How does an uninterruptible power supply work?

What your computer expects to get from the power grid is 120/127/208/220/230/240-volt AC power oscillating at 50/60 Hertz. A computer can tolerate slight differences from this specification, but a significant deviation will cause the computer’s power supply to fail. An uninterruptible power supply generally protects a computer against four different power problems:

1- Voltage surges and spikes – Times when the voltage on the line is greater than it should be
2- Voltage sags – Times when the voltage on the line is less than it should be
3- Total power failure – Times when a line goes down or a fuse blows somewhere on the grid or in the building
4- Frequency differences – Times when the power is oscillating at something other than 50/60 Hertz

There are two common systems in use today: standby UPS and continuous UPS. A standby UPS runs the computer off of the normal utility power until it detects a problem. At that point, it very quickly (in five milliseconds or less) turns on a power inverter and runs the computer off of the uninterruptible power supply UPS’s battery. A power inverter simply turns the DC power delivered by the battery into 120/127/208/220/230/240-volt, 50/60-Hertz AC power.

Continuous UPS

In a continuous UPS, the computer is always running off of battery power and the battery is continuously being recharged. You could fairly easily build a continuous UPS yourself with a largish battery charger, a battery and a power inverter. The battery charger continuously produces DC power, which the inverter continuously turns back into 120/127/208/220/230/240-volt AC power. If the power fails, the battery provides power to the inverter. There is no switch-over time in a continuous UPS. This setup provides a very stable source of power.

Standby UPS

Standby UPS systems are far more common for home or small-business use because they tend to cost about half as much as a continuous system. Continuous systems provide extremely clean, stable power, so they tend to be used in server rooms and mission critical applications.