of switching power supplies, particularly older ones, also have
fairly lousy power factor: it can easily be 0.5 or worse for some units.
This might account for some of the difference between the output power
of 75W and the input "power" of 230VA.

This also means that you need to be a little bit careful when trying to
measure the AC current drawn by such a PSU, since some multimeters will
give somewhat misleading results when presented with non-sinusoidal
waveform.

p.