Power Supplies - Upgrading and Repairing PCs - page 908

through a particular output. Values are expressed as individual amperages for each output
voltage. With these figures, you can calculate not only the total amount of power the power
supply can supply, but also how many devices using those various voltages it can support.
• Minimum Load Current —The smallest amount of current (in amps) that must be drawn from a
particular output for that output to function. If the current drawn from an output falls below the
minimum, the power supply could be damaged or automatically shut down.
• Load Regulation (or Voltage Load Regulation) —When the current drawn from a particular
output increases or decreases, the voltage changes slightly as well—usually increasing as the
current rises. Load regulation is the change in the voltage for a particular output as it transitions
from its minimum load to its maximum load (or vice versa). Values, expressed in terms of a +/-
percentage, typically range from +/-1% to +/-5% for the +3.3V, +5V, and +12V outputs.
• Line Regulation —The change in output voltage as the AC input voltage transitions from the
lowest to the highest value of the input range. A power supply should be capable of handling
any AC voltage in its input range with a change in its output of 1% or less.
• Efficiency —The ratio of power input to power output, expressed in terms of a percentage.
Values of 65%--85% are common for power supplies today. The remaining 15%--35% of the
power input is converted to heat during the AC/DC conversion process. Although greater
efficiency means less heat inside the computer (always a good thing) and lower electric bills, it
should not be emphasized at the expense of precision, stability, and durability, as evidenced in
the supply's load regulation and other parameters.
• Ripple (or Ripple and Noise, or AC Ripple, or PARD [Periodic and Random Deviation]) —
The average voltage of all AC effects on the power supply outputs, usually measured in
millivolts peak-to-peak or as a percentage of the nominal output voltage. The lower this figure,
the better. Higher-quality units are typically rated at 1% ripple (or less), which if expressed in
volts would be 1% of the output. Consequently, for +5V, that would be 0.05V or 50mV
(millivolts). Ripple can be caused by internal switching transients, rectified line frequency
bleed-through, or other random noise.
Power Factor Correction
In order to improve power line efficiency and to reduce harmonic distortion generation, the power
factor of PC power supplies has come under examination. In particular, new standards are now
mandatory in many European Union (EU) countries that require harmonics to be reduced below a
specific amount. The circuitry required to do this is called power factor correction (PFC) .
The power factor measures how effectively electrical power is being used and is expressed as a
number between 0 and 1. A high power factor means that electrical power is being used effectively,
whereas a low power factor indicates poor utilization of electrical power. To understand the power
factor, you must understand how power is used.
Generally, two types of loads are placed on AC power lines:
• Resistive —Power converted into heat, light, motion, or work
• Inductive —Sustains an electromagnetic field, such as in a transformer or motor
A resistive load is often called working power and is measured in kilowatts (KW). An inductive
load, on the other hand, is often called reactive power and is measured in kilovolt-amperes-reactive
(KVAR). Working power and reactive power together make up apparent power , which is measured