Thursday, May 04, 2006

Power Factor

How much power a PC system draws from the AC outlet is often a good indication of how much heat the system dissipates and consequently how much work is needed to keep the system cool or for silent PC enthusisasts, gives us a rough idea how quietly it can be run. In all my previous posts, I have been measuring the amperage of the AC current being drawn by the system and multiplying it by the AC voltage to calculate the power consumed in watts, however this method is actually not entirely correct. For a DC circuit, Power=Voltage x Current, however for AC circuits with a reactive component there are other factors that must be considered.

In fact what I have been measuring is "Apparent Power" which has the units VA (volt-amperes). Apparent Power is made up of two components, "True Power" and "Reactive Power". "True Power" also called "Real Power" is the actual amount of power being dissipated or consumed by the system and is measured in Watts. This is the figure that I thought I was measuring. "Reactive Power" is the additional power being drawn by the reactive components of the system but not actually dissipated or consumed by the system. The "Reactive Power" affects the phase of the AC current but is returned and absorbed back to the AC power line. A more detailed and exact explaination can be found in this chapter of Lessons in Electric Circuits as well as in this SPCR article.

Power Factor defines the ratio between True Power and Apparent Power (PF = True Power / Apparent Power). For a purely resistive circuit (one without any reactance) the PF = 1, and consequently Apparent Power = True Power. However most generic PC power supplies do not have any power factor correction (pfc), and their PF is probably below 0.60 meaning if the system is dissipating a True Power of 60 Watts, the Apparent Power draw would be measured at 100VA or more.

More and more high-end PC power supplies are now being offered with pfc to improve PF, and also there are regulations in the EU requiring power factor correction. There are two types of pfc, Active and Passive. Active pfc often yields a PF very close to 1.0 whereas Passive pfc offers PF in the 0.6-0.8 range.

For all the external brick AC/DC adaptors I used, these are universal input voltage, switching power supplies which do have active pfc and therefore their PF is probably in the range of 0.95-0.99. So for all measurements given for the ASUS Pundit, PW200M, PicoPSU, and Mac Mini systems the True Power being consumed is slightly less than the Apparent Power figures I posted but should be within 5%. The stock Antec Aria PSU is also similarly equipped with active PFC. The AOpen XC Cube stock PSU is marked "with pfc" but I assume passive. This helps explain for the surprisingly large difference in Apparent Power draw compared with the PicoPSU. The True Power consumed with the stock PSU should be 80% or less of the measured Apparent Power, which brings it closer to the power draw of the PicoPSU. For the Tagan PSU however I am not clear whether or not it has any pfc. It does have universal input voltage, a common characteristic of active pfc, however it's specs are not marked with any indication of pfc and the measured Apparent Power is significantly higher than the Pico/PW200M psus suggesting that it does not have pfc.

Thanks to Linus and Devon at SPCR for setting me straight.


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