One of the basic measures of any ATX power supply is the unit’s overall physical weight. This may seem rather simplistic but it generally holds that more industrial grade components and larger heatsinks equal a better PSU. The following graph illustrates how the SilverStone Zeus 850W compares to some other popular power supplies.
(click to enlarge)
Note: As you might expect, the Zeus 850W is slightly heavier (174g) than the Zeus 750W PSU.
DC Output Voltage Load Regulation
To simulate real world and maximum loading conditions, the Zeus 850W PSU was connected to the load tester, supplied with 115 VAC, and allowed to burn-in for 24 hours before voltage readings were taken. In this test we are interested in seeing how well a PSU can maintain the various output voltages while operating under different loads. The DC output voltages were measured with a FLUKE digital multimeter.
The new ATX12V V2.2 tolerance for voltages states how much each output (rail) is allowed to fluctuate and has tighter tolerances for the +12V outputs.
The following table lists the DC voltage results at different loads for the Zeus 850W PSU while operating on 115 VAC, 60 Hz.
The Zeus 850W PSU produced excellent voltage regulation on all of the outputs across a broad range of loads; even when delivering up to 850 watts of DC power. All of the major outputs stayed within Â±3%, which is very good!
AC Ripple (electrical noise) on DC Outputs
The amount of AC ripple present on the outputs was checked using an oscilloscope. This AC component may be present in the KHz range where most switching power supplies operate or it may be more prevalent at the 60 Hz line frequency. I adjusted the O-scope time base to look for AC ripple at both low and high frequencies.
The new ATX12V V2.2 specification for DC output noise/ripple is defined in the ATX12V Power Supply Design Guide.
Ideally we would like to see no AC noise on the DC outputs — the cleaner the better! But in reality there will always be some present. I measured the amplitude of the AC signal (in millivolts, peak-to-peak) to see how well the power supply complied with the ATX standard. The following table lists the ripple/noise results during all of the load tests for the seven main output voltages of interest.
The Zeus 850W power supply exhibited good AC ripple suppression under light to moderately heavy loads, but under very heavy loads, the AC noise on the +3.3V and +5V rails increased substantially. In reality this is normal — as the DC load increases, typically so does the AC ripple. And this is not unique to the SilverStone power supply. I saw the same thing in the last 1kW PSU I tested and I expect the trend will continue with all the newer high output units. The AC noise on the +12V outputs also went up as the load increased.
This illustrates one of the advantages of having a power supply with plenty of reserve capacity. In general, it’s a good idea to operate a PSU at 50% to 60% of its maximum capacity. Doing so allows the PSU to run cooler/quieter, minimizes AC ripple, and places it closer to the point of peak operating efficiency.