Power Supply Help


Shaun B

Hi I have an ASUS A7V8X-MX SE mobo and have a 20 pin 400W Power Supply for
it, what is best for it ie 350W 400W 450W or 500W PS?? What is the diference
if it can run on a 350W ps why have higher Watt Power supplys?
Thanks Shaun B.



Mike T.

Shaun B said:
Hi I have an ASUS A7V8X-MX SE mobo and have a 20 pin 400W Power Supply for
it, what is best for it ie 350W 400W 450W or 500W PS?? What is the
if it can run on a 350W ps why have higher Watt Power supplys?
Thanks Shaun B.

That's the wrong question to ask. The number on the box of the power supply
is usually exaggerated. That is, some 350W power supplies are better than
some 500W power supplies. A typical home system built today should not
require more than 400W. HOWEVER, that assumes that the power supply is a
good name brand like fortron/source, enermax or seasonic. If it's NOT a
good name brand, then it might be rated as 400W, and only able to actually
push 250W.

If you want to know more about what your SPECIFIC system requires, check the
specs. for your CPU and the documentation for your video card. BOTH should
have recommendations. For example, the CPU might specify 350W minimum, but
the video card might specify something like "30A on the 12V rail". Then
when you go to look at power supplies, you will find that the power supply
that gives you 30A on the 12V rail is actually 450W. How do you know the
12V rail rating? It should be listed in the advertisement for the power
supply, on the spec. sheet of the power supply manufacturer's web site, AND
on a label on the side of the power supply. You will see something like
"+12V 30A". Or you might see something like "+12V1 15A, +12V2 14A". That
would be 29A on the 12V rail, but the power supply actually has TWO 12V
rails. (so just add the amps together)

The reason you check the specs for the CPU and video card is that those two
components alone use most of the power in your system. If your power supply
complies with the recommendations for the CPU and video card, all other
components will have PLENTY of power, as the video card and CPU
manufacturers consider other typical components in the system when making
their recommendations about power supplies. -Dave


Hi I have an ASUS A7V8X-MX SE mobo and have a 20 pin 400W Power Supply for
it, what is best for it ie 350W 400W 450W or 500W PS?? What is the diference
if it can run on a 350W ps why have higher Watt Power supplys?
Thanks Shaun B.

Graphics cards and processors have both become more
power hungry. And a lot of people are using more drives
than they used to. TigerDirect has a simple guide on their
site. I don't have the link offhand but this is their list:

Component Wattage Required
Motherboard 15-30
Low-End CPU 20-50
Mid To High-End CPU 40-100
RAM 7 per 128MB
PCI Add-In Card 5
Low To Mid-Range Graphics 20-60
High-End Graphics 60-100
IDE Hard Drive 10-30
Optical Drives 10-25

I'm not an expert about this, but I think the
big numbers that people talk about for power
supplies are mostly coming from gamers who have
high-end systems and enjoy doing that as a
hobby, like having a high-performance sports car.
So you have to consider it in context. If you just need
to drive like most people drive then you won't need
fancy tires.

For instance, according to the list above you should
add 40-50 watts to your calculation of what you need
if you're using "high end" graphics. On the other hand,
for most people who are not big on trying the latest games,
using on-chip graphics or an old graphics card is more
than enough to do the job. All they need is enough
graphics memory to give them 4 bytes per pixel of
display, so that they can use 32-bit color. Generally
that's 4-8 MB. And PCI graphics is fine.

I just bought an Antec 330 (or maybe 350?)
for a new system. I do software programming,
web design, browsing, writing, etc. I don't use
games and I don't play video. I also don't have
any special sound requirements. I got on-chip
sound and video because it's adequate for my
uses and saves a lot of money. And I only need
256 MB of RAM because I'm running Win98SE
and Linux, both of which are very happy with
that amount. I also have 2 IDE hard disks
and 1 CD writer.
So I don't need a higher wattage power supply.
But some of the descriptions I've read in this group
are of machines with 3 times as much hardware
hooked up as what I have.


