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What Is A UPS?
Short for uninterruptible power supply, a UPS is a power supply that includes a battery to maintain power in the event of a power outage. Typically, a UPS keeps a computer running for several minutes after a power outage, enabling you to save data that is in RAM and shut down the computer gracefully. Many UPSs now offer a software component that enables you to automate backup and shut down procedures in case there's a power failure while you're away from the computer.

There are two basic types of UPS systems: standby power systems (SPSs) and on-line UPS systems. An SPS monitors the power line and switches to battery power as soon as it detects a problem. The switch to battery, however, can require several milliseconds, during which time the computer is not receiving any power. Standby Power Systems are sometimes called Line-interactive UPSes.

An on-line UPS avoids these momentary power lapses by constantly providing power from its own inverter, even when the power line is functioning properly. In general, on-line UPSs are much more expensive than SPSs.
UPS Technologies
Offline UPS Diagram

 Offline UPS technology is typically found in home use or for powering individual computer workstations and other non-critical applications that only require power outage protection. Offline products sometimes have surge suppression and/or "buck and boost" circuits to compensate for high or low input voltage, but otherwise do not provide any significant input power conditioning.

Line-Interactive UPS Diagram

 Line-Interactive technology resembles the offline product, but inserts a transformer or capacitors in series between the utility power source and the load. This inline capacitor enables the UPS inverter to "interact" with incoming power and provide some power conditioning to the load. This "buck-and-boost" circuitry helps with high and low input voltage conditions. Commonly used for smaller loads under 5kVA.

On-Line UPS Diagram

 On-line technology isolates utility power from your equipment loads. AC power is rectified to DC power to supply the internal DC bus (battery bank) of the UPS. The output inverter takes the DC power and converts it back to computer grade AC power to support the critical load. Batteries attached to the DC bus are float charged during normal operation. When the input power voltage is out of spec, the batteries provide power to support the inverter and critical load. (AC Double Conversion).


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