# How much amperage do the 7800GTX and 7800GT need ?

Discussion in 'Nvidia' started by Skybuck Flying, Nov 11, 2005.

1. ### Skybuck FlyingGuest

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.

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.

Skybuck Flying, Nov 11, 2005

2. ### Roger HamlettGuest

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

Best Wishes

Roger Hamlett, Nov 11, 2005

3. ### farmuseGuest

1 godzillabiggagigawatt

farmuse, Nov 11, 2005
4. ### JEBEDIAH DUPREEGuest

typical black american response

JEBEDIAH DUPREE, Nov 12, 2005
5. ### Jim ThompsonGuest

And your response? You sound like a village idiot to me.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

Jim Thompson, Nov 12, 2005
6. ### Steve CarrollGuest

I believe that you need to read the formula more carefully.

Steve Carroll, Nov 12, 2005
7. ### Skybuck FlyingGuest

Indeed this site uses yet another formula:

http://www.ebtx.com/mech/ampvolt.htm

Watts = Amps x Volts x Length

Bye,
Skybuck.

Skybuck Flying, Nov 12, 2005
8. ### John LewisGuest

With that name he probably is one.

John Lewis
- Technology early-birds are flying guinea-pigs.

John Lewis, Nov 12, 2005
9. ### spodosaurusGuest

Or the governor of Florida...same difference...

--
spammage trappage: remove the underscores to reply

I'm going to die rather sooner than I'd like. I tried to protect my
neighbours from crime, and became the victim of it. Complications in
hospital following this resulted in a serious illness. I now need a bone
marrow transplant. Many people around the world are waiting for a marrow
transplant, too. Please volunteer to be a marrow donor:
http://www.abmdr.org.au/
http://www.marrow.org/

spodosaurus, Nov 12, 2005
10. ### Jim ThompsonGuest

Or French ?

Have you heard what trouble the French are into now? Seems the
terrorists burned down the white flag factory ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

Jim Thompson, Nov 12, 2005
11. ### Fred AbseGuest

You owe me for a new coffee cup, keyboard, shirt, pair of pants and carpet

Fred Abse, Nov 12, 2005
12. ### Rich, Under the AffluenceGuest

.
The early bird gets the worm, which is fine if you like worms for
breakfast. ;-)

Cheers!
Rich

Rich, Under the Affluence, Nov 13, 2005
13. ### David MaynardGuest

Makes on wonder who surrenders first when both sides are French.

David Maynard, Nov 13, 2005
14. ### First of OneGuest

http://www.xbitlabs.com/articles/video/display/ati-vs-nv-power_2.html
http://www.anandtech.com/video/showdoc.aspx?i=2451&p=20

The 6800Ultra AGP uses 72W under load (measured with shunts on the power
dongle and AGP pins tapped), drawing 3.7A on the 5V rail and 4.2A on the 12V
rail. 4.2A is evidently too much for a single Molex 4-pin connector, so
nVidia equipped the card with two. The PCIe version draws more juice from
the slot. However, overall power consumption should be no different.

The 7800GTX uses less power than the 6800Ultra. If the 7800GTX uses only the
12V rail, you are looking at 72W/12V = 6.0A. This is *just for the video
card*. Those "18A - 24A" figures you saw elsewhere account for other
high-powered equipment, like a hot Prescott CPU and a RAID5 array. Certain
components like DC motors in disk drives cause large current spikes at
startup.

First of One, Nov 14, 2005
15. ### bowgusGuest

bowgus, Nov 14, 2005