"Shaun B" said:
Hi I have an ASUS A7V8X-MX SE mobo and have a 20 pin 400W Power Supply for
it, what is best for it ie 350W 400W 450W or 500W PS?? What is the diference
if it can run on a 350W ps why have higher Watt Power supplys?
Thanks Shaun B.

There is a lot of mythology around power supply selection, and it
has a lot to do with the inability of consumers to get good
info about what they are buying.

For example, there are 500W supplies for $20 and 500W supplies
for $100. How can that be ? In one funny account I was reading,
in a Newegg review, a guy bought four cheap power supplies, as
each one only lasted about 2 weeks before dying. He could have
bought a more expensive supply, and not had to waste a lot of
personal time ordering and reinstalling the things.

In terms of brands to avoid, the "skum" at the bottom of the
food chain, keep changing the brand names. In the power supply
industry, there is a lot of rebranding, so I can buy a Deer
power supply, slap a coat of paint on the outside of the power
supply case, and set the retail price at $100. It may take
users a while to conclude my "new" brand is crap. In which case
I get to sell a lot of supplies, leaving the users with the
headaches. The cheap supplies want to maintain a low profile,
and not be reviewed by web sites, for fear they will be
discovered and the game is up.

In the latter case, there is not a lot that users can do to
protect themselves. On the one hand, you can buy from
reputable name brands (Enermax, Fortron/FSP/Sparkle, Antec,
PCPowerandCooling), but you may be paying extra for the name.
And virtually every brand has some gotcha (there are some people
on USENET who have opened up some of these things, and on occasion
they'll tell us what they found).

So, to start, the rating printed on the side of a cheap power
supply, may not be sustainable for long periods of time. Reading
the user reviews on Newegg, for a given power supply, can give you
an early warning about quality issues, but if a brand has just
been "freshly introduced", not much may be known.

As for the web sites that recommend power supplies, some of these
sites have an axe to grind. Some of them sell power supplies, and
by giving you an estimate on the high side, they seek to profit
from the result. Other sites, pretend to offer info on power
supplies, but all they are really doing, is getting advertising
profits from the adverts on the web page. You would do as well
by just randomly selecting a supply, as following their advice.
The one good site, in terms of the approach they used to power
supply selection, is unfortunately out of business, leaving nothing
of value to take its place. (That was takaman.jp)

The Takaman site, worked out the power supply requirements in
spreadsheet form. Every component in the computer, needs some
amount of 3.3V, 5V, or 12V. Takaman had estimates for common
components, and the spreadsheet would add them up. The results
are the "total amperes" for each voltage. This is the right way
to do the calculation, and can be used to provide a realistic
estimate of the power supply required. I copied the results
from the site, for one system I was looking at, and so this
is an example of what they would tell you:


The current generation of components, tend to draw major amounts
of power, from the +12V output. By the time you purchase a supply,
with enough amperes on +12V, usually you are covered for +3.3V and
+5V. There can be exceptions to the rule, but not a lot of people
are sticking antique PCI cards with a lot of high power 5V components
on them into the computer, so for the most part the +3.3V and +5V
are not that demanding.

The current output, in amperes, is printed on the label on the side
of most shipping power supplies. There are the odd skum power supply,
with no label on the side, but they are the exception rather than the
rule. Since Newegg has pictures on their web site, and usually also
lists the individual current output levels for the supply, that
site is a gold mine in terms of selecting a supply. You can always
buy the supply elsewhere when the selection process is finished.

One reason I'm writing this, is I hate to see people buy supplies
based on the simple-minded "350W" label. And, if I'm not careful,
I can lapse into generalities as well. For example, for a computer
with a low end video card, one disk drive, one CD, a basic minimal
system, I might recommend finding a supply with 12V @ 15A output.
(I would recommend that, if for example, a user refused to give
a list of the hardware they were putting in the computer. A
detailed list...) Most of that current is used by the processor, and
if you had an 89W processor, you can see that makes a good dent in
the 180W potential output of the supply on the 12V rail. (The processor
powering circuit draws current from +12V, and that is where the 89W
would come from.)

