It means the supply is designed with the expectation that one
rail is more heavily loaded than the others.
http://images17.newegg.com/is/image/newegg/17-139-006-Z05?$S640W$
The supply is a 750W total. The 12V rail is rated at 12V @ 60A or
720W. You are allowed to draw that 60A if you want, but if you
did, there would be little remaining total power allowed to be
drawn from the other rails. You'd have 30W left for the other
rails, which for the computer, may not quite cover all the
loads present. It is highly unlikely you could arrange to draw
the 60A, as the other rails may need more than the 30W figure.
That label really spells it all out for you. For example, the
180W limit, says that, although you have 3.3V @ 30A and 5V @ 28A
to work with, the total of those two is limited to 180W. Perhaps
they're sharing a resource inside the power supply. You could draw
99W from the 3.3V side, and have only 81W left from the 5V side,
so that would be 5V @ 16A or so. Either the 3.3V or the 5V rail
could be loaded up to its max, but that would leave less than
the max for the other partner. And obviously, if the 3.3V and 5V
are drawing 180W, then you're no longer going to be allowed to
draw 12V @ 60A, because the sum total would be over 750W.
You'll notice that the label is kinda arranged in "layers". The
750W at the bottom, is an overriding concern. The next level up,
partitions the rails into smaller groups of concern. Finally,
you get to the individual rails and their maximum limits.
To work out what is available on your 750TX, you do a budget
for the computer. I've done a bunch of these in the past for
people, as an illustration of how you can go about it. For
example, I allocate 50W for the motherboard and RAM, and
assume it comes from either the 3.3V or the 5V rail, without
being more precise about it. (The 50W is based on my experience
measuring just two of my motherboards, so there is little basis
for the estimate. I checked an AthlonXP and a P4 board, to come
up with the figure.) So, if we took your 750W, and subtracted 50W
from it, now we have 700W left. If the remaining budget was allocated
to the 12V rail, that would now be less than 60A. We can add in
contributions for hard drives, optical drives, processor and so on,
until the allocation for video is all that is left.
There are also web sites that do power calculations, but
I don't like any of the current sites that are available,
due to their habit of hiding details. I feel it is important
for the estimation process to be completely exposed (as in,
a spreadsheet format), as then it is easier to verify
just how bad the estimation method is. For example, Asus
offers a web page for power supply calculations, but it
works in increments of 50W. It is quantized. You could
have 49W worth of load, and it would say "zero". You could
add 1W more load and then it would read "50W". Real stupid.
By doing so, and rounding, it isn't possible to accurately
see what each component is contributing to the total. Such an
approach, to me, does not encourage a sense of trust. As in,
the ability to verify the math is being done correctly.
The Takaman site in Japan was the only site I liked, and it
closed years ago. I think they used to sell power supplies,
and that was their reason for offering a calculator.
One web site people used to use, offered estimates that
were roughly double the real value. I think at one time,
it may have estimated a stick of RAM as 25W. When I
see stuff like that, I encourage people to use their
imaginations. For example, imagine a 100W light bulb
where your four sticks of RAM are. Would you burn
yourself on the RAM, the same way you would on the
light bulb ? If the amount of power computed, doesn't match
your experience with component temperatures, that is a
very rough way to do a reality check. Modern RAM sticks,
according to Kingston data sheets, are somewhere around
2W a piece now. Which is why I can easily hide them
in my "50W for chipset and RAM sticks" figure, without
really worrying about it.
As for "how does a power supply work", here is one schematic
to illustrate. What is shown here, is very old, but still
gives you some idea how it works. There is a text description,
so no point in me repeating it
http://www.pavouk.org/hw/en_atxps.html
The nice feature of those, is they're transformer isolated for
safety. The "Hi-POT OK" on the label of your power supply,
means someone tested the transformers for a certain level of
breakdown protection. So even that little thing, is there
for a reason. You assume they're Hi-POT tested, but it is
nice to see it in print.
Some details on Hi-POT testing are mentioned here, in the
"Hi-Pot Requirements" section.
http://www.ramtechno.com/medsafety.php
http://en.wikipedia.org/wiki/Hipot
HTH,
Paul
