[mythtv-users] Help with new Video Card (power supply requirements)

[Corne Beerse]

On 12-3-2010 21:19, Paulin wrote:
> All,
>
> I just purchased a new video card (Jaton 498pci - 
> http://www.newegg.com/Product/Product.aspx?Item=N82E16814139046). 
>  Anyway I was getting ready to install it and one of the requirements 
> on the box says minimum 450W power supply.   Anyway I checked my dell 
> box and it told me the V 115 with 7 amp.  So simple math sells me 700+ 
> watts.  Then I opened up the box and on the power supply there is a 
> label.  It says that max power is 300 watts.
>
> So my question is my thinking correct, in that, I can't use this card 
> because my power supply is maxed at 300w.
>
> Since V x A = W why does the power supply say 300 max?
>
> So should I return the card and look for something else or will this work?
>

For a starter you made some reasonal assumptions. However...

Your dell box was labeled 115V and 7A separately, which indicates that 
it operates nice at 115 Volts (give or take 10 %) and that the max 
current it will require is about 7 Amps, hence the fuse must be at least 
7 Amps, otherwise it might blow in normal operations.

For a powerbox, with volts and amps both at the imput and the output, 
you have to note the efficiency of the powerbox. If it produces 12 volts 
with 2 amps and 5 volts at 5 amps, it outputs 24 and 25 watts. With 
that, it will consume more than 49 watts at the input, the remaining 
power is converted to heat and maybe some light (a power-led or such) 
and other radiations (distorsion on radios) too.

Then for the real electricians, while dealing with ac, there is also the 
'cosinus phi' factor:  If you start calculating on stuff like ups-es and 
such, then there is a difference between 'volt-ampere' and 'watt'. I've 
read an article on this at te APC website: www.apc.com.

For your calculation you only need to have a look at the provided power 
by the power supply and the total consumed power of the connected parts. 
Donnot use a way-to-large power supply as they operate best between 
(roughly) 50 and 90 % of their max values.


CBee