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Uninterruptible Power Supply (UPS)
What your computer expects to get from the power grid
(in the United States) is 120-volt AC power oscillating
at 60 Hertz. A computer can tolerate slight differences
from this specification, but a significant deviation
will cause the computer's internal power supply to fail.
A UPS generally protects a computer against four
different power problems:
- Voltage surges and spikes - Times when
the voltage on the line is greater than it should be
- Voltage sags - Times when the voltage on
the line is less than it should be
- Total power failure - Times when a line
goes down or a fuse blows somewhere on the grid or
in the building
- Frequency differences - Times when the
power is oscillating at something other than 60
Hertz
There are two common systems in use today: standby
UPS and continuous UPS.
A standby UPS runs the computer off of the normal
utility power until it detects a problem. At that point,
it very quickly (in five milliseconds or less) turns on
a power inverter and runs the computer off of the UPS's
battery. A power inverter simply turns the DC power
delivered by the battery into 120-volt, 60-Hertz AC
power.
In a continuous UPS, the computer is always running
off of battery power and the battery is continuously
being recharged. You could fairly easily build a
continuous UPS yourself with a largish battery charger,
a battery and a power inverter. The battery charger
continuously produces DC power, which the inverter
continuously turns back into 120-volt AC power. If the
power fails, the battery provides power to the inverter.
There is no switch-over time in a continuous UPS. This
setup provides a very stable source of power.
Standby UPS systems are far more common for home or
small-business use because they tend to cost about half
as much as a continuous system. Continuous systems
provide extremely clean, stable power, so they tend to
be used in server rooms and mission critical
applications.
While not limited to
protecting any particular type of equipment, a UPS is
typically used to protect computers, data centers,
telecommunication equipment or other electrical
equipment where an unexpected power disruption could
cause injuries, fatalities, serious business disruption
or data loss. UPS units range in size from units
designed to protect a single computer without a video
monitor (around 200 VA rating) to large units powering
entire data centers, buildings, or even cities.
A unit designed for a
single computer is nearly the cost of a good surge
protector with added protection. Let's say you purchase
an expensive television, which is electronic circuit
boards much the same as a computer. Protecting it with a
UPS could be a good idea. Supplying electronic equipment
with good clean power will lengthen it's lifespan.
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