UPS Keeps Computers Humming (.PDF Download)

April 25, 2017
UPS Keeps Computers Humming (.PDF Download)

In the 1970s, a Popular Mechanics article described how to build your own UPS using a car battery charger, a car battery on a rubber drain board, and a 12-Volt inverter. The charger kept the battery charged as long as power was present, and the battery ran the inverter on its own charge for a while when the main power disappeared. It was an ungainly device and delivered brutal square-wave power that would be troublesome for a lot of today’s more delicate equipment. Still, it embodied the same principles that underpin modern UPS designs.

The primary elements of all contemporary UPSs are a power-line monitor, battery, battery charger, and inverter. Let’s take a look at each:

Monitoring was once as simple as an electromechanical relay, which is still common in many simpler, lower-cost UPSs. Over time, brownouts (voltage drops that could last from a few minutes to few hours, resulting from the power company not being able to generate enough power to satisfy the demand) became more common. This led electrical engineers to realize that power feeds from utilities could vary much too widely from nominal values to limit UPSs to provide power only in the absence of any power. So designers improved UPS monitoring to the point where it could recognize any power feeds outside acceptable limits in terms of voltage and frequency. Acceptable, in this sense, refers to the designer’s determination of what voltage and frequency range UPS-protected equipment could use without damage.

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