X-bit labs

ATX Power Supply Units Roundup. Part IV (page 5)

Category: Other

by Oleg Artamonov

[ 04/10/2006 | 09:02 AM ]

As I have written in my other reviews, power supplies with multiple +12V outputs have in fact only one +12V power rail inside which is split in several lines to comply with the requirements of a safety standard. The standard demands that the current in each line was not higher than 20A (the manufacturers even safeguard themselves setting the limit at 18A). Thus, none of the individual lines can be said to have any peak current at all since any current below 18A is normal for it, any current over 18A triggers the protection. They can put the limit of 18A on all the +12V output lines and the power supply will still be compliant with the standard. It wouldn’t require any work on the developer’s part: the PSU’s internals remain the same, and only the protection-triggering thresholds are changed. And the power supply wouldn’t become more powerful for that, as you see.

Thus, a peak current can only be specified for the common +12V rail, which is inside the PSU before it is divided into the separate outputs. But it is for this rail that there is no peak load specified here.

For the same very reason, there is no technical meaning in the words of PSU manufacturers about extra stability, extra wattage, and any other extras that multiple +12V lines bring about. You’d better just filter out this white noise you hear from the marketing departments. Stability, wattage and other characteristics all come from the “basic” +12V power rail and do not depend at all on how many lines this rail is split into on the PSU’s output. In other words, a PSU with a single 12V/36A line is not any worse than a PSU with two 12V/18A lines.

The RealPower RS-550-ACLY has the following cables and connectors:

• Mainboard cable with a 24-pin non-separable connector (an adapter for a 20-pin connector is included); it is 49cm long
• CPU cable with an 8-pin connector; 50cm
• CPU cable with a 4-pin connector; 51cm
• Two graphics card cables with 6-pin connectors; 50cm
• Cable with three Molex connectors and one floppy mini-plug on each; 49cm to the first plug and then 15cm more to each next plug
• Cable with three Molex connectors; 49cm+15cm+15cm
• Cable with three SATA connectors; 49cm+15cm+15cm
• Cable for the power consumption indicator; 60cm

There’s nothing I could cavil at. The PSU offers all connectors necessary for a modern computer, even a separate 8-pin CPU connector and as many as two power connectors for graphics cards, for SLI or CrossFire configurations. Of course, the SATA power connectors have a +3.3V voltage unlike the deficient connectors of the above-described Ryanpower2. I only think that perhaps it would be simpler for the user to have splittable CPU and mainboard connectors (one 4+4 connector instead of the two 4- and 8-pin ones and one 20+4 connector instead of the 24-pin one plus the adapter).

Lacking independent voltage regulation, the PSU still yields very stable voltages. The output voltages remain within acceptable limits almost across the entire diagram. But like with the Ryanpower2, the maximum output power can only be achieved here if all the rails are fully loaded, which is virtually impossible in a real computer, as opposed to our specialized testbed.

The load being 525W, the output ripple was 23, 54 and 14 millivolts on the +5V, +12V and +3.3V rail, respectively. There is almost no low-frequency pulsation (at the double mains frequency or 100Hz in our case).

The RS-550ACLY uses a blue-highlighted CoolerMaster A12025-25BB-2AN-PI fan (which, however, bears its native marking “Protechnic Electric MGA12012HB-O25”). The graph I got is an exact copy of the one you will find in the user manual. The fan speed remains constant at about 1200rpm under loads of below 200W. At higher loads the speed grows up linearly until it reaches 2400rpm.

The graph is shaped in the same way as the fan speed graph of the RS-450-ACLY model, but is shifted upwards by 400rpm (the speed was varied within a range of 800-2000rpm in the RS-450-ACLY), so the new model is noisier than its predecessor under the same load. Still the noise characteristics of the new PSU are acceptable; I guess you will be satisfied with it, if you do not seek for absolute silence. If you do, consider other models, perhaps even the mentioned junior model from CoolerMaster.

The unit boasts good efficiency (over 80% on average and 84% at the maximum) and power factors.

My impressions about the RealPower RS-550-ACLY are generally on the positive side. This is not an exceptional product and, despite its high total output power, corresponds rather to 400-450W models from other manufacturers in its effective load capacity, i.e. considering the way the load is distributed along the different power rails in a modern computer. But it has good electrical characteristics (excellent stability of output voltages; low level of pulsations; high efficiency) and high quality of manufacture. The CoolerMaster RS-550-ACLY is going to be a good choice for many users, except for those who value silence above everything else. Its noise characteristics are rather average and it is a little louder than the junior model from the same manufacturer (but otherwise the RS-550-ACLY has done better than the RS-450-ACLY in my tests).