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

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

  1. 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.
     
    Skybuck Flying, Nov 11, 2005
    #1
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  2. 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
     
    Roger Hamlett, Nov 11, 2005
    #2
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  3. Skybuck Flying

    farmuse Guest

    1 godzillabiggagigawatt
     
    farmuse, Nov 11, 2005
    #3
  4. typical black american response
     
    JEBEDIAH DUPREE, Nov 12, 2005
    #4
  5. Skybuck Flying

    Jim Thompson Guest

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

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)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
    #5
  6. I believe that you need to read the formula more carefully.
     
    Steve Carroll, Nov 12, 2005
    #6
  7. 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
    #7
  8. Skybuck Flying

    John Lewis Guest

    With that name he probably is one.

    John Lewis
    - Technology early-birds are flying guinea-pigs.
     
    John Lewis, Nov 12, 2005
    #8
  9. Skybuck Flying

    spodosaurus Guest

    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
    #9
  10. Skybuck Flying

    Jim Thompson Guest

    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 Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)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
    #10
  11. Skybuck Flying

    Fred Abse Guest

    You owe me for a new coffee cup, keyboard, shirt, pair of pants and carpet :)
     
    Fred Abse, Nov 12, 2005
    #11
  12. .
    The early bird gets the worm, which is fine if you like worms for
    breakfast. ;-)

    Cheers!
    Rich
     
    Rich, Under the Affluence, Nov 13, 2005
    #12
  13. Makes on wonder who surrenders first when both sides are French.
     
    David Maynard, Nov 13, 2005
    #13
  14. Skybuck Flying

    First of One Guest

    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
    #14
  15. Skybuck Flying

    bowgus Guest

    bowgus, Nov 14, 2005
    #15
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