Hardware Reference
In-Depth Information
and more efficient to spread the load out among multiple smaller and thinner wires.
The digital electronic components and circuits in the system (motherboard, adapter cards, and disk
drive logic boards) typically use the +3.3V or +5V power, and the motors (disk drive motors and any
fans) use the +12V power. In addition, voltage regulators on the motherboard or in other components
convert these standard voltages to others as necessary.
Table 18.1
lists the devices typically powered
by the various voltage rails.
Table 18.1. Voltage Rail Usage in a PC
You can think of each rail as a separate power circuit, kind of like a power supply within the power
supply. Normally, each rail is rated for a specified maximum amount of current in amperes. Because
the extreme amount of 12V current required by newer CPU voltage regulators and high-end video
cards can exceed the output of common 12V rails, some power supply designs use multiple +12V
rails. This means that essentially they have two or more separate 12V circuits internally, with some
wires tapping off of one circuit and others tapping off of another. Unfortunately, this can lead to
power problems, especially if you fail to balance the loads on both rails or to ensure you don't
exceed the load capacity on one or the other. In other words, it is far better to have a single 12V rail
that can supply 40 amps than two 12V rails supplying 20 amps each because with the single rail you
don't have to worry which connectors derive power from which rail and then try to ensure that you
don't overload one or the other.
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 independent than they should be.
This manifests itself in voltage regulation problems in which 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 indirectly 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 Rambus 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,
