Power Supplies - Upgrading and Repairing PCs - page 1164

Standby Power Supplies
A standby power supply is known as an offline device : It functions only when normal
power is disrupted. An SPS system uses a special circuit that can sense the AC line cur-
rent. If the sensor detects a loss of power on the line, the system quickly switches over
to a standby battery and power inverter. The power inverter converts the battery power to
120V AC power, which is then supplied to the system.
For a stand-by (switching) type UPS to work, the hold-up time of the power supply
has to be longer than the switching time of the UPS. For example, I have a TrippLite
SMART1000LCD,whichisastand-bytypeUPSwitha4msswitchingtime.Becausethis
iswellbelowthe16mshold-uptimecalledforintheofficialPowerSupplyDesignGuide,
it should switch well before any properly designed and functioning power supply allows
the system to reset. Unfortunately if the power supply in a PC is poorly designed or over-
loaded, it may be disrupted by even a 4ms switch, causing the system to shut down or
reset anyway, and all unsaved work to be lost.
A truly outstanding SPS adds to the circuit a ferroresonant transformer, which is a large
transformer with the capability to store a small amount of power and deliver it during the
switch time. This device functions as a buffer on the power line, giving the SPS almost
uninterruptible capability.
Tip
Look for SPS systems with a switch-over time of less than 10 milliseconds (ms). This is
shorter than the hold-up time of typical power supplies.
SPS units also might have internal line conditioning of their own. Under normal circum-
stances, most cheaper units place your system directly on the regular power line and of-
fer no conditioning. The addition of a ferroresonant transformer to an SPS gives it extra
regulation and protection capabilities because of the buffer effect of the transformer. SPS
devices without the ferroresonant transformer still require the use of a line conditioner for
full protection. SPS systems usually cost between a hundred and several thousand dollars,
depending on the quality and power-output capacity.
UPSs
Perhaps the best overall solution to any power problem is to provide a power source that
is conditioned and that can't be interrupted—which is the definition of an uninterruptible
power supply. UPSs are known as online systems because they continuously function and
supply power to your computer systems. Because some companies advertise ferroreson-
ant SPS devices as though they were UPS devices, many now use the term true UPS to