X-bit labs

ATX Power Supply Units Roundup. Part VII (page 14)

Category: Other

by Oleg Artamonov

[ 09/04/2007 | 10:40 AM ]

The RT550-135-BK doesn’t yield very stable voltages. The +5V voltage is higher than necessary in the bottom part of the diagram and the +12V is not too stable, either. Anyway, the PSU copes with every load combination successfully, leaving the allowable limits only when there is a strong load misbalance that can hardly occur in a real PC.

At full load there is a weak high-frequency pulsation: about 20 millivolts on the +5V rail, 45 millivolts on the +12V, and small spikes on the +3.3V.

Alas, the picture grew worse when I switched the oscilloscope’s resolution from 10 microseconds/div to 2 milliseconds/div. Besides high-frequency pulsations, there is a strong low-frequency ripple on the +5V and +12V rails (at the doubled frequency of the mains, i.e. 100Hz). The combined ripple amounts to 50 millivolts on the +5V rail (i.e. exactly equal to the allowable maximum) and to 80 millivolts on the +12V rail (the allowable maximum is 120 millivolts).

I couldn’t identify the manufacturer of the fan. It bears Rosewill stickers on both sides without any additional information. I can only tell you it’s an 11-blade 135mm fan with blue highlighting.

The fan speed is adjusted linearly from 1080 to 1650rpm. Thus, the Rosewill power supply is somewhat noisier than average (it would be average if the fan speed did not increase at all up to a load of 300-400W).

The efficiency of this PSU is rather low by today’s standards. It hardly reached 80% and fell back to 76% at full load. The power factor is 0.65 on average just as you can expect from a PSU with any PFC (compare this to the power factor of 0.7 and higher of PSUs with passive PFC and 0.95 and higher of PSUs with active PFC).

Thus, the Rosewill RT550-135-BK is a mainstream power supply, unexceptional in any of its parameters. It has average voltage stability, average noisiness, an acceptable selection of connectors, etc. Its 135mm fan distinguishes it from previous-generation models, yet this fan doesn’t guarantee silence. The buzz of its impeller and the noise from the airflow is quite audible at speeds higher than 1000rpm.