Testing (Cont’d)



The efficiency of a power supply is defined by the power output divided by the power input and is usually expressed as a percentage.  If a PSU were a 100% efficient (which none are) 400 watts of AC power going in would result in 400 watts of DC power coming out.  In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process.


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Newer revisions of the ATX12V Power Supply Design Guide V 2.2 have continued to increase the efficiency recommendations for PC switching mode power supplies.  And the latest revision now lists both required and recommended minimum efficiencies.


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I measured the AC power input to the Spire SP-500W power supply with the WattsUp? Pro watt meter and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps) for five different DC loads.


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The overall efficiency of the Spire SP-500W power supply was good (by old standards), and easily exceeded the ATX design requirements but fell short of meeting the new recommended 80% efficiency mark.


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The 80 Plus Computer Power Supply Program


There is a growing awareness among users, PC manufacturers and electric utilities regarding the money and natural resources that could be saved by adopting higher efficiency power supplies.  One group that is spearheading this new movement is Ecos Consulting.  You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.


Spending a little more money up front to purchase a high efficiency power supply may very well pay for itself over the lifetime of the PC… 🙂



Differential Temperature and Noise Levels


The differential temperature across the Spire SP-500W power supply was calculated by subtracting the ambient room air temperature (T in) from the temperature of the warm exhaust air flowing out of the power supply (T out). 


Thermocouples were placed at the air inlet and exhaust outlets of the power supply.  The ambient room air temperature was 23ºC (74ºF) +/- 0.5ºC during testing.


T in = temperature of air entering power supply

T out = temperature of air exhausting from power supply

ΔT = T out – T in


Sound pressure level readings were taken 3′ behind the PSU in an otherwise quiet room.  The power supply was placed on a foam rubber mouse pad during testing.  The ambient noise level was ~30 dBA. 


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The SpirePower SP-500W power supply was very quiet while operating under light loads.  The rear 80mm fan runs at a constant low speed at all times and the 120mm fan only turns on or speeds up when temperatures inside the power supply increase.  At higher loads, the 120mm fan became audible.  When the 120mm fan was forced on at full speed with the green button on the rear panel, the sound level measured 3′ away was 47.8 dBA.


One thing I noticed was that the 120mm fan speed appears to ‘hunt’ continuously at the higher loads (speed up, slow down) and never really reached a constant steady state speed.  This could become annoying at certain power levels.


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