Although the case determines the size, shape, and construction quality of a UPS, the big differences are inside. The design of the core electronics determine how the UPS operates, how much power it can handle, and how much protection it affords.

The UPS core electronics serve two basic functions. The power circuitry handles the heavy-duty electricity that powers your computer and its peripherals. It takes care of the power conversions–from AC to DC to charge batteries, and from DC to AC to convert battery power to computer-compatible voltages. The control circuitry tells the power circuitry what to do–when to switch from line power to the battery supply, how much current to use to charge the batteries, when to limit power or otherwise protect the electronics of the UPS and your computer. In addition, the control circuitry lets you monitor the operation of the UPS. The control circuitry also links with your computer so you can remotely monitor your UPS and the UPS can warn your computer of power failures.

UPSs typically combine power and control circuitry onto a single circuit board so there is only one part to manufacture. For example the APC SmartUPS uses one large glass-epoxy circuit board to hold both the power and control electronics, as shown in Figure 5.

Figure 5

In the figure, the control circuitry takes up most of the left side of the circuit board. The power circuitry is on the right. You can see the difference–power-handling components are naturally heftier.