Hardware Reference
In-Depth Information
operating from the battery, 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.
When the AC current supplying the battery charger fails, a true UPS continues functioning 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 possible. 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 execute an orderly system shutdown. Based on an appropriately
scaled storage battery, the UPS functions continuously, generating power and preventing unpleasant
surprises. When the line power returns, the battery charger begins recharging the battery, again with
no interruption.
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 switch time exists, the unit
can't be a true UPS because UPS units never switch. 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
protected 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
designed 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 purchase a UPS that
provides enough power to run your system and peripherals and enough time to close files and provide
an orderly shutdown. Remember, however, to manually perform a system shutdown procedure during
a power outage. You will probably need your monitor plugged into the UPS and the computer. Be
sure the UPS you purchase can provide sufficient power for all the devices you must connect to it.
Because of a true UPS's almost total isolation from the line current, it is unmatched as a line
conditioner and surge suppressor. The best UPS systems add a ferroresonant transformer for even
greater power conditioning and protection capability. This type of UPS is the best form of power
protection available. The price, however, can be high. To find out just how much power your
computer system requires, look at the UL sticker on the back of the unit. This sticker lists the
