Ask A Scientist© Engineering Archive

Index Key:   ENG028
Author:      tommie
Subject:     Uninterruptable power supply
Text:        How exactly do UPS (Uninterruptable Power Supplies) work?  Surely 
one cannot "store" AC like one would DC.

Response #:  1 of 2
Author:      proach
Text:        You are right.  You cannot store AC the way you store DC.  But 
the UPS stores the AC in the form of DC. It rectifies the AC to get DC, and 
stores this in a battery.  Then when the UPS is needed (the AC source has 
failed), the DC from the battery in converted back to AC in a device known as 
an invertor.  The bigger the battery, the longer the UPS can supply power.



Response #:  2 of 2
Author:      dipper
Text:        UPS's generally operate in the following manner:  The most 
efficient way (and the most fail safe) is to simply use the AC power to charge 
a battery.  This is done by rectifying the AC and filtering and regulating to 
a DC voltage.  The battery is then used to power the computer by the use of a 
DC-AC invertor that synthesizes the AC through the use of some fairly common 
and simple analog circuitry and a set of power transistors.  That is the truly 
UPS systems.  Other systems are simply systems with a large transformer with a 
large amount of iron in the core.  While these are not uninterruptable, they 
do help filter out some of the "power glitches" that come through the line, by 
creating a large field in the transformer that takes  a relatively long time 
to break when the current disappears.  These, however, are not true UPS's and 
should not be used as one.  (By the way -- instead of batteries, some UPS's 
now use large capacitors to store the power.  These are not as long lasting as 
batteries yet, but they are getting much better, so look for some big 
advancements soon.)