One thing that is common to all UPS technologies is that the incoming AC power
must be converted into DC power for use inside the UPS and then converted back into
AC power for use by the PC (which will convert it back to DC for use by its internal com-
ponents). All of this power conversion may seem redundant, but keep in mind that the
UPS is there to simulate the normal AC power source.
There are three basic UPS technologies, along with several hybrids, used to protect
against or solve different types of power issues: standby power supply (SPS), line-inter-
active UPSs, and on-line UPSs.Standby Power Supply (SPS) A standby power supply (SPS), also known as an off-line
power supply, is a pass-through unit that is inactive until the power fails. It shares the in-
coming power with its devices to charge its batteries and, as long as the electrical power
source is available, power is passed through the unit to its outlets and the devices
pluggedintothem.Figure23-4,intheprecedingsection,illustratesthegeneralconfigura-
tion of an SPS unit. When a brownout or blackout occurs, the unit switches over to the
battery to provide power to its outlets. Because of the time involved to switch its modes,
which varies by manufacturer and model, SPS units are not good for dealing with power
sags because the reaction time of the unit is usually longer than the duration of these
events. This type of UPS is an inexpensive solution for stand-alone, noncritical PCs and
peripherals and is generally not suitable for servers. However, because of their lower
cost, they are often used to protect desktop workstations.
Standby UPS technology is typically very reliable and switches modes fast enough to
prevent serious problems when the power source blacks out. The key specification when
considering a standby UPS is the voltage range that the UPS accepts as its normal operating
range. Whenever the voltage level of the incoming power is outside this range, the UPS be-
gins drawing on its battery. You want a standby power supply to have a wide operating
range, but not too wide. If the voltage range is too wide, your PC may be running on low or
highvoltageforextendedperiods.However,youwanttherangewideenoughtominimize(^610) PC Hardware: A Beginner’s Guide
Figure 23-4. The basic structure of an uninterruptible power supply (UPS)