X-bit labs

FSP VGA Power: Power Supply Unit for Your Graphics Card (page 6)

Category: Other

by Oleg Artamonov

[ 05/16/2006 | 12:31 PM ]

Practical Use

The main question is whether the user actually needs a VGA Power. The parameters of the PSU are within the acceptable limits as I have checked in the previous section, so it will work correctly in a computer. But what load can it take over from the main PSU? How many cards can it power up? What else can you do with it?
In most hardware reviews the total consumption of a graphics card is mentioned because when the whole system is powered by a single PSU, it is the total consumption that matters. It doesn’t matter which connector the current flows through because this doesn’t affect the amount of dissipated heat or noise.

But there are two power supplies in our case. The graphics card has two connectors, too. The PCI Express slot it is plugged into not only gives it control signals, but also power, up to 75 watts. By the way, graphics cards with a maximum consumption of 50W and less are not even equipped with an additional power connector lately because the PCI Express slot alone gives them more power than they need.

I asked my colleagues from the Video lab to additionally measure the power consumption of today’s top-end graphics cards, dividing it into the consumption through the PCI Express slot and through the additional power connector. Here are the results:

So you can see that graphics cards do consume up to 50 watts of power from the PCI Express slot (and you may also note that the weakest of the tested cards – GeForce 7900 GT – doesn’t in fact need external power). Everything above that is consumed from the additional power connector. The consumption from the +12V rail is only shown in the diagram because it bears most of the load; the share of the +3.3V rail is negligible.

The VGA Power can only be connected to the graphics card’s additional power connector whereas the mainboard and the PCI Express slot are powered by the main PSU. So you shouldn’t expect the VGA Power to fully unload the main PSU as if there were no graphics cards at all – if you’ve built a SLI or CrossFire subsystem with two top-end graphics cards, it will consume as much as 90-100 from the main PSU.

The maximum load on the VGA Power that can be achieved in a modern system is a little less than 150W (if you’ve got two Radeon X1900 XTX) or half its maximum load capacity.

It means that:

1. The wattage of the VGA Power is not just sufficient, but almost twice redundant for modern systems. The VGA Power is even powerful enough for Nvidia’s upcoming quad SLI systems, at least assembled on the existing 7900 series chips.
2. The VGA Power cannot fully unload the main power supply of your computer. Two top-end graphics cards are going to put a load of about 100W on the main PSU.

Here I’d want to return to the beginning of the article where I said that on modern mainboards the CPU power connector is electrically separate from the mainboard’s main connector, and another power supply can be connected to it without any problems (of course, you should check this out on each particular mainboard with a multimeter – if there’s a near-zero resistance between the 12-volt contact of the ATX12V connector and the 12-volt contact of the main connector, they are linked together). But if the VGA Power is redundant even for a SLI system, why can’t it also be used to power the CPU? What you need is a new cable that can be made in 15 minutes if you’ve got just the basic skills.