Testing (Cont’d)
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.
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 an Extech power analyzer. The Zeus 850W power supply incorporates active power factor correction circuits, which resulted in all PF readings being at or close to 1.0.
Note: A power supply with active PFC is more environmentally friendly (doesn’t pollute the AC transmission grid) and will draw 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.
Efficiency
The overall efficiency of a power supply is very important, especially when delivering up to 850 watts. The less waste heat generated the better! Efficiency 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 (with no waste heat to dissipate). In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process.
The latest revisions to 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 (Ver 2.2) now lists both required and recommended minimum efficiencies.
I measured the AC power input to the Zeus 850W PSU with the Extech power analyzer and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps) for five different DC loads.
The overall efficiency of the SilverStone Zeus power supply is good and meets the recommended efficiency levels. The overall efficiency would most likely be slightly higher when operating on 230 VAC instead of 115 VAC, which would put it closer to SilverStone’s claim of greater than 80% efficiency.
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, especially when you are using this much power… 🙂
Differential Temperature and Noise Levels
The differential temperature across the Zeus 850W 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 outlet. The ambient room air temperature was 24ºC (75ºF) +/- 1ºC 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′ in front of 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.
Note: I was unable to measure the SPL during the final three load tests because the programmable DC load unit’s cooling fans were all running at high speed.
Below 400W output, the Zeus 850W PSU was quiet and ran relatively cool. Above 500W, the exhaust air temperature became quite warm and the cooling fans ramped up noticeably in speed.
