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Question
How does an Uninterruptable Power Supply (UPS) work on a personal computer? Does the computer always run from the battery? Thanks!!!

Answer
What your computer expects to get from the power grid (in America) is 120 volt AC power oscillating at 60 hertz (see the HSW power grid page for more information). A computer can tolerate slight differences from this specification, but any significant deviation will cause the computer's power supply to fail. A UPS generally protects a computer against four different power problems:

Voltage surges and spikes - times when the voltage on the line is greater than it should be.
Voltage sags - times when there is power on the line but it is less than it should be.
Total power failure - times when a line goes down or a fuse blows somewhere on the grid or in the building
Frequency differences - times when the power is oscillating at something other than 60 hertz.

There are two common systems in use today: standby UPSs and continuous UPSs. A standby UPS runs the computer off of the normal utility power until it detects a problem. At that point it very quickly (in 5 milliseconds or less) turns on a power inverter and runs the computer off of the UPS's battery (see the HSW battery page for more information). A power inverter simply turns the DC power delivered by the battery into 120 volt, 60 hertz AC power.

In a continuous UPS the computer is always running off of battery power and the battery is being continuously 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 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 UPSs 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.

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