Archived from groups: alt.comp.periphs.videocards.nvidia (
More info?)
'NoRemorse' wrote, in part:
| If there's only 1 rail,
| then all your devices are connected in parallel anyway, so what am I
missing
| here?
_____
You are missing a small factor. There is a certain amount of resistance in
copper wire, and a certain amount of contact resistance at each connection.
At high currents and low voltages this resistance can cause a significant
voltage drop. The move from 5 volts to 12 volts for on-motherboard CPU
DC-DC voltage downconvertors/regultors is to help reduce this effect. The
same is true for the direct 12 volt DC connection to high end graphics
cards. ATX +12V power supplies regulate the voltages as measured at a point
inside the power supply, not at the point of delivery. The difference in
the two voltages is tiny for low current circuits, but is larger for high
current cirucits. A hard drive and a graphic adapter on one + 12 volt line
from the power supply shouldn't be a problem, but there will be a slightly
higher voltage drop. Even if the resistance of the ground wire + 12 volt
wire + contact resistance is only 0.01 ohms, the voltage drop will be more
than 0.1 volts if the current drawn is 10 Amperes. At some point this could
become a problem - if contact resistance is higher and/or current draw is
higher. (The Molex connectors used on power supplies have a limited current
rating, that's why for the + 12 volt CPU connector there are two + wires and
connector pins and two - wires and connector pins, otherwise the current
ratings could be exceeded.
For the same reasons, automotive electrical systems are moving to ~ 48 volts
from 12 volts; lower voltage drops, less wasted power, and smaller wires and
connections.
E=IR
"NoRemorse" <m-jastrzebski@comcast.net> wrote in message
news:1_KdnaUGt8ZR_p7eRVn-gg@comcast.com...
> "vellu" <velimala@hotmail.com> wrote in message
> news:ddvjnm$bdu$1@phys-news1.kolumbus.fi...
>> Prof Rumpo kirjoitti:
>>> I have been meaning to upgrade for a while and was going for either an
>>> Radeon X800XL or a Nvidia 6800GT. I'm now tempted by the new 7800, but
>>> wonder if it's a waste of cash with my processor, a P4 3.4ghz (800) with
>>> a 460W power supply in my PC. I keep reading that these cards are CPU
>>> limited? Also can anyone tell me if I can connect the 6800/7800 power
>>> connector up to just one power cable from my power supply or are two
>>> power cables needed to connect up to the two molex connectors that I see
>>> with the adaptor supplied with most cards. Many thanks for any feedback
>>>
>>
>> You CPU is definitely more then adequate, it's far above average today.
>> Your PSU too is well above average so it should be enough aswell. It is
>> however recommended that you do not share the power connector with other
>> devices (with hard drives or cd/dvd drives). According to NVidia website
>> FAQ 350W is enough for a single 7800 card (500W recommended for SLI).
>> Also a single power connector is used, the adapter you are referring to
>> is most likely a connector splitter in case all connectors are in use
>> already.
>
> From an electrical standpoint, it shouldn't make any difference whether
> you connect the video card to a separate connector or to a connector
> that's shared with other devices unless you have a power supply with
> multiple rails and you know which connectors are on which rail. If there's
> only 1 rail, then all your devices are connected in parallel anyway, so
> what am I missing here?
>
> --
> NoRemorse
> "Expect me when you see me."
>
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