ccording to Jacobs, AMD had started seeing similar shutdowns with high-end, dual-graphics card setups recently. He was concerned that readers might attribute the problem to the power draw of the FX-60, which would be an incorrect conclusion. Jacobs pointed the finger squarely at dual-graphics cards, noting that the 512MB 7800 GTX can draw in excess of 11 amps when running full bore. If the two cards share a power supply rail, that means that more than 20A is being pulled from one power supply rail, which is a recipe for disaster. According to another source, AMD has started recommending 700W power supplies with high-end SLI or CrossFire setups to alleviate potential problems.

A Question of Balance

It turns out that throwing bigger power supplies at the problem isn't necessarily the issue. Part of the issue revolves around power supply design. The rapid escalation of power usage by today's high performance graphics cards has taken some manufacturers by surprise. Kelt Reeves of Falcon Northwest was caught unawares when another Web site reviewed one of his systems, which had been configured with a fairly light load-out—except for two 512MB 7800 GTX cards. Falcon Northwest uses the 600W version of the same Silverstone power supply and was surprised when the reviewer experienced system shutdowns.

"We've loaded up systems with two 512MB 7800 GTX cards, four 10,000 RPM hard drives and two optical drives and never had a problem with the Silverstone power supplies," Reeves noted in a phone conversation.

What happens is that some power supplies are designed with a shared power plane. According to Tony Ou, of Silverstone technical marketing, in an email:

"I am sure you already know that PC power supplies we have today have three main rails, +3.3V, +5V, and +12V, which are required to power various components. However, what most people don't know is that many power supplies are designed with shared power plane (it is very common to have +5V and +12V rails linked together) to help reduce cost and obtain higher maximum power. If you have seen our retail box for our ST60F 600W model or read the manual, you will see this:

+5V min. load is 10A when +12V output is 30A to 38A
+5V min. load is 15A when +12V output is 38A to 42A

This means that in order for our power supply to generate maximum power for +12V rail optimally, the +5V must also be loaded up. Normally this is not a problem because most systems do draw enough power from both +5V and +12V rails evenly to make cross loading requirement a non-issue. Even when SLI came out in late 2004, the most powerful gaming system at the time would rarely draw more than 30A from the +12V rail..."

In other words, if you balance the loads on the different rails, then you won't have this kind of problem. So we took PCI Express power adapter cable to give this idea a test. These cables consist of two Molex four-pin connectors on one end, and a six-pin PCIe connector on the other end.

Using the same Silverstone 650W power supply as before, we ran the second 512MB 7800 GTX with the adapter connected to a different cable. At this point, everything ran without a hitch.