"jime" said:
I will have 3 HDs, a DVD, DVD-RW, 2 printers, 1GB memory, P4 2.8 Northwood.
I have a case with a 300W power supply. Will this supply be large enough?
You didn't mention the video card.
Intel has numbers for one of their 875 chipset boards. The
power numbers, unfortunately, aren't just for the components
on the motherboard, which means they cannot be relied on
by themselves.
ftp://download.intel.com/design/motherbd/bz/C3176501.pdf (D875PBZ)
DC Power +3.3V +5V +12V -12V +5VSB
Minimum loading 199.00W 5.00A 10.00A 10.00A 0.03A 0.60A
Maximum loading 284.00W 10.00A 14.00A 13.30A 0.10A 1.40A
Power supplies have a rating in amps (A) for each output voltage
of the major supplies +3.3, +5, and +12. In principle, you
should calculate the totals for all three supply voltages,
to determine what are minimum acceptable values.
What I recommend, is for the most part, on the +3.3V and
+5V, go with an estimate like the ones from the Intel
motherboard datasheet. You can bump up the numbers as you go.
First, let's list some sample supply specs:
Antec Truepower specifications -
VOLTAGE +5V +12V +3.3V -5V -12V +5VSB
TRUE330 30A 17A 28A 0.5A 1.0A 2.0A
TRUE380 35A 18A 28A 0.5A 1.0A 2.0A
TRUE430 36A 20A 28A 0.5A 1.0A 2.0A
TRUE480 38A 22A 30A 1.5A 1.0A 2.0A
TRUE550 40A 24A 32A 0.5A 1.0A 2.0A
So, first of all, the +3.3V is easy to meet.
On the +5V, you need to add maybe 1 amp per drive.
If you have an ATI9800 or FX5950, add 10 amps for +5.
Total 14 + 1*5 + 10? = 29A (All Truepower meet this.)
If you have a video card without aux power cable, strike
the 10A add-on.
For +12V, the 2.8GHz/FSB800/512KB cache processor draws
55.9A @ 1.5V operating voltage. The power is converted
from +12V. 55.9*(1.5/12) = 7 amps. At 80% efficiency
conversion, then 8.7 amps is drawn from +12V. 1 amp
for fans, 0.5 amps per idling drive, 2A for high end
video card => 8.7 + 5*0.5 + 1 + 2 = 14.2A from +12V.
When the computer first starts, the three HD drives
spin up, and each draws 2 amps from +12V until the
drives are up to speed. So, at startup, the disks
draw more current, and the video card draws less current,
and for the most part, the slight increase isn't important,
as it only lasts for seconds.
From this exercise, you may notice that the +12V supply
is pretty critical. The +5V is next, and generally isn't
an issue until you throw in a high end video card. (The
big unknown here, is what Intel included in their 14A
of consumption number. I hadn't expected to even get
close to the limit on +5V, so maybe something is getting
double counted here.)
On the surface of it, a True330 meets the bill, and a
380 or 430 leaves a little breathing room.
You cannot shop by total power number alone. For
example, Powmax used to ship a 500W power supply
with 12V@10A. While the total power is good, the
+12V wasn't sufficient for even a basic P4 system.
That is why, it pays to work up some estimates on your
own.
There are web sites that have estimators on them,
but they tend to over-estimate the power required
(presumably so you'll buy a big ass supply).
http://www.jscustompcs.com/power_supply/
You can look up power numbers for many of the products
you buy, and then compare them to the estimators, to
see how far out they are.
Now, a little sanity check. Total up all the estimates.
(I'll include the high end video card numbers.)
3.3*10 + 5*29 + 12*14.2 = 33+145+170.4=348W
If you actually measure the wall power, you'll probably
find slightly more than 200W coming from the wall plug.
That will give you some idea of how screwy my numbers
are
If doing this exercise on an AMD board, you have to
verify where the processor power comes from, as some
boards use +5V and others use +12V as the source. This
messes with the PSU selection process a bit.
Also, when the next gen video cards hit the streets,
like 6800 or x800, they are going to be busting the
power supply calculation big time. Be prepared to
fork out more cash for a new power supply to go with
one of those video cards, when some genius finally
measures the real consumption, so we can take it
into account in the estimates. The review sites did
us a dis-service by not measuring the current on the
aux power cable, when they had a chance. Now we'll
have to find an end-user to do the measurement.
HTH,
Paul
