Articles: Cases/PSU

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As I have written in the introduction, there was a sticker with the text “+12V/18A MAX” on the ATX-300GTF model. So, I decided to check up if the PSU could really yield such a current. The higher-wattage ATX-350GTF unit didn’t have such a sticker, so the diagrams below are somewhat paradoxical: the higher-wattage PSU has a smaller load on the +12V rail. But I want to remind you once again that I don’t pursue the goal of finding the limit the power supply is going to crash at. I don’t go beyond the max ratings indicated on the PSU’s label, and the limit for the 12V rail of the ATX-350GTF is declared to be 15 amperes.

As you can see, both PSUs successfully – for their class, of course – maintain stability of the +12V as well as +5V rail. The ATX-300GTF quite easily held the 18amp load (yes, the voltages go out of the acceptable limits when there’s a tenfold or bigger misbalance between loads on different rails, but it is not a serious drawback considering the category this PSU belongs to). The PSU also had no problems maintaining the 18amp load on the +12V rail for a long time. I dismantled the PSU and found a SBL2060CT diode pack on the +12V rail. It can hold such a current all right.

The oscillograms of the pulsations on the outputs of both PSUs are absolutely identical, so I only publish the performance graph of the ATX-350GTF. At the maximum load power the ripple is no more than 20 millivolts on the +5V rail and 35 millivolts on the +12V rail. This is far on the safe side of the limits.

The two models display a different correlation between the fan speed and the temperature of the PSU due to the use of different control circuits. The fan of the ATX-300GTF works perceptibly quieter, especially at low loads. At loads below 200W this fan is practically silent whereas the fan of the ATX-350GTF accelerates to 2000rpm at a 100W load already. Both PSUs are not quiet at high loads. But as I mentioned above, it is a separate card that controls the fan speed. It can be easily replaced, so different batches of these PSUs may display varying speed/load correlations.

The efficiency and power factors of the PSUs are absolutely identical, so I publish the ATX-350GTF graphs only. These characteristics of the PSUs just fit into the requirements of the standard, but not more: the efficiency factor at full load equals the allowable minimum of 68%. The passive PFC device helps to fit into the EU requirements (EN 61000-3-2) as concerns the harmonics in the consumed current. It but slightly increases the power factor proper, so it’s of small practical use.

Thus, the ATX-300GTF and the ATX-350GTF are high-quality units for entry-level systems. They are almost blameless in their own class, having stable voltages and small pulsations. The units are not noiseless. At high loads their fans accelerate to rather high speeds (this is in part due to the low efficiency factor, of course), but their noise is going to be quite acceptable in a relatively low-power computer (by today’s standards, I mean).

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