After these tests we tried to pull even more power from StarTech.com WattSmart 650 W, but the main problem was noise. Pulling 650 W we were already too close to the 120 mV limit set by the ATX standard and by pulling just a little bit more noise surpassed this value.

So we have two results for maximum power. The first, shown below, is with the power supply working within ATX specs, i.e. with noise level below 120 mV at +12 V (during this test room temperature was 50.7º C and the power supply housing was at 48.8º C). Then the second result is the maximum power we could pull but with noise level outside specs.

With the power supply running under this configuration noise level 115 mV at +12V1 and 120 mV at +12V.

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Figure 20: Noise level at +12V1 input of the load tester.

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Figure 21: Noise level at +12V2 input of the load tester.

The problem was that we could pull more power from this unit, but the noise level was above the maximum admissible and a user doesn’t have a way to know that this is happening. We could pull up to 793 W with this power supply by pulling 33 A from +12V1 and 27 A from +12V2, but under this scenario we saw a 149.4 mV noise at +12V1 and 152 mV at +12V2. We were pulling 995 W from the wall, so efficiency was 79.7%. Room temperature was at 50º C and the power supply housing was at 49º C.

We tried to pull even more power from the unit, but it wouldn’t turn on – showing us the over power protection (OPP) in action, which is great: it allows us to go over above the power supply limit but not high enough to the point where we would burn it.

On this unit over current protection (OCP) is disabled or is set at a value over 33 A. We made a simple test here, we set +12V1 at 3 A and then increased +12V2 to 33 A, and the power supply would work just fine. Since the label states a maximum current of 18 A per rail the power supply should not allow this, as we were pulling 33 A from the power supply +12V1 virtual rail (don’t get confused here, +12V1 and +12V2 mentioned above are the name of the inputs located on our load tester), since we connected the EPS12V connector from the power supply to the +12V2 input from our load tester and kept the ATX12V connector disconnected from the tester.

Short circuit protection (SCP) worked fine for both +5 V and +12 V lines.

During our tests we could see the speed of the power supply fans changing as the power supply temperature increased. Below 30º C it spun slowly, making almost no noise, and after this temperature it started increasing its speed, which also increased noise level.

We were impressed by the cooling system used by this power supply. Its two 80-mm fans were able to keep the power supply housing temperature at room temperature or one degree Celsius below the temperature inside our hot box. Usually during our tests the temperature of the power supply housing is between 2º C to 5º C above the temperature inside our hot box.