An uninterruptible power supplies (UPS) serves as essential electrical backup to ensure that critical equipment stays running during power failures. When power is lost, the UPS automatically switches to its battery. Any electrical or electronic devices plugged into the UPS continue to work without a problem.
Electronic circuits in the UPS continually monitor the power line's voltage. Normally, the alternating current (AC) power runs at 110 to 120 volts. If the voltage sags more than a few percent, for even a few thousandths of a second, the UPS switches over to battery power. The switching action is so fast that even sensitive equipment never registers a problem. When the AC line voltage returns to normal, the UPS switches the battery off, providing power straight from the outlet. The UPS uses the battery only when needed.
DC to AC Conversion
The electric power available at a standard outlet is alternating current, or AC. This electricity reverses direction at 60 cycles per second. Any appliance plugged into an outlet needs AC at the correct voltage. The UPS's battery produces direct current (DC) at a much lower voltage. An electronic circuit called an inverter converts the DC battery power to 120-volt AC that appliances can use.
The UPS sounds an audible alarm when the power goes out, indicating that it's running on battery power. Depending on the size of the UPS and the devices running on it, the battery can last anywhere from 5 minutes to several hours. If you have a smaller UPS unit running a computer, for example, this gives you enough time to save your files and shut your computer down in an orderly fashion so you don't lose your work.
In addition to battery backup, a UPS removes electrical surges from the power line from lightning and other causes. These spikes last millionths of a second but can damage sensitive electronic equipment such as computers. Your UPS may have separate sockets for surge protection. These sockets do not provide the advantage of battery backup, but will protect less-critical devices from power surges.
During a power outage, the battery drains as the UPS uses its electricity. When the utility restores the power, the UPS automatically recharges the battery. If several outages take place within a short time, the UPS may exhaust the battery and shut down. But as long as power stays on for an extended period, the UPS will fully recharge the battery.