Articles: Cases/PSU

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Seasonic SS-250SU

The first of the compact PSUs I am going to discuss in this review follows the FlexATX form-factor which was introduced not long ago, in 2007. Although the namesake mainboard form-factor developed back in 1999 and representing something in between microATX and mini-ITX (FlexATX has two expansion slots and is compatible with microATX in terms of the mounting holes) never really took off, FlexATX power supplies can be found in compact system cases such as the rather popular Cooler Master Elite 100. Moreover, this PSU from Seasonic has a wattage rating of 250 watts whereas the native PSU of the Elite 100, only 150 watts. Of course, it’s hard to think of components that could be fitted into the tiny Elite 100 and would require over 150 watts, but what if you've really got them?

The SS-250SU is a small box you can easily grasp with one hand. It measures 150 x 81.5 x 40.6 centimeters. There are even cutouts in the top and bottom panels of the case to accommodate its 40mm fan, so compact this power supply is.

As you might expect, the interior is filled far more densely than any ATX power supply. There is no empty spot on the PCB even the size of a square centimeter. A soft heat-conductive electrical-insulation pad is laid on top of the L-shaped heatsinks to transfer the heat to the top panel of the case. Of course, the steel panel with a sticker is not much of a heatsink, but every means of cooling is welcome in a PSU with such a high component density.

The PSU is cooled with a 40mm fan (Superred CHA4012DB-M) that has a max speed of as high as 8900 RPM. Hopefully, it won't rotate at its maximum. The fan is 20mm thick (the native PSU of the abovementioned Cooler Master Elite 100 has a thickness of 15 millimeters), so we can expect it to be rather efficient.

The output cables are obviously designed for OEMs. The mainboard power cable is quite normal but the rest of the cables can be called modular. There is a 10cm tail going out of the PSU which ends in a 12-pin connector a bunch of cables is attached to. Thus, the SS-250SU can be instantaneously tailored by Seasonic to the particular client's needs.

My sample of the PSU has the following cables and connectors:

  • One mainboard cable with a 20+4-pin connector (35 cm)
  • One CPU cable with a 4-pin connector (38 cm)
  • One cable with two PATA power connectors (38+15 cm)
  • One cable with one PATA and one floppy-drive connector (32+15 cm)

The only problem I can see is the lack of SATA connectors. There is not much space for adapters in those system cases this PSU is designed for.

Despite its low wattage rating and limited applications, the Seasonic SS-250SU is quite a modern product. It boasts 80+Bronze certification and can deliver over 200 watts across its +12V rail.

This PSU had no problems working with my APC SmartUPS SC 620, which is no wonder considering its 250-watt rating.

The cross-load diagram looks good. The PSU copes with misbalanced and near-zero loads.

The “+1” in the diagram marks the measured power consumption of a computer system with a Core 2 Duo and a Sapphire Radeon 4850 which corresponds to quite a high-performance gaming configuration of some three years ago. Considering that modern CPUs and midrange graphics cards consume the same amount of power, this indicates that you can build a fast enough computer with this power supply.

The output voltage ripple is very low even at the maximum load.

The PSU is over 80% efficient at loads up to 50 watts and over 85% efficient at loads above 65 watts. The 80+Bronze standard requires that the PSU be as efficient as 82% and higher at loads of 20% (i.e. 50 watts). My measurements produced a lower number, which might be due to measurement inaccuracies of our testbed at low loads.

The standby source does its job well, delivering up to 2 amperes without lowering the output voltage much.

Unfortunately, I could not measure the speed of the PSU fan. My tachometer could not catch a stable reflection from its tiny blades even after I had glued a piece of shiny film to one of them. Subjectively, the PSU gets noisy at loads above 150 watts but the noise is not very discomforting. It is a soft whisper of the air rather than the irritating hiss typical of such small fans.

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