Testing…

Testing — Physical Weight

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 components and larger heatsinks equal a better PSU. 

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In addition to being quite heavy the ST75ZF is also longer than most ATX power supplies, measuring a full 180mm long — make sure your case can accommodate the extra length before you purchase one.

Testing — DC Output Voltage Load Regulation

To simulate real world operation, the Zeus 750W PSU was connected to my home made load tester, supplied with 115 VAC, and allowed to burn-in for 24 hrs 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 +12V1 and +12V2 outputs. 

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The following table lists the DC voltage results at different loads for the ST75ZF PSU while operating on 115 VAC, 60 Hz.

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The Zeus 750W PSU produced excellent voltage regulation on all of the outputs across a broad range of loads.  Note: The only time the +12V2 rail was loaded (11.6A, 140 watts) was during the final 471watt combined load test.

Testing — 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.

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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 our 263 w load test for the five main output voltages of interest.

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The Zeus 750W power supply exhibited very good AC ripple suppression on all of the measured outputs, which was well within the ATX12V V2.2 specification. 

Testing — Power Factor (PF)

Power factor (PF) is one of those mysterious properties of AC that even most electrical engineers have a hard time explaining.  A thorough technical discussion goes beyond the scope of this review (not to mention this author’s understanding).  For a more detailed discussion about PF, please look here.

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Power factor is defined as the ratio of true power (measured in watts) to apparent power (measured in Volt Amps).  It measures how effectively AC power is being used by a device.  The difference between true power and apparent power is expressed as the power factor and results from the way true power and apparent power are measured.  Ideally we would like to have true power and apparent power equal to one another, which would result in a PF of 1.00 or 100% effective power utilization. 

I measured the AC Power Factor with a WattsUp? Pro power analyzer.  The Zeus 750W power supply incorporates active power factor correction circuits, which resulted in all PF readings being close to 1.0. 

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Note: A power supply with active PFC is more environmentally friendly (doesn’t pollute the AC transmission grid) and will use less current, but it will not save you money on your monthly electric bill unless you are a commercial user whose bill is based on PF and usage.   
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