Standby Power Supplies

A stand-by power supply, or SPS, uses utility power to supply electricity to computers under normal conditions. When utility power is unavailable due to a momentary interruption, the SPS continues to supply power through a battery system for a limited time. These units are available for small computer systems such as personal computers or work stations.

The main reason to purchase an SPS is to protect against the momentary outages or "blinks" that often accompany storms and windy conditions. The SPS will provide you with enough time to save data and shutdown your PC in an orderly fashion.

The electricity you receive from the utility is alternating current or AC. Batteries such as the one in your car, provide direct current or DC. The SPS takes the AC power from the power company and supplies it directly to your computer as long as it is available. When the AC gets too low or disappears, the SPS automatically switches over to an onboard battery. The SPS converts the battery DC to AC for use in your computer. The length of time that Q power is available from the battery will depend on the size of the battery and the load of your computer. It is important to remember that the SPS does not condition the power in any way during normal utility operation.

When considering an SPS there are three questions that must be answered. First, how long does the battery need to keep the system running, and how long does it take to store up sufficient energy? Let's say there's an interruption once a day, and a backup time of at least 5 minutes is needed. If the SPS provides the 5 minutes, but takes 3 days to recharge, then it will not meet the stated needs.

Second, what is the transfer time to go from source to battery? Depending on this time, the SPS may or may not keep the system running during an interruption. If the electronic load can only ride-through an interruption of 8 milliseconds, and the SPS requires 10 milliseconds to transfer, then this SPS does not help.

Third, what determines when the transfer is made from source to backup, and from backup to source? It is possible that the sensing circuitry may respond to sags which the load could easily ride-through. By responding, though, it uses some of its energy. After some amount of time, the next sag or real interruption may find the SPS unable to provide any energy, and the load crashes.