Uninterruptible Power Supply (UPS). A UPS is a device that delivers power to a destination in the event of power interruptions or problems with electric power quality. In general, UPSes consist of a battery and an inverter that generates a reasonable approximation of good quality AC power. If the inverter is not run constantly, the UPS will contain line conditioning circuitry. For the most part, the purpose of UPSes is to keep computer or other critical equipment operating during short power interruptions and to provide power long enough for an orderly shutdown or switchover of the affected system during long interruptions.

UPSes vary widely in cost. The principle factors affecting cost are the quality of the output power; the amount and duration of power they can deliver; and their ability to avoid glitches when switching on and off.

In general, inexpensive UPSes will have small batteries with limited capacity and will deliver inverter waveforms that are easy to generate by digital switching, but rich in high frequency harmonics. More expensive UPSes have larger batteries and generate better approximations of a sine wave. Inexpensive UPSes will generally pass through line power -- probably with noise peaks clipped -- until some threshold is passed after which they will deliver power derived from their battery until the wall power becomes safe again. More expensive UPSes may augment the wall power when it is low in voltage but otherwise OK. Highest quality UPSes run their inverters constantly and use wall power only to charge their battery.

UPSes must deliver enough voltage and current to drive all devices attached to them. For reactive loads (including PCs) this will require a higher rating than the actual power consumed.

Many UPSes have a provision to deliver a shutdown signal to attached devices -- typically over a serial interface. A typical arrangement would be for the UPS to deliver power for five minutes, then issue a shutdown order and continue to deliver power until the battery is exhausted.

Common problems with UPSes include:

Some FAQs

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