Power Supply Testing Methodology
at EXTREME Overclocking

ATX12V v2.x In-Depth:

The biggest area of concern with the latest ATX12V specs is how the 12V rails are split. Below lists how the two rails are *supposed* to be divided, however manufacturers don't always follow guidelines. Because of this inconsistency we will have a page listing which connectors are on each 12V rail for each power supply. The main reason for splitting the rails is for safety (UL/EN code lists a maximum of 240VA per circuit), and to try and keep line noise at acceptable levels.

• 12V1 - ATX Main Power, SATA Connectors, Peripheral & Floppy Connectors, (PCIe)
• 12V2 - +12V Power Connector

Note: The 6-pin (2x3) PCI-Express connector was added & removed from the official ATX12V power supply specifications very quickly, however that hasn't stopped manufacturers from using it. Since there is no official spec, there seems to be almost no rhyme or reason as to which rail the PCIe connector is on (if a PSU has one). Motherboard & video card manufacturers add to the confusion sometimes by requiring additional power via different connectors, which may or may not be on the 12V rail that you would prefer to keep the load balanced.

Split 12V Rails & SLI:

Here's some interesting points to ponder (this is all just my thoughts on the matter). For most PCs split 12V rails is good (for the reasons mentioned above), however with the advent of SLI (dual video cards) 12V power consumption made a noticeable jump. If you consider the peak power consumption from a pair of top-of-the-line video cards can reach 12-16A, that doesn't leave much power (if any) for the rest of the devices on that rail. UL/EN spec says a maximum of 240VA (translates to 20A on each 12V rail, or even more stringent is 18A per ATX12V spec).

So what is a power supply company supposed to do (for both short-term and long-term)? The reality is unfortunately the UL/EN safety spec will be in direct conflict with what can be done with respect to a power limit of 240VA per rail. More often than not, a PSU manufacturer will simply join the two rails together on the supply-end to maximize available 12V power.

The SLI market is relatively small compared to the entire PC market, take a subset of that number for users that have the top-end video cards, and then divide up that small percentage among all the various PSU makers. The end number that would be roughly each PSU manufacturer's share of the market is extremely small when compared to the mass-market of more "traditional" requirements. So, creating a special power supply just to cater to this nice market really isn't cost-feasible (at this time).

Until some new official guidelines are published, and there is a clear direction that both video card & motherboard manufacturers are going to take, I don't think we are going to see any huge change in power supply design. The current route of having a combine 12V rail seems to be the more cost-effective solution than something like a 3rd 12V rail dedicated for video.

What We Use To Test Power Supplies:

In order to do a more thorough job of testing power supplies than the traditional "hook it up to a PC" test, we have built a special dummy load rig inside a full tower case. In order to keep things simple, but reproducible, various resistors were wired to switches which allow incremental load levels on each rail. The maximum total capacity for this device is about 800W, give or take.

• Maximum selectable current (under nominal voltage levels) for each rail (approx):
  • 3.3V - 38A
  • 5V - 50A
  • 12V1 - 18A
  • 12V2 - 18A
  • 5Vsb - .5A (fixed)
  • -12V - .2A (fixed)

The reason this devices is built inside a case and not an open-bench design is because we wanted to take one extremely important factor into consideration, HEAT. As the ambient temperature increases, the maximum output capacity of a power supply will decrease. ATX12V specs clearly state that a power supply should be able to operate under full load in a 50° C ambient environment, however not all manufactures take this into consideration. Some manufactures may specify a derating factor so the user knows the maximum temperature & load of that particular power supply. While doing some initial testing with this dummy load rig, the ambient temperature inside the case was pretty consistent at each load level, peaking around 48° C at full load conditions. Any PSU that has an adjustable fan setting is set to its lowest or auto setting (since that is what most people would do in real life situations).

The chassis has two intake and two exhaust fans, several fans are mounted above the resistors to help circulate air and prevent the resistors from getting too hot. A panel of switches was built into the top of the case, each switch activates one or more resistors. The front bezel has been wired so that voltage & current readings can be taken without having to open the case.

The connectors on the rig all have gold plated contacts to minimize resistance at the plug. Some high quality power supplies use gold plated connectors, though tin plated is more common. Most motherboards and peripheral devices only have tin plated contacts (I suppose it is a cost consideration). There are two separate 12V load circuits, the +12V wires on the main connector is on one circuit, and the +12V (2x2) connector is on the other circuit. There is an additional peripheral connector on each circuit to accommodate for power supply wiring differences (since not all power supplies follow proper wiring guidelines).