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