Skybuck Flying said:
Hello,
On this power supply calculator it says:
http://extreme.outervision.com/index.jsp
The 7800GTX needs about 118 watts of power.
The 7800GT needs about 97 watts of power.
Somewhere else I also read
Current = Watts / Voltage.
Since these cards are supposed to be powered by a single 12v+
rail/connector
(?) this would mean the amperage is about:
118 / 12 = 9.8 amperage.
97 / 12 = 8.0 amperage.
So a power supply delivering 18A on each 12v+ rail should be enough ???
However I have seen many threads say at least 18A or even 24A...
So I am a bit confused...
How much amperage/ampere do these cards really need ??
Bye,
Skybuck.
The problem is a question of how you 'measure' a current 'rating'. If you
look back in time, to the old IBM supplies for the original PC-AT, these
were 'rated' at 135W, conversely many of the 'clone' supplies at the same
time, were rated at 220W, yet the IBM supplies often ran systems fine,
while the latter showed problems with poor regulation. The reason was that
the IBM 'rating' was what the supply could deliver at the connectors, with
less than 5% change in regulation, while the 'clone' suppliers were uing
10% change, and often at the supply itself. Given that a couple of rails
required regulation better than 7%, on the actual board, the IBM supply
was often giving better performance than the supposedly more powerful
'clones'. The same trend has continued since, with the manufacturers with
'good' reputations, having much more margin in their specifications, than
other units.
Now the big problem with cards, CPU's etc., is not only their 'continuous'
power consumption in heavy use (the 97W/118W figure), but the rate at
which this can change. If (for instance), the card switches from an almost
idle state, to doing a heavy rendering task, the load can jump from just a
couple of amps, to nearly 10A, in a few uSec. The question is how well a
supply can maintain regulation when there is this sudden change?. Supplies
built for heavier outputs, will tend to have larger reservoir capacitors
(slowing the rate at which the rail voltage changes), and heavier driver
components, allowing the circuit to actually cope better when there is
such a change. A good analogy, would be wooden boards on a scaffolding.
Heavier boards, not only allow bigger loads to be supported, but if a load
is suddenly dropped onto the boards, they 'spring' less, with the sudden
change. It is possible to design supplies to give better behaviour for
this type of change, but 90% of PC supplies on the market, are sold with
ratings that they can only just generate, and small margins...
Hence for good reliability, it becomes necessary to build 'margins' into
the specification, and 'overrate' supplies by perhaps 20%. Some
manufacturers do give 'real' figures, and (for instance), it is perfectly
reasonable to use a Artic Cooling 350W supply on a system that needs over
300W, while conversely, many of the 500W 'clone' supplies, can only just
about handle the same requirements...
Best Wishes
