Eric said:
Hi,
Well if I understood your question correctly, then you want to know what is
going on with your power supply if not all 450 watts are being used, such as
when the computer is idle. Well if your power supply is a pretty good one
then it will only use whatever is needed however if it's a lower quality
power supply then it will always be using 450 watts. You should always check
if a power supply is effecient like that before buying it.
Regards,
Eric
The +5VSB on some supplies (this is the rail that maintains RAM content
when the computer sleeps), is only 50% efficient. But since the power
drawn might be 5VSB @ 1A or perhaps 2A, then the 50% isn't that important.
So the standby output on a supply, isn't very good in terms of numerical
efficiency. But the waste heat because of that, is rather small.
The main power supply rails are 68% to 85% efficient. The efficiency
varies a bit with the percentage of full rating being used, but for
the purposes of estimation, treat the efficiency as a constant. Power
supplies come in classes, where 68% is the default for efficiency.
But more efficient (and more expensive) ones are available.
That means, if you draw 100 watts from the output (say a Q6600 operating
with a decent computing load on it),
100W
---- = 147W
0.68
In other words, whether it is a 450W power supply or a 900W power supply,
the "wall power" used, is fixed by the function above. If you draw
100W DC combined from the 3.3/5/12/-12/5VSB rails, then the wall power
used is about 147W.
If you draw 200W output from the ATX supply DC rails, then the wall power
would be 294W
200W
---- = 294W
0.68
If you push that particular power supply, all the way to its rated output load
of 450W, then the power drawn at the wall is actually more than the
rating. The rating is the maximum power that can be drawn at the output.
But at the wall, the efficiency still comes into play.
450W
---- = 661W (ouch!) from the wall
0.68
Returning to the 200W output loading case (since that is more likely
to be the loading of a decent desktop computer), say we don't like the
wasted power conversion heat that this 294W - 200W = 94W of heat represents.
That is a lot of heat coming from the fan on the power supply. So we go
to the Newegg catalog, and buy an 85% efficient supply. We connect up our
200W desktop load again, to the new power supply.
200
---- = 235W
0.85
With our new 85% efficient supply, the waste heat is 235W - 200W = 35W.
That is a lot less heat coming from the power supply vent. As a result,
the fan blade spins slower, and the air temp is a bit cooler than
it was before, on the power supply exhaust.
The power rating of the power supply, is the maximum output it can
sustain. You need a big enough supply, to withstand the largest
loading the computer could place on the supply. If the supply is
excessively large, the additional capacity isn't really helping
us. (A 900W supply running a 200W computer, is a waste of money on
the power supply.)
But the important factor is the efficiency, as a means of working
out the "wall watts". When the efficiency is unstated, it is
assumed to be about 68%. Better supplies, will justify their
higher price, by quoting their improved efficiency numbers.
Under no circumstance, will the power supply arbitrarily decide
to suck down 450W, independent of the DC loading place on it
by the computer. It has to behave in proportion to the load
on its output. If you place a short on the output, good
supplies shut down, and survive to run another day.
The ATX supply consists of two sections, and a run-of-the-mill
supply would look like this in terms of efficiency. But
for simplicity, I ignored the 5 or 10 watts on the upper
section, and just treated the whole load as coming from
the 68% efficient part. Frequently, the actual efficiency
of the 5VSB is not presented in marketing information, and
only the main section has a stated efficiency. And that
is because the manufacturer would be embarrassed to report
that the tiny standby supply is only 50% efficient.
--------------------------------+ Standby supply.
+---| +5VSB (50% efficient) 2A max |------- To RAM and
| --------------------------------+ Wake-On-LAN etc
To wall ----+
| --------------------------------+
+---| 3.3V / 5V / 12V / -12V |------- Goes
| Main supply. (68% efficient) |------- Everywhere
--------------------------------+
Paul
