Hardware Reference
In-Depth Information
describeatrulyonlinesystem.AtrueUPSsystemisconstructedinmuchthesamewayas
anSPSsystem;however,becausethecomputerisalwaysoperatingfromthebattery,there
is no switching circuit.
In a true UPS, your system always operates from the battery. A voltage inverter converts
from +12V DC to 120V AC. You essentially have your own private power system that
generates power independently of the AC line. A battery charger connected to the line or
wall current keeps the battery charged at a rate equal to or greater than the rate at which
power is consumed.
WhentheACcurrentsupplyingthebatterychargerfails,atrueUPScontinuesfunctioning
undisturbed because the battery-charging function is all that is lost. Because the computer
was already running off the battery, no switch takes place and no power disruption is pos-
sible. The battery begins discharging at a rate dictated by the amount of load your system
places on the unit, which (based on the size of the battery) gives you plenty of time to ex-
ecute an orderly system shutdown. Based on an appropriately scaled storage battery, the
UPSfunctionscontinuously,generatingpowerandpreventingunpleasantsurprises.When
the line power returns, the battery charger begins recharging the battery, again with no in-
terruption.
Note
Occasionally, a UPS can accumulate too much storage and not enough discharge. When this
occurs, the UPS emits a loud alarm, alerting you that it's full. Simply unplugging the unit
from the AC power source for a while can discharge the excess storage (as it powers your
computer) and drain the UPS of the excess.
Many SPS systems are advertised as though they are true UPS systems. You can tell a
Standby Power Supply from a true UPS by the unit's switch time. If a specification for
switchtimeexists,theunitcan'tbeatrueUPSbecauseUPSunitsneverswitch.However,
true UPS systems are very expensive, and a good SPS with a ferroresonant transformer
can virtually equal the performance of a true UPS at a much lower cost.
Note
Many UPSs and SPSs today come equipped with a cable and software that enables the pro-
tected computer to shut down in an orderly manner on receipt of a signal from the UPS. This
way, the system can shut down properly even if the computer is unattended. Some OSs de-
signed for server environments contain their own UPS software components.
UPS cost is a direct function of both the length of time it can continue to provide power
after a line current failure and how much power it can provide. You therefore should pur-
