PCI Express: Measuring Power Consumption

Power consumption issues were seen to by the developers of the PCI Express bus. It surpasses the Accelerated Graphics Port (AGP) in this respect. As a response to the higher power consumption of modern system components, the PCI Express specification lists advanced energy-saving functions, and a graphics card in the PCI Express x16 slot can receive much more power than one installed into the AGP. The developers of the new standard accounted for the unavoidable growth of the power consumption of graphics cards and realized that the maximum of 42 watts as provided by the AGP wouldn’t be enough.

So, the maximum amount of power a graphics card may get from the PCI Express x16 slot is about 75W, although originally a smaller value (like 60W) was proposed. The developers must have miscalculated the rate of the power consummation growth of modern graphics cards and had to adjust the specification, assigning more contacts for transferring electricity and raising the consumption bar to 75W.

Graphics cards in the “desktop” PCI Express variant are powered by two lines, 3.3v and 12v. The specification of the PCI Express x16 slot sets the maximum currents on these lines at 3amp and 5.5amp, respectively. The total power consumption on these lines, i.e. the maximum consumption of a graphics card without additional power connectors, is easily calculated to be 75.9W.

NVIDIA’s GeForce 6600 GT reference card has no additional power plugs – there’s place left for it on the PCB, but it is not soldered in:

So, the power consumption of the GeForce 6600 GT cannot exceed 75.9W. But how can we measure it with more precision? It’s done simply, in the same way as I tested the AGP cards in my previous reports. I isolate the power contacts of the PCI Express x16 slot and send this power directly to the card, bypassing the slot.

The PCI Express x16 slot is divided into two parts: the bigger part contains signal lines, and the smaller part consists of power lines. On the front side, the smaller part of the slot contains three 12v contacts and one 3.3v contact.

The back side of the slot has two 12v contacts and two 3.3v contacts.

In total, we have five contacts of the 12v line which has a higher maximum current value (5.5amp) and only three contacts of the 3.3v line (a maximum current of 3amp).

I measured the power consumption of the GeForce 6600 GT in two modes: Idle and Burn. There were no running applications in the Idle mode; the Windows Desktop was on the screen in 1280x1024x32bit@75Hz display mode. For the Burn mode I started up Doom 3 in 1600x1200 resolution with the maximum graphics quality settings and forced 4x full-screen antialiasing and 8x anisotropic filtering.