# Re: Psu - 500 Watt?

In article <tink.236byd@xxxxxxxxxxxxxxxxxxxxxxx>, tink
<tink.236byd@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

Hi I'm building my own PC and I require a suitable power supply. Thespec is:

Pentium 4 @ 3.0Ghz
GeForce Nvidia 6600GT
1 CD-DVD burner
2x hard drives
a few usb devices
1 PCI card for video capture

I think this will require no less than 350W. A local shop has
no 400Wunits but they do have a 500W unit on special offer
(cheaper than the350W they stock). I was hoping to get a 400W
unit. Will it be OK touse a 500W unit even if it's full potential
is not required? Will it overload my motherboard????-- tink

Power doesn't work that way. A 500W power supply will not

This plot compares two power supplies. The plot is for the
voltage on one of the wires of the power supply. The 350W
power supply is marked with the "+" symbols. The 500W supply
is marked with the "x" symbol.

Voltage
|
|++++++++++++xxxxxxx
| + x
| + x
|___________+______x_ current

Notice that for both supplies, the voltage is constant. The
motherboard cannot "feel" any difference between the two
supplies, because the voltage level from both supplies is
exactly the same.

Now, the horizontal axis is current. You can draw more and
more current from the supply, up to its limit. Then the voltage
will drop to zero, because the power supply detects the
overload and stops operation before something burns (we hope).

So, the 500W supply allows more current to be drawn, before
the supply reaches its output limit.

A computer like yours, will only be drawing 150W to 200W when
you are playing games. But the purpose of a 500W supply, is
not the fact that it has 300 watts more reserve power. You
have to consider how the power is distributed on the 3.3V,
5V, and 12V outputs. A modern computer draws a lot of current
from +12V (for the processor and perhaps for the video card,
if the video card has a separate power cable). Now, there
were some cheap 500W supplies a couple years ago, that only
offered 12V @ 10A maximum, and even though the supply is
rated at 500W, you could not even run an email/web surfing
PC from the supply. The 12V would overload, while the
5.5V and 3.3V rails would have hardly any load at all -
the problem is the 500W is not distributed where it is
needed. Therefore, "watts" do not tell the full story.

Have a look at this thread before you buy a supply. Now,
unfortunately, this thread does not list all of the less
desirable brands, but at least someone had the guts to
try and list them. It could be that your retailer is
trying to sell you one of those cheap-to-make 500W
supplies. If you are unsure about brands, you can
always post back and ask people what they think
of a particular brand. (If you include the URL to
a page describing the product, so much the better,
as that makes it easier to evaluate the product.)

HTH,
Paul
.

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