Uninterruptible Power Supply Overview

Before delving into the details of how an uninterruptible power supply works, let's take a quick look at the basics of this type of equipment. The fundamental purpose of a UPS is to provide an uninterruptible source of power for the equipment it protects. How exactly is this done? An electric device plugged into the wall (or into a surge suppressor plugged into the wall) has only one source of power. If there is a blackout, the electricity is cut and the device obviously goes off immediately. A UPS changes this equation by providing its equipment two sources of power.

UPSes are designed so that there is one source of power that is normally used, called the primary power source, and another source that kicks in if the primary is disrupted, called the secondary power source. The power from the wall is always one of these sources, and the battery contained within the UPS is the other. A switch is used to control which of these sources powers the equipment at any given time. The switch changes from the primary source to the secondary when it detects that the primary power has gone out. It switches back from the secondary power source to the primary when it detects that the primary power source has returned.

Very basic block diagram of a UPS, showing the basic design:
two power sources, controlled by a switch.

Contrary to what you might think, the wall AC power is not always the primary power source and the battery the secondary. Which source is primary and which is secondary depends on the type of UPS.

Of course, the power that comes from the wall is AC, and your PC uses AC power as well. All batteries, however, provide DC power. Therefore, circuitry is provided within all UPSes to convert AC power to DC to charge the battery. A device called an inverter is also provided to change the battery's stored DC electricity to AC to run your equipment. These components of the UPS, and others, are discussed in detail in the section covering the various parts of the UPS.

UPSes come in many different sizes and shapes. The size of the UPS is primarily dictated by the size of the battery; the larger the battery, the more time your equipment can run on battery power before shutting down. Larger units not only can power equipment for more time, they can also handle a larger total demand for power. Different UPSes have various other additional features, including warning signals, PC control software, and conditioning circuitry for the AC power source. Most newer UPSes also include a feature to shut down your PC in the event that both of its power sources fail, to avoid possible operating system problems caused by the power going out suddenly to the PC. UPS features are discussed in this section.