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Uninterruptible Power Supply (UPS)

A UPS (uninterruptible power supply), sometimes called an online (or true) UPS or a line interactive UPS, provides power to a computer or other device for a limited amount of time when there is a power outage. A UPS provides enough time to save work and safely shut down the computer. Some operating systems do not operate properly if power abruptly cuts off and the computer is not brought to a logical stopping place. A network server, the main computer for a network, is a great candidate for a UPS. Network operating systems are particularly susceptible to problems during a power outage. Some UPSs have a connection for a cable and special software that automatically maintains voltages to the computer, quits all applications, and powers off the computer. Some UPS units have USB and/or network connections as well.

A UPS also provides power conditioning for the devices attached to it. The AC power is used to charge a battery inside the UPS. The battery inside the UPS supplies power to an inverter. The inverter makes AC for the computer. When AC power from the outlet fails, the battery inside the UPS continues to supply power to the computer. The battery inside the UPS outputs DC power, and the computer accepts (and expects) AC power. Therefore, the DC power from the battery must be converted to AC voltage. AC voltage looks like a sine wave when it is in its correct form, but cheaper UPSs produce a square wave (especially when power comes from the battery) that is not as effective. Some computer systems and peripherals do not work well on a 120VAC square wave, modified sine wave, or quasi-sine wave. Figure 4.33 illustrates a sine wave and a square wave.

Figure 4.33

Figure 4.33. Sine wave and square wave

A UPS can be the best protection against adverse power conditions because it protects against overvoltage and undervoltage conditions, and it provides power so a system can be shut down properly. When purchasing a UPS, be sure that (1) the amount of battery time is sufficient to protect all devices; (2) the amount of current the UPS produces is sufficient to protect all devices; and (3) the output waveform is a sine wave.

To install a UPS, perform the following steps:

  1. Connect the UPS to a wall outlet and power it on. When a UPS is first plugged in, the battery is not charged. See the UPS manufacturer’s installation manual for the specific time it will take to charge.
  2. Power off the UPS.
  3. Attach device power cords, such as the PC, to the UPS. Ensure that the UPS is rated to supply power to the number and type of connected devices.
  4. Power on the UPS.

A UPS has a battery inside that is similar to a car battery (except that the UPS battery is sealed). Because this battery contains acid, you should never drop a UPS or throw it in the trash. Research your state’s requirements for recycling batteries. All batteries fail after some time, and most UPSs have replaceable batteries.

UPS troubleshooting is not difficult. In addition to following the manufacturer’s recommendations for troubleshooting, try the following guidelines:

  • If a UPS will not power on, check the on/off switch. Verify that the UPS is attached to an electrical outlet. Ensure that the outlet has power and that the circuit breaker for the outlet has not been tripped. Ensure that the battery is installed properly.
  • Check whether the UPS unit has a self-test procedure and include a self-test button.
  • With some UPS units, a beep indicates that a power interruption has occurred. This is a normal function.
  • Some UPS units beep at a different rate when the battery is low. Others have a light indicator to indicate that it’s time to recharge or replace the battery.
  • If a UPS is overloaded—that is, has too many devices attached—the UPS may shut off, trip a circuit breaker, beep, or turn on a light indication for this problem.

Figure 4.34 shows the front of an American Power Conversion UPS. Notice the diagnostic lights on it.

Figure 4.34

Figure 4.34. Front of an American Power Conversion UPS

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