I am thinking of upgrading my power supply and have a couple
questions. 1) the cheaper power supplied seem to have inflatedf max
amp ratings. I am having a hard time finding a single 12v rail over
20amps on the quality cards, yet there are plenty of cheap ones with
this rating. I assume the cheap PS's are not putting out the claimed
power? Similar to cheap amplifiers, etc.
I was looking at the Thermaltake ToughPower W0105 700 watt and again,
it only shows a 18amps for the 12v. The cheap PS I have lists 19amps.
Will 19 amps be enough for a powerful pci-e video card, 2 hard drives,
and one dvd-drive?
The power supply standards are supposed to adhere to IEC60950.
As I understand it, that spec tells the power supply makers, not
to put more than 12V @ 20A on a single output rail. It has something to do
with fire safety, but I've never found any more information on the
topic that I can point you to.
There are two ways to limit the current. One way, would be for a
quad rail power supply, to have four actual separate circuits.
Very few power supplies have done this, and one clue that they have,
is the case of the power supply will be a lot longer than normal.
PCPowerandcooling used to make one with completely independent
output circuits.
A more common practice, is to make a giant single output, and then
put current limit detection on each output wire or group of wires.
The theory is, if the user draws more than 20A from one of the sets
of wires, the power supply turns off. There doesn't seem to be much
evidence that they do that.
It is also possible for the manufacturer to just flat out tell a lie.
Jonnyguru.com tests power supplies, and has found some of them to
use one common output, and have no current limiter at all. So when
the output says 18A and 20A on the two outputs, you could probably
draw 36A from one of the outputs.
PCpowerandcooling is honest about their Silencer 750W. It has a
single 12V @ 60A output, and that output feeds all of the 12V wires.
At least you know exactly what you are getting there. Maybe that unit
has good enough thermal sensing, that it could detect if there was
a fire inside the PSU. What I do wonder about, is if there was a
partial short on the output, you could have a motherboard burned up,
and the power supply would hardly even notice. (I've seen one
motherboard, that burned all around the CPU socket, and it was
quite impressive looking.)
So, what you'll find is, for dual rail 12V supplies, the manufacturers
will pretend to be adhering to the 20A limit. When the supply has
triple or quad 12V outputs, they'll tell you any story they feel
like, in terms of how the supply works. And not many review sites
will open them up, and tell you how they really work.
If you give a complete hardware inventory of your system, I can give
you an estimate. For example, a Core2 Duo is 65W, which is 12V @ 6A
when you take the Vcore efficiency into account. An 8800GTX is 145W,
which is 12V @ 12A. A hard drive is 12V @ 0.6A at idle. A CDROM
is 12V @ 1.5A or so (I've measured one at 1.0A). Case fans might be
0.5A for three of them. On a dual rail supply, 12V2 powers the CPU.
Leaving 12V1 for the other 15A or so. I'm still waiting for Xbitlabs
to measure an actual 8800GTX and give some real numbers - that is
where I get most of the numbers I use for estimates.
Paul
