Articles: Cases/PSU

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This PSU is declared to comply with the ATX12V 2.0 standard, but you can see from the table below that it only corresponds to 300W units as concerns the load current on the +12V rail (that is, 22 amperes in total) whereas a 350W PSU should provide a combined current of 25 amperes.

The label on the PSU also declares the peak load currents, particularly 18amp and 16amp for the two +12V outputs (denoted as 12V1 and 12V2). This seems to surpass the capabilities of all typical power supplies described in the ATX12V 2.0 standard (the most powerful of them is a 400W unit), but it’s not quite so. The typical ATX12V 2.0 power supply must be capable of maintaining the maximum current on both 12V outputs simultaneously (in other words, the 400W unit should be ready to yield up to 14amp+15amp=29amp on its +12V rail), but the RS-450-ACLY limits the combined load current on the +12V at 22amp.

I already said at the beginning of this article that the splitting of the +12V rail in two was only required to comply with the safety regulations that limited the maximum power on user-accessible rails. And it was implemented by means of two independent current sensors. But there is actually only one +12V rail inside the power supply and it is its load capacity that limits the combined load current on the +12V1 and +12V2 outputs. Thus, the typical 400W ATX12V 2.0 power supply has a load capacity of 29 amperes on this rail whereas the CoolerMaster RS-450-ACLY – only 22 amperes. The declaration of the peak load currents of 18amp and 16amp only means that the over-current protection of this PSU is set to these values. They could have declared them as sustained rather than peak currents with the reservation that their sum should always be lower than 22 amperes.

The cross-load characteristics of the PSU indicate that it oriented towards modern systems that are mostly powered by the +12V rail, notwithstanding my complaints about the load capacity of this rail expressed above. Unlike the previous model, this PSU is not equipped with independent regulation of the output voltages. You could have guessed that by the lack of magnet amplifier coils on its output.

The output ripple is rather strong here, being very close to the allowable limit of 120 millivolts on the +12V rail and being even higher than the norm (the norm is 50 millivolts or one grade of the oscillogram) on the +5V rail. When the load power was reduced to 350 watts, the pulsation diminished to 40 millivolts on the +5V rail and to 60 millivolts on the +12V rail, which is acceptable.

The fan rotation speed is quite effectively controlled, so the PSU is almost perfectly silent under small loads. When the load is higher, the fan speeds up to 2000rpm, and the noise from the air stream is perceptible. By the way, the speed of the fan is adjusted depending on the load on the PSU rather than on its temperature.

The efficiency of the PSU is high – over 80% in all the range of loads almost. The power factor is 2-3% lower than that of the competitors, but this small difference isn’t very important in practice. Like the Ryanpower2, the unit uses its active PFC to support all input voltages from 90 to 240 volts without manual switching.

So, the CoolerMaster RealPower RS-450-ACLY is a quality product. Its advantages include active PFC, stable voltages (despite its classic design without auxiliary regulators), a quiet 12cm fan with highlighting, and long cables with numerous plugs suitable for any system case. But the manufacturer is not quite honest talking about the compliance of this PSU with the ATX12V 2.0 standard. The load characteristic of this PSU on the +12V rail is only comparable with 300W models as described by this standard. The RS-450-ACLY will suit perfectly for modern midrange systems, but you may want to consider other PSUs, with higher currents on the +12V rail (they don’t necessarily have to have a higher wattage), if you’re building up some extreme configuration.

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