Power Supplies - Upgrading and Repairing PCs - page 1062

Whereas the +3.3V, +5V, and +12V rails are technically independent inside the power
supply, many cheaper designs have them sharing some circuitry, making them less inde-
pendentthantheyshouldbe.Thismanifestsitselfinvoltageregulationproblemsinwhich
a significant load on one rail causes a voltage drop on the others. Components such as
processors and video cards can vary their power consumption greatly by their activity.
Transitioning from sitting at the Windows desktop to loading a 3D game can cause both
the processor and video card to more than double the draw on the +12V rail. On some
cheaper power supplies, this can cause the voltages on the other rails to fall out of spec
(drop greater than 5%), making the system crash. Better designed power supplies feature
truly independent rails with tighter regulation in the 1% to 3% range.
Voltage Regulators
The power supply must deliver a good, steady supply of DC power so the system can
operate properly. Devices that run on voltages other than these directly must then be in-
directly powered through onboard voltage regulators, which take the 5V or 12V from the
power supply and convert that to the lower voltages required by various components. For
example, older DDR (double data rate) dual inline memory modules (DIMMs) and Ram-
bus inline memory modules (RIMMs) require 2.5V, whereas DDR2 and DDR3 DIMMs
require 1.8V and 1.5V, legacy AGP 4x/8x cards require 1.5V, and current PCI Express
cards use only 0.8V differential signaling—all of which are supplied by simple onboard
regulators. Processors also require a variety of voltages (as low as 1.3V or less) that are
supplied by a sophisticated voltage regulator module (VRM) that is either built into or
plugged into the motherboard. You'll commonly find three or more different voltage reg-
ulator circuits on a modern motherboard.
Note
When Intel began releasing processors that required a +3.3V power source, power supplies
that supplied the additional output voltage were not yet available. As a result, motherboard
manufacturers began adding voltage regulators to their boards, which converted +5V to
+3.3V for the processor. When other chips began using 3.3V as well, Intel created the ATX
power supply specification, which supplied 3.3V to the motherboard. You would think that
having 3.3V direct from the power supply would have eliminated the need for onboard
voltage regulators, but by that time, processors, memory, and other components began run-
ning on a voltages lower than 3.3V. Motherboard manufacturers then included adaptable
regulator circuits called voltage regulator modules to accommodate the widely varying pro-
cessor voltage requirements. Additional regulators are also used to power other devices on
the motherboard that don't use +3.3V, +5V, or +12V.
See “ CPU Operating Voltages ,” p. 83 ( Chapter 3 , “ Processor Types and Specifica-
tions ”).