Output from a line-interactive UPS varies with the input voltage, but regulation extends the voltage range of input power the UPS can use without going to battery. Typically, the regulator is a transformer with multiple taps that can be switched in or out, depending on line conditions. Like changing gears, the tap switching boosts voltage up when it gets low (brown-out protection) or trims it down when it gets high.
If you plot input against output voltage, you get a sawtooth pattern, as represented by our test results with Deltec's PowerRite Pro 1000VA (
see the figure
). Lowering input voltage from 115 VAC (to the left), output voltage declines. At 110 VAC on input, the UPS switches to another transformer tap, and output jumps to 118
VAC. (The "less than abrupt" output change that takes place during a tap switch isn't evident here because we took readings at 5-V intervals.) The sawtooth just below 100 VAC is where the unit switched to battery.
Proceeding up from 115 VAC, the output voltage climbs until about 125 VAC, where a tap switch bucks it down to about 115 VAC. Again, as the input voltage continues rising, output rises until just around 140 VAC, when the battery kicks in and output drops to 117 VAC. Some units provide just a single tap point for low voltage. Others provide more than two -- as does Tripp Lite's Smart 1050 -- to provide finer regulation and another tooth in the plot.
-- Rick Grehan
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With a line-interactive UPS, such as Deltec's PowerRite Pro, output voltage varies with input voltage. Regulation by the UPS's transformer keeps output within a tolerable range, but not at a constant level.
Boost and Buck
