"Shawn said:
Finally got my P4P800-E Deluxe and P4-3.2 Northwood CPU.
I'm going to buy a new case, since I don't want to disassemble my old system
and I don't think it can drive a P4 motherboard anyway.
How much power do I actually need? I have an oldish GeForce video card,
will be driving at the very most 3 IDE HDDs and a couple of USB peripherals,
one of which is a disk with its own dedicated PSU.
Some case/PSU combos seem to go as low as 300W and some as high as 500W.
300W seems kinda low, 500W seems kind of excessive, but I can't really tell
what I ought to use.
Any case/PSU brands specifically worth *avoiding*? I'm likely going to buy
over the counter at Microcenter just so I can more easily give it the
once-over, and at last glance they seemed to have a fair selection of cases.
A typical minimal system (i.e. doesn't have 10 disk drives in it)
using high end components, might draw 200-250W total power while
gaming. So, if the only parameter needed was just the raw power
provided by the PSU, then practically anything would work, and
that is not the case.
Take the case of a certain Q-tec power supply. It was rated for
over 500W, but only provided something like 12V @ 10A maximum.
Now, P4 based motherboards tend to use +12V for the processor
Vcore power circuit, and a P4 plus a high end video card, can
exceed this capability. So, you would be sitting there looking
at your 500W+ power supply, being unable to power your system.
Instead, you have to examine the label on the side of the power
supply. It will state the maximum current that can be drawn from
+3.3V, +5V, and +12V. The -5 and -12V are generally weak outputs,
and motherboards use way less than 1 amp from these supplies. The
+5VSB is used for keeping the computer working while it is "asleep",
and 2A or more is handy for keeping DRAM refresh operating, plus
keeping a LAN chip listening for a magic (wake on LAN) packet.
(Your motherboard manual has estimates of how much +5VSB is needed
when various options are enabled, so you can see how much of the
2A will be needed.)
In terms of estimating power consumption, complete information is
not available to work out power consumption on all three power rails.
One way to get estimates, is to download an Intel motherboard manual,
for a motherboard that best matches an Asus equivalent. The only
downside of the Intel numbers, is Intel doesn't state what components
have been included in their calculation, so you never know whether
any power dissipations are being double counted or not. I think
possibly Fujitsu motherboards might have had power numbers as well.
There are power estimation web sites on the net, that attempt to
estimate power, but they uniformly overestimate the requirements.
Even if you read some of the estimates that I've done, they are
overestimated as well.
The main thing to be concerned with, is the rail most likely to
be overloaded. On a P4 or an Athlon64, this would be +12V. On
an AthlonXP board, it might be +5V.
Some supplies, if you total all the V*I numbers, the total
exceeds the total power spec for the supply. What this means,
is the power supply has the flexibility to feed one heavily
loaded rail, as long as the others aren't loaded to their
maximums. If it wasn't for this effect, you would not be able
to move a P4 power supply into an AthlonXP computer, and you'd
have to stock two different kinds of supplies.
Some numbers:
ftp://download.intel.com/design/Pentium4/datashts/29864312.pdf
3.2C/FSB800/512KB cache 82W thermal design power (TDP)
82W/12V=6.8A, Vcore conversion 90% efficient 6.8/0.9=7.5A
Three fans in the case 12V@1A
Hard drive 12V@2A (spinup, temporary)
[email protected] stable 5V@1A logic
CDROM Allocate something in the same neighbourhood.
Video card:
Original estimates were 70W for a high end video card. Later
refined to 5V@10A and 12V@2A. I've actually measured my ATI9800
Pro 128MB and it measures
[email protected] and 12V@1A. Quite a difference.
You don't have to compensate for spinup current on the disk
drives, unless you have a large number of drives or something.
So, 10.5A would be a starting point for +12V. I recommend perhaps
12V@15A, as there should be some slack in case these estimates
are too tight. If you have one of the current high end cards,
like 6800 or whatever, then you'll have to do some more research,
as none of the reviews I read bothered to measure the aux power
feeding the cards.
Now, time to look at some more numbers:
Here are some Antec Truepower specs:
VOLTAGE +12V +5V +3.3V -5V -12V +5VSB
MAX. LOAD
TRUE330 17A 30A 28A 0.5A 1.0A 2.0A
TRUE380 18A 35A 28A 0.5A 1.0A 2.0A
TRUE430 20A 36A 28A 0.5A 1.0A 2.0A
TRUE480 22A 38A 30A 1.5A 1.0A 2.0A
TRUE550 24A 40A 32A 0.5A 1.0A 2.0A
Intel D875PBZ motherboard (similar to a P4C800-E)
ftp://download.intel.com/design/motherbd/bz/C3176501.pdf
Mode DC Power +3.3V +5V +12V -12V +5VSB
Minimum loading 199.00 W 5.00A 10.00A 10.00A 0.03A 0.60A
Maximum loading 284.00 W 10.00A 14.00A 13.30A 0.10A 1.40A
You can see from the Intel estimate, that my +12 current is
on the low side. The main value of the Intel numbers, is to
see that there is no need to panic about +3.3 or +5V power
consumption. Even adding in some current from +5V for the
video card, there is still room.
In terms of the Antec Truepower, any of those supplies will
handle your board. Take these current estimates with you
when you go to the store, and take the side off the cases,
to see the label on the PSU with the currents listed.
HTH,
Paul
