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Introduction - Part One


ATX Power Supply Group Test

Just over two months ago we were given the opportunity to go to Germany to test PC power supplies units (PSUs). Now you might wonder why we had to go all the way to Germany just to test PSUs. It can’t possibly be that hard to test power supplies can it? Well, in fact, testing them properly is quite a challenge. The equipment we used costs in the region of $20,000 a piece, somewhat beyond our own equipment testing budget. Thanks to Nanopoint UK, we were given access to the test equipment used at its headquarters in Germany, where it is known as Maxpoint. This is where Tagan power supplies are designed.

The test equipment consisted of two Fast Auto FA-828ATE PSU load testers as well as some additional equipment that was used with power supplies that featured two 12V rails. The PSU load testers were only able to place a load on one of the 12V rails and thus some extra equipment was needed to properly test the latest generation of power supplies.

Each PSU was tested at 25, 50, 75 and 100 per cent load on the 12V rail(s), with the 5V and 3.3V rails making up for the additional drain. Those who know something about power supplys, might wonder why we did it this way. However, the explanation is quite simple. All modern computers put a much heavier load on the 12V rail than the 5 or 3.3V, so this is the most important part to test.

We stayed within the manufacturer’s wattage rating for the 12V rail to make sure that we didn’t push any of the units too far. The 5V and 3.3V made up for the additional wattage to meet the manufacturers spec.

To calculate the appropriate load levels multiply the amp rating with the voltage for the specific power rail - the result will give you the correct wattage for that rail. Deduct the correct percentage from the amp rating and perform the same calculation; you will then know by how much you should lower the amps in order to achieve 25, 50 and 75 per cent loads. For instance, to get a 75 per cent load you would deduct 25 per cent from the amp rating.

However, if you have an older (Pentium III or an early Athlon system) these test results are not going to be quite accurate. This is because older computers relies more on the 5V rail, but as they have much lower power requirements than current machines, all of the power supplies on test should in reality work just fine.

Several changes have recently been made to the ATX PSU specification and some of them are fairly easy to spot, either on the packaging itself or by looking at the connectors. Some of the other changes aren’t that obvious, so we are going to list the most important ones.

Dual 12V rails are one of the recent additions and the main reason behind this change is to protect users PCs. With increasing power requirements the amps on the 12V rail have constantly increased, and if this had continued unchecked a PSU failure could potentially result in a fatal accident. With the new design this risk has been greatly reduced and we should see a transition to dual or even quad 12V rails on all high-rated PC power supplies this year.

One other change that is good news for your wallet is that all new power supplies must have a minimum efficiency of 70 per cent at normal load (around 50 per cent load and upwards). It should also be no lower than 60 per cent at light loads (around 20 per cent load or below).

As we'll demonstrate in our tests, the rating on the PSU has nothing to do with how much power your PSU actually draws from the wall socket. What is of more significance is its efficiency. The difference between how much power a PSU draws and how much it actually delivers to the motherboard determines its efficiency. The closer together these figures are, the less power is wasted, saving you money on your electricity bill in the long run. In the most extreme case during our testing one of the power supplies had an input/output differential of over 300W, which is highly wasteful. All the figures are outlined in our graphs, both in percentage and in actual watts.

Other additions to newer PSUs includes a 24-pin EPS style power connector, instead of the 20-in ATX power connector. This has been designed for the new Intel Pentium 4 processors, but has been a server and workstation standard for some time. Added to this is a new eight pin 12V connector, which will eventually replace the four pin 12V connector, but this is unlikely to happen for a few months yet. A six pin connector for PCI Express graphics cards should start appearing shortly as well. Some of the power supplies on test came supplied with this six pin connector as a small converter, which would work with pretty much any power supply.

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