PC Hardware A Beginner’s Guide

(^612) PC Hardware: A Beginner’s Guide
An on-line UPS supplies power continuously from an AC to DC power inverter.
There is no switchover when the power fails because the outlets on the UPS are powered
from the battery at all times. The incoming power source is conditioned to protect the cir-
cuitry of the UPS, but the result is that any device connected to the on-line UPS is isolated
from power problems.
An on-line UPS produces the best quality power of any of the UPS technologies. It pro-
duces a near-perfect power stream that is free of even the smallest fluctuations. Standby
(off-line) and line-interactive UPS technologies reduce the severity of spikes, surges, and
sagsbyclampingthemdownintothenormaloperatingvoltagerangeoftheunit,butfluctu-
ationswithinthenormalrangeareunaffected.Theon-lineUPSisabletodealwithover-and
under-voltage events without using its battery, which can extend the battery’s life. Like the
line-interactive UPS, the on-line UPS draws a small amount of the incoming power to keep
its battery charged. This type of UPS technology is usually applied to mission-critical net-
works and high-availability devices, such as disk arrays and network access servers.
Sizing a UPS
Typically, a UPS needs to be able to provide between 5 and 15 minutes of good power or
enough power for a long enough period to allow the systems to be successfully shut
down, either manually or automatically, or for the power source to be restored. The UPS
is chosen for a given use based on its technology, load size, and its battery’s capacity.
The load size, or the amount of power a UPS can deliver, is specified in volt-amperes
(VA), which are commonly called volt-amps. Virtually all PC power supplies are rated in
watts. Technically, a watt is one am pof electrical current flowing at 1 volt. However, the
power rating of a PC power supply is typically only around 70 percent of the volt-amp
rating of the UPS.
To calculate the load size needed for a particular situation, you must first calculate the
volt-amprequirementsofeachdevicetobeprotected.Todothis,multiplyadevice’svolt-
age requirement by its ampere requirement. Or, if the device’s power requirements are
given in watts, multiply the watts required by 1.4, or to be truly safe, by 1.5. Once you
have the total volt-am prequirements of the devices to be attached to the UPS, you can be-
gin shopping for a UPS.
A number of UPS size calculators are available on-line on UPS manufacturers’ Web
sites. American Power Conversion has several interactive calculators available at
http://www.apcc.com/sizing/selectors.cfm, and Tripp Lite Power Protection has an interactive
sizing guide atwww.tripplite.com/sizing/. You can use the VA rating of the products
recommended by these proprietary tools as an indication of the UPS you need for your
situation. More than likely your budget will determine which technology you will
choose, but carefully consider the electrical environment of your location and match it to
the technology that best suits your needs.
UPS Warnings
There are at least two devices that you should never plug into a UPS: a laser printer or a
surge suppressor. A laser printer draws a tremendous amount of power when it starts up