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Input Voltage Requirements and Tolerances

Input specifications refer to what the power supply requires for its electrical power input--in other words, what it wants to see coming from the wall, or from your UPS. Most electrical input specifications will be provided as a range, because while the power supply may require 115 V as input it of course doesn't need exactly 115 V. The range of acceptable values on a specification are sometimes called the specification's tolerances.

Input Voltage Range: Acceptable range of input voltages. Since most power supplies can function on nominal 115 V or 230 V electricity, you will usually see two sets of numbers. For example: "85 to 135 V AC" and "170 to 270 V AC". The input range is not usually all that critical in determining the suitability of a power supply, because most utility power stays fairly close to the nominal level under normal circumstances. However, the minimum voltage level can have some impact on how well the power supply rides through brownouts.

Voltage Selection: If the power supply supports both 115 V and 230 V nominal voltage, does it automatically select between them, or is there a manual switch?

Frequency: Acceptable frequency of input power (50 Hz, 60 Hz, or 50 and 60 Hz). Alternately, a range of acceptable frequencies (for example, 48-62 Hz). Most power supplies can handle both nominal 50 Hz and 60 Hz input.

Power Factor: The power factor that the power supply presents as a load to the utility power line. Normal power supplies will be in the 60% to 70% range (0.6 to 0.7). Power-factor-corrected supplies will have a number like "0.99". Sometimes, the spec will just say "power factor corrected". Read more about power factor in the discussion of the power supply's conversion circuitry, or in the Electrical Basics power page.

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