The heart of our testing is our load tester, also known as ATE (Automatic Test Equipment), a SunMoon SM-268, which can be seen on Figures 1, 2 and 3. The basic function of this equipment is to pull the maximum power possible from the power supply being reviewed, but this machine does much more than that, as we will explain.

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Figure 1: SunMoon SM-268.

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Figure 2: SunMoon SM-268.

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Figure 3: SunMoon SM-268.

This load tester allows us to type in five different load patterns, called I1 thru I5 (see these buttons on Figure 3). For each load pattern we can set the current the tester will pull from each individual power supply output (+12V, +5V, +5VSB, +3.3 V and -12 V, which on the machine are labeled VA thru VF; VG and VH aren’t used – see the displays on Figure 1).

Here it is important to explain something before people get confused on our reviews. This equipment has two separated +12 V inputs, labeled +12V1 and +12V2, which does not necessarily relate with the power supply multiple rails (+12V1, +12V2, +12V3, etc). All plugs that provide +12 V (main motherboard power connector, peripheral power connectors, video card power connector and EPS12V/ATX12V connector) are connected to the machine +12V1 input. The second input is connected only to a second EPS12V/ATX12V connector that is available.

When testing power supplies with single rail design we don’t need to worry much about how we will connect all the plugs. On power supplies with multiple rails, however, we need to think about load distribution, as the machine has only two +12V inputs and the power supply may have more than two +12V rails. What we will basically do is to connect the EPS12V or ATX12V that is connected to an individual rail to +12V2 and all the rest on +12V1 and put this input to draw more current. This should work fine as the current should split evenly between the several connectors (Kirchoff's Law #2).

On reviews with power supplies with multiple rails we will have to explain how the power supply was connected to the load tester.

So the first step is to program the load tester with the currents (and thus power, as power is given multiplying the current by the voltage of each output) we want to pull from each output. This will depend on each particular power supply, as each power supply has its own particular specs.

In our methodology we decided to make six load tests. First we will test the power supply with 20%, 40%, 60%, 80% and 100% of its labeled power. Then we will try to see what the maximum power the unit is capable of delivering is, as some good units can deliver more power than what is labeled. All these six loads are pulled from the power supply immediately, meaning that on our methodology the power supply have to be able to deliver these loads as soon as it is turned on.

Just a real example to clarify. Suppose we are reviewing a 500 W power supply. We will conduct complete tests with 20% load (100 W), 40% load (200 W), 60% load (300 W), 80% load (400 W) and 100% load (500W). Then we will check what the maximum power this power supply can deliver right after being turned on is.

During our tests we will concentrate the load on the 12 V outputs, especially on high wattage units (i.e. above 500 W), in order to reflect a typical power supply usage today, as ATX12V, EPS12V and video card connectors have only 12 V wires. Thus in a high-end PC the most part of the power is pulled from the 12 V outputs.

All our load tests will be conducted with a room temperature between 45º C and 50º C (113º F and 122º F). This is a very important aspect of our reviews. The capacity of semiconductors delivering current (and thus power) drops with temperature, a phenomenon called de-rating (click here to read more about this). Many power supplies are labeled at 25º C (77º F), a temperature that is too low and impossible to achieve inside a computer. Because of that many units labeled at 25º C cannot deliver its rated power when used in a real-world environment. We will talk more about temperature later.

Because of the difference between our methodology and the methodology used by some manufacturers a given power supply not passing our load tests doesn’t necessarily mean that the reviewed power supply is bad. For example if we discover that a given 600 W power supply can only deliver 520 W, this doesn’t necessarily mean that this unit is bad; depending on other factors it can be considered a good 520 W model – if the user knows that he or she is bringing home a unit that in real life delivers less that the label says. Of course if the unit is labeled as 600 W and it can only deliver 200 W then it is a completely different story…

The load tester tests a lot more things besides checking if the power supply can deliver its rated power. By pressing a button on its panel we can immediately see if voltages are within the correct range, i.e. if the outputs are stable. The equipment not only shows the current voltage for each output, but also shows an alert whenever any output is out of range.

In our reviews instead of listing the voltages of each output during each load test, we will simply say whether the power supply passed or not on the voltage stability test; if the power supply fails, then we will report values and talk about them. We will consider a 3% tolerance margin for each output, detailed on the table below. This margin is lower than the standard 5% margin (see second table below), so we will be using a tolerance lower than normal. This will help us to qualify the voltage stability of a power supply: if all outputs are below 3% from their nominal voltages this means that we have an excellent power supply. If they are above 3% but below 5% this means that we have a good power supply, but it could have an even better stability. If the power supply is out of the 5% then we are obviously facing a bad power supply, which can even damage your equipment.

With the load tester we can also test some of the power supply protections. During our load tests we automatically test two of them: over current and over power protection. If the power supply doesn’t have these two protections it will literally burn when we try to go over its rated specs (bad power supplies will burn even within its specs). The load tester also provides separated short-circuit tests for the +12 V and +5 V outputs by just pressing a button. Of course we will also test this feature.

There is one known "flaw" with our load tester that we should talk about. Each of its 12 V inputs has a 33 A limit, meaning that the maximum we can pull from the +12 V outputs using this machine is 792 W (33 A x 2 x 12). This makes our system not suitable for testing power supplies above 900 W. Power supplies between 900 W and 1,000 W can be tested, but pulling more power from +5 V and +3.3 V outputs than from +12 V, a situation that we don't like since we try to concentrate power on the +12 V outputs, as explained before.