Hardware Reference
In-Depth Information
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 recti-
fiers.
Theswitching supply,ontheotherhand,usesaswitching circuit that chopsuptheincom-
ing power at a relatively high frequency. This enables the use of high-frequency trans-
formers 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 re-
lay 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.
Some power supplies have minimum load requirements for both the +5V and +12V sides.
According to IBM specifications for the 192-watt power supply used in the original AT,
a minimum load of 7.0 amps was required at +5V and a minimum of 2.5 amps was re-
quiredat+12Vforthesupplytoworkproperly.Aslongasamotherboardwaspluggedin-
tothepowersupply,themotherboardwoulddrawsufficient+5Vatalltimestokeepthose
circuits in the supply happy. However, +12V is typically used only by motors (and not
motherboards), and the floppy or CD/DVD drive motors are usually off. Because floppy
or optical (CD/DVD) drives don't present +12V load unless they are spinning, systems
without a hard disk drive could have problems because there wouldn't be enough load on
the +12V circuit in the supply.
