CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [144]
UPS An uninterruptible power supply (UPS) protects your computer (and, more importantly, your data) in the event of a power sag or power outage. Figure 10-10 shows a typical UPS. A UPS essentially contains a big battery that provides AC power to your computer regardless of the power coming from the AC outlet.
Figure 10-10 Uninterruptible power supply
* * *
NOTE There are two main types of UPS: online, where devices are constantly powered through the UPS’s battery, and standby, where devices connected to the UPS only receive battery power when the AC sags below ~80–90V. Another type of UPS is called line-interactive, which is similar to a standby UPS but has special circuitry to handle moderate AC sags and surges without the need to switch to battery power.
All uninterruptible power supplies are measured in both watts (the true amount of power they supply in the event of a power outage) and in volt-amps (VA). Volt-amps is the amount of power the UPS could supply if the devices took power from the UPS in a perfect way. Your UPS provides perfect AC power, moving current smoothly back and forth 60 times a second. Power supplies, monitors, and other devices, however, may not take all of the power the UPS has to offer at every point as the AC power moves back and forth, resulting in inefficiencies. If your devices took all of the power the UPS offered at every point as the power moved back and forth, VA would equal watts.
If the UPS makers knew ahead of time exactly what devices you planned to plug into their UPS, they could tell you the exact watts, but different devices have different efficiencies, forcing the UPS makers to go by what they can offer (VAs), not what your devices will take (watts). The watts value they give is a guess, and it’s never as high as the VAs. The VA rating is always higher than the watt rating.
Because you have no way to calculate the exact efficiency of every device you’ll plug into the UPS, go with the wattage rating. You add up the total wattage of every component in your PC and buy a UPS with a higher wattage. You’ll spend a lot of time and mental energy figuring precisely how much wattage your computer, monitor, drives, and so on require to get the proper UPS for your system. But you’re still not finished! Remember that the UPS is a battery with a limited amount of power, so you then need to figure out how long you want the UPS to run when you lose power.
The quicker and far better method to use for determining the UPS you need is to go to any of the major surge suppressor/UPS makers’ Web sites and use their handy power calculators. My personal favorite is on the American Power Conversion Web site: www.apc.com. APC makes great surge suppressors and UPSs, and the company’s online calculator will show you the true wattage you need—and teach you about whatever new thing is happening in power at the same time.
Every UPS also has surge suppression and power conditioning, so look for the joule and UL 1449 ratings. Also look for replacement battery costs—some UPS replacement batteries are very expensive. Last, look for a UPS with a USB or serial port connection. These handy UPSs come with monitoring and maintenance software (Figure 10-11) that tells you the status of your system and the amount of battery power available, logs power events, and provides other handy options.
Figure 10-11 APC PowerChute software
Table 10-1 gives you a quick look at the low end and the very high end of UPS products (as of late 2009).
Table 10-1 Typical UPS Devices
Supplying DC
After you’ve assured the supply of good AC electricity for the PC, the power supply unit (PSU) takes over, converting high-voltage AC into several DC voltages (notably, 5.0, 12.0, and 3.3 volts) usable by the delicate interior components. Power supplies come in a large number of shapes and sizes, but the most common size by far is the standard 150 mm × 140 mm × 86 mm desktop PSU shown in Figure