Date   : Tue, 01 Sep 2009 00:16:54 +0100
From   : robert@... (Rob)
Subject: Leccy @ Acorn World '09

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!


> Couple this with the fact that some stuff just plain lies. My satellite
> receiver takes 30W. It says so. In actual fact, it runs about 3W in
> standby and 20-22ish when running (prob. different LNB polarisation
> power accounts for the difference?). Well, it's around 1/3 less than
> quoted. Might not sound like a lot, but imagine if everybody at the
> exhibition had one of them, the difference between quoted and actual
> could be enough to allow a couple of coffee-makers to be included!

Indeed. Most ratings on any equipment will be the absolute maximum it
could take if it's running full blast with all options enabled etc.
Plus a little extra capacity just in case.

> Would it be better to take LCD displays instead of "authentic" Cubs? The
> power consumption is less, yes?

If you have one that will accept the right sorts of input!  Which
generally means LCD TVs rather than VGA monitors.

Rob.