Hardware Reference
In-Depth Information
current ones are capable of putting out up to 1,000 watts (1 kilowatt) or more.
Note
NVIDIA has trademarked the term
SLI
as meaning
scalable link interface
, but its primary
competitor, AMD, uses similar dual-graphics card technology called CrossFire to achieve
comparable performance improvements.
If your existing power supply doesn't feature PCI Express auxiliary power connectors, you can use
Y-adapters to convert multiple peripheral power connectors (normally used for drives) into a single
6-pin or 8-pin PCI Express auxiliary power connector. Note, however, that these adapters will not
help if the power supply is not capable of supplying the total power actually required.
Power Supply Specifications
Power supplies have several specifications that define their input and output capabilities as well as
their operational characteristics. This section defines and examines most of the common
specifications related to power supplies.
Power Supply Loading
PC power supplies are of a
switching
rather than a
linear
design. The switching type of design uses a
high-speed oscillator circuit to convert the higher wall-socket AC voltage to the much lower DC
voltage used to power the PC and PC components. Switching-type power supplies are noted for being
efficient in size, weight, and energy compared to the linear design, which uses a large internal
transformer to generate various outputs. This type of transformer-based design is inefficient in at least
three ways:
• The output voltage of the transformer linearly follows the input voltage (hence the name linear),
so any fluctuations in the AC power going into the system can cause problems with the output.
• The high current-level (power) requirements of a PC system require the use of heavy wiring in
the transformer.
• The 60Hz frequency of the AC power supplied from your building is difficult to filter out inside
the power supply, requiring large and expensive filter capacitors and rectifiers.
The switching supply, on the other hand, uses a switching circuit that chops up the incoming power at
a relatively high frequency. This enables the use of high-frequency transformers that are much smaller
and lighter. Also, the higher frequency is much easier and cheaper to filter out at the output, and the
input voltage can vary widely. Input ranging from 90V to 135V still produces the proper output
levels, and many switching supplies can automatically adjust to 240V input.
One characteristic of all switching-type power supplies is that they do not run without a
load
.
Therefore, you must have something such as a motherboard and hard drive plugged in and drawing
power for the supply to work. If you simply have the power supply on a bench with nothing plugged
into it, either the supply burns up or its protection circuitry shuts it down. Most power supplies are
protected from no-load operation and shut down automatically. Some of the cheapest supplies,
however, lack the protection circuit and relay and can be destroyed after a few seconds of no-load
operation. A few power supplies have their own built-in load resistors, so they can run even though
there isn't a normal load (such as a motherboard or hard disk) plugged in.
