Debunking Power Supply Myths

Connectors and the 12V Rail Issue

Okay, we've chosen a few power supplies for our three test systems, but so far we've only looked at performance. There are still two additional issues that can affect your choice of power supply. We have previously shown that it is possible to run even high-end systems with a much lower rated power supply than you might expect. However, look at the offerings from the various manufacturers and you will frequently find that there are insufficient connectors for some configurations. The second issue involves the 12V rail(s), which is primarily responsible for powering the processor and graphics card(s).

Let's start with the first issue: having enough connectors. It is certainly possible to run a GeForce 8800 Ultra using only a 400W power supply; however, no one makes a 400W PSU with the necessary two PEG connectors. In fact, some power supplies in this range might not even have a single PEG connection. You could always use a Molex to PEG adapter(s), but you're probably better off selecting a different power supply.

Our entry-level system used the ATI Radeon HD 3650, which is a nice choice because it doesn't require any PEG connector. In the future users might want to upgrade graphics cards, however. The good news is that all of the entry-level PSUs be selected include a single 6-pin PEG connector, so they should be sufficient for powering up to a midrange (8800 GT/HD 3850) graphics card. If all you need is an average computer system, these power supplies will work fine.

The second issue is the amount of power the unit needs to provide in order to port graphics cards that use a single 6-pin jack. We could have included more cards, but for this example we've selected ATI's HD 3850/70 and NVIDIA's GeForce 8800 GTS and 9600 GT. We've created a table showing how much power these cards consume and where this power comes from under full load.

We need to check if the power supplies come with either one or two 12V rails. PSUs with two 12V rails usually have one rail connected to the 6-pin PEG connector and the other running the 24-pin ATX connector. The 24-pin ATX connector has two yellow cables that supply 12V, and both of these cables supply the PCI-E slots with power. The increasing power requirements of modern GPUs was the driving force behind the switch from 20-pin ATX connectors to 24-pin connectors. Some power supply manufacturers include different color markings on the 12V cables to differentiate rails, so if you have such a power supply you should make sure you connect components to a rail that has sufficient remaining juice.

It's important to have one 12V rail supply the CPU with power and the second rail for the PCI-E slots and 6-pin connector. Unfortunately, many companies make a tremendous mistake when it comes to power distribution. We have seen several power supplies that use one 12V rail for the 6-pin PEG connector and then a second 12V rail for the CPU and 24-pin ATX connector. That means if you have a graphics card that doesn't include a 6-pin jack, both the CPU and GPU will use the same 12V rail for power. In this case, the second 12V rail goes completely unused, and users risk drawing too much current on the remaining 12V rail. In addition, how much power a GPU draws from the 6-pin connector and how much it takes from the PEG slot varies.

Checking the labels of the entry-level units, we see that our selected power supplies should all have no difficulty running any of the above GPUs. Power supplies like the Corsair VX450W that has a single 12V rail have the advantage of being able to fully utilize the rated 33A.