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.  


Seasonic S12-600 watt Power Supply - Cases and Cooling 29



Testing — DC Output Voltage Load Regulation


To simulate real world operation, the S12-600 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 under a moderately heavy load.  The DC output voltages were measured with a FLUKE digital multimeter at the ATX connector. 


The ATX tolerance for voltages states how much each output (rail) is allowed to fluctuate.  Seasonic specifies ±5% for all outputs except for the -12 V output, which is ±10%.


Seasonic S12-600 watt Power Supply - Cases and Cooling 30


The following table lists the DC voltage regulation results for the Seasonic S12-600 PSU.


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As you can see, all of the DC outputs were held within the ATX specification while operating under a 263 watt combined load when measured at the 24-pin power connector.  However, with the 20-pin adapter cable inserted, the primary voltages dropped slightly due to the added resistance of the connector.  This was particularly noticeable on the 3.3V rail as all the 3.3V power (~10 Amps) during testing flows thru this connector. 


The voltage drops caused by the added connector illustrates how important maintaining good connections with minimal resistance are.  Even very small resistances can cause noticeable voltage drops due to the high current load.  This is why I am not personally a big fan of power supplies that incorporate removable cables or require the use of adapter cables.


E = I x R = 10 amps x 0.01 ohms = 0.1 volts


Applying Ohm’s Law, we can confirm that a very small resistance can cause a measurable drop in voltage.  In the example above, if a connector has 0.01 ohms resistance and 10 amps of current flow thru the connector, the resulting voltage drop will be 0.1 volts.



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.  In each case, I adjusted the O-scope time base to look for AC ripple at both low and high frequencies. 


Seasonic S12-600 watt Power Supply - Cases and Cooling 32


60 mV P-P on the +12 VDC output


The ATX 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.  The four main output voltages of interest (+3.3 V, +5.0 V, +12 V and +5 VSB) were recorded after the 24 hr burn-in period.


Seasonic S12-600 watt Power Supply - Cases and Cooling 34


The Seasonic S12-600 power supply exhibited good AC ripple suppression on all of the measured outputs but was noticeably higher than previous Seasonic PSUs I have tested. 



Testing — Input Voltage Line Regulation


During the first load test we set the AC input voltage to 115 VAC.  This is an optimum value for most of the power supplies under test.  To find out how well the Seasonic S12 power supply can handle under and over voltage conditions on the AC mains, I lowered the input voltage to 100 VAC and then raised it to 130 VAC with a Variac (variable autotransformer).  Once again we are interested in seeing how well the PSU can maintain the various output voltages as the input line voltage fluctuates +/- 13% (+/- 15 VAC).


Seasonic S12-600 watt Power Supply - Cases and Cooling 35


The S12-600 produced excellent line regulation with varying input voltage.  It showed no more than a few one hundredths of a volt change as the line input went from 100 VAC through 115 VAC all the way up to 130 VAC.


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