At another extreme, say a system with an FX-60, and two highest
end ATI video cards, the estimate would be 10 amps for the processor,
and 10 amps for each video card. That is 12V @ 30A total. But few
people can afford such extravagance, so even mentioning buying a
supply with a 30A rating, is silly for most people. If you are
building a high end system, you may need that kind of power, but
for systems at total cost of $1000 or less, chances are a lot
less than that is required.

So calculating the power required is the right way to do the
selection. Start with a list of _all_ the components in the computer.
Try to find numbers for the currents consumed. Total up the amps
for +3.3V, +5V, +12V outputs, and find a supply that matches the
requirement, with a few amps left over for expansion, or aging, or
whatever you will.

I've measured current used on a couple of my computers.

P4C800-E Deluxe, 4x512MB memory, 2.8Ghz Northwood, motherboard consumption

12V @ 5.9A (CPU), 3.3V @ 13.8A, 5V @ 0.56A (motherboard connector)
12V @ 0.4A fan current
12V @ 0.5A 5V @ 5.5A (Ati 9800Pro video card)

You can see the current is pretty well spread across the output rails.
The disk drives draw [email protected] when they first spin up, and draw
[email protected] and [email protected] when idle in Windows, and drive currents were
not included in the above.

You can see that a supply with 12V @ 15A (a "350 watter" if you will),
could well be good enough.

The second system is an A7N8X-E Deluxe with AthlonXP 3200+.

12V @ 0.53A (CPU fan), 3.3V @ 6.4A, 5V @ 16.6A (CPU uses the +5V rail)
12V @ 0.5A, 5V @ 5.5A (Ati 9800Pro video card)

Here, the power comes mostly from the +5V rail. An "AMD" supply is
called for, where the +5V rail has a rating of at least 25A or so. Now,
current generation motherboards don't do this any more, namely
draw CPU power from +5V. And that is why we can concentrate on
the 12V output rail rating when shopping today. If you had an older
Athlon system, you may see more +5V load than +12V.

So, while I cannot write a complete book in this posting, on the
exact right supply for your application, that is the kind of
approach to take when selecting a supply:

1) Do a detailed calculation of power consumption. Info sources:

processorfinder.intel.com (Intel processor power)
www.amdcompare.com (AMD Athlon64 or later, processor power)

X1900XTX, X1900XT, X1800XT OC, 7800GTX 512MB
X1800XT, X1800XL, 7800GTX, 7800GT
X850XT, X800XL, X800GTO16, X800GTO, 6800GS
X800GT, X1600XT, 6800, 6600GT
X1300Pro, X700Pro, 6600, S3 Chrome S27


X800XTPE, X800Pro X800Pro Extreme, 9800XT, 9800Pro, 9600XT, 9600Pro
http://www. xbitlabs .com/articles/video/display/ati-powercons.html

6800 Ultra, 6800GT, 6800, FX5950 Ultra, FX5900 Ultra, FX5700 Ultra, FX5700
http://www. xbitlabs .com/articles/video/display/ati-vs-nv-power.html

2) Use well known brand names, maybe check the odd review, check Newegg
customer reviews to find duds etc.


I'm still waiting for a web site to come up with a decent calculation
tool, that gives a spreadsheet style output. The spreadsheet style helps
expose really stupid estimates, and that is why the web sites that
do offer a "power calculation", won't show the numbers they use. They
don't want to be ridiculed.





Shaun B said:
Hi I have an ASUS A7V8X-MX SE mobo and have a 20 pin 400W Power
Supply for
it, what is best for it ie 350W 400W 450W or 500W PS?? What is
the diference
if it can run on a 350W ps why have higher Watt Power supplys?
Thanks Shaun B.

I just did that. A 350 Antec came with the case. I took it out and
replaced it with the 500w. Why? It's like a car. You can drive at
50mph on a 4 cylinder, but if I want to, I can 'go'. Besides that.
it's a minor expense.

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question