Answer 7
A more modern problem is the prevalance of equipment containing a brute-force capacitor input power supply. This includes the primary portions of switch-mode power supplies such as those used in PCs and other electronic equipment.

Such power supplies tend to draw all of their current in short bursts at the peaks of the AC waveform instead of the sinusoidal shape characteristic of a purely resistive load. This distorted current waveform has a higher RMS value than that of a sine wave, and this RMS current causes additional power dissipation within the power distribution system just as reactive current does.

Therefore, another way to define power factor is the ratio of the equivalent sine wave RMS current to the actual (distorted) RMS current.

A resistor at a given power level draws a certain amount of current:

Suppose a power supply consuming the same amount of power draws current during only a fraction (the duty cycle) of the AC cycle. This current can be modeled as a train of rectangular pulses. The peak voltage of the AC waveform is:

The average current is:



and the peak current is:



The RMS value of this current is:

The power factor is the ratio of these two RMS values:

Contributor: David Tweed

Published July 2003