UPS

UPS

Uninterruptible Power Supply. A battery backup device that supplies AC power automatically in the event of a power failure.

There are two basic types of UPS, "stand-by" and "on-line":

Stand-by power supply or SPS. A device powered by an SPS is normally powered directly from the mains, with the battery charging. The batter is the stand-by power supply. Should the power fail then the power supply is switched to the battery (via an inverter). SPS has the advantage of being cheaper than the on-line version but does not provide an instant change over of power nor does it provide any mains filtering and therefore they are not suitable for all applications.
On-line. Here the battery to AC inverter is always connected to the output of the UPS. While mains power is available the battery will recharge, and should the mains power fail the switch to batter power is almost instantaneous. Thus there is almost no risk of interruption of supply should the power fail. They normally also provide some form of mains filtering but are generally more expensive than the SPS variety.