# p4 and psu amperage

Discussion in 'Asus' started by Guest, Feb 5, 2004.

1. ### GuestGuest

what would be the minimum amperage on the 12 volt line for a p4 on a
p4p800?

Guest, Feb 5, 2004

2. ### Stephan GrossklassGuest

Approx. 15 A on a good quality 300W PSU. (At least that's about the
minimum I'd use.)

Stephan

Stephan Grossklass, Feb 5, 2004

3. ### DaveWGuest

Which P4 did you have in mind?

DaveW, Feb 6, 2004
4. ### PaulGuest

You can do a rough estimate for yourself.

There should be a design max current consumption number in the datasheet
and that is where you start.

Imax * 1.5/12 * 1/0.80 = I for Vcore circuit coming from +12V supply.

This equation is just the ratio of Vcore to the supply voltage and
the second term is correcting for the 80% efficiency of the Vcore
circuit. For example, a 3.06Ghz/533 P4 draws 52.5 amps from Vcore, and
this would be 8.2 amps from +12V. One of the new Prescotts would be
quite a bit more.

For disk drives, during spinup they draw 2A from +12V. When the spindle
is finished spinning up, the drive will draw about 0.5A. Most
manufacturers have data on this, so you can use your own, real numbers.
Note that during spinup, the processor may not be running flat out,
so it would be pessimistic to take max for the processor and max
for the disk drives at the same time.

Current to run the CPU and case fans is frequently printed on the fan
itself. If it isn't, estimate 1 amp to run all of them.

High end video cards have a drive power connector, and the highest
consumption I know of, is the FX5900 at about 70W or so. A
pessimistic estimate then, would be to include [email protected] to run a card like
this. Cards without the drive power connector, draw their power from
+5 or +3.3, so those video cards don't enter into the calculation
of +12V consumption.

As Stephan says, for a small install, 15A should cover it. If you
have a high end video card, or a lot of disk drives, you might want
to look at the calculation in more detail.

HTH,
Paul

Paul, Feb 6, 2004