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Standby Power Supply (SPS)

A device similar to a UPS is an SPS (standby power supply). An SPS contains a battery like the UPS, but the battery provides power to the computer only when it loses AC power. It does not provide constant power, like the UPS. An SPS is not as effective as a UPS because the SPS must detect a power-out condition first and then switch over to the battery to supply power to the computer. As a result, SPS switching time is important. Any time under 5 milliseconds is fine for most systems. Figures 4.35 and 4.36 show the differences between how SPSs and UPSs work.

Figure 4.35

Figure 4.35. SPS/line interactive UPS operation

Figure 4.36

Figure 4.36. Online UPS operation

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