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| Uninterruptible
Power Supply Overview |
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Before
delving into the details of how an uninterruptible power supply
works, let's take a quick look at the basics of this type of equipment.
The fundamental purpose of a UPS is to provide an uninterruptible
source of power for the equipment it protects. How exactly is
this done? An electric device plugged into the wall (or into a
surge suppressor plugged into the wall) has only one source of
power. If there is a blackout, the electricity is cut and the
device obviously goes off immediately. A UPS changes this equation
by providing its equipment two sources of power.
UPSes are designed so that there is one source of power that is
normally used, called the primary power source, and another source
that kicks in if the primary is disrupted, called the secondary
power source. The power from the wall is always one of these sources,
and the battery contained within the UPS is the other. A switch
is used to control which of these sources powers the equipment
at any given time. The switch changes from the primary source
to the secondary when it detects that the primary power has gone
out. It switches back from the secondary power source to the primary
when it detects that the primary power source has returned.
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Very
basic block diagram of a UPS, showing the basic design: two power
sources, controlled by a switch.
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Contrary
to what you might think, the wall AC power is not always the primary
power source and the battery the secondary. Which source is primary
and which is secondary depends on the type of UPS.
Of course, the power that comes from the wall is AC, and your
PC uses AC power as well. All batteries, however, provide DC power.
Therefore, circuitry is provided within all UPSes to convert AC
power to DC to charge the battery. A device called an inverter
is also provided to change the battery's stored DC electricity
to AC to run your equipment. These components of the UPS, and
others, are discussed in detail in the section covering the various
parts of the UPS.
UPSes come in many different sizes and shapes. The size of the
UPS is primarily dictated by the size of the battery; the larger
the battery, the more time your equipment can run on battery power
before shutting down. Larger units not only can power equipment
for more time, they can also handle a larger total demand for
power. Different UPSes have various other additional features,
including warning signals, PC control software, and conditioning
circuitry for the AC power source. Most newer UPSes also include
a feature to shut down your PC in the event that both of its power
sources fail, to avoid possible operating system problems caused
by the power going out suddenly to the PC. UPS features are discussed
in this section.
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