Chapter 14: Power Supply and Electrical Issues^333
UPS Device Characteristics
Here are some of the features commonly found on a UPS: Information displays All UPS devices will issue a warning before its battery
is completely discharged, but the better devices have information displays to
provide information on the charge level of the battery, the amount of power
being demanded by the PC, and other information you need to decide how
much time you have before the battery is dead.
Monitoring systems Many UPS devices include a serial cable that is attached
to a serial (COM) port on the PC, which is used by software running on the
PC to monitor the “heartbeat” of the UPS. The UPS sends a signal at regular
intervals over the serial cable. These signals are monitored by a software
program running in background on the PC. If the UPS fails to send too many
signals, the software assumes that the UPS is gone and begins to shut down
the PC. The software program that monitors the UPS is usually supplied
by the manufacturer of the UPS. There are advanced monitoring systems that
can display console messages, send e-mail, or dial a pager to notify the system
administrator.
Line conditioners A line conditioner, also commonly called a power
conditioner, eliminates line noise from the incoming power and keeps voltage
within normal levels. Line conditioners don’t protect against blackouts, but they
do smooth out any low or high-voltage conditions on the incoming power line.
Alarm systems Most UPS and line conditioning devices sound an alarm
when the input power source drops below a certain level or if the power
becomes unreliable.Watts and Volt-amps Ratings
Most UPS devices are rated in volt-amps, but the power requirements of most PC devices,
including that of the power supply, are generally stated in watts. To determine the
right-sized UPS device for your system, you need to understand the difference (and simi-
larities) of these two electrical measures: Watts The real power used by an electrical device. It is the power actually
taken from the AC input source.
Volt-amps (VA) The VA rating of a device is computed as the volts it uses times
the amount of current in amps it draws from the circuit. The volt-amps rating is
used for determining the size of wiring, circuit breakers, and UPS devices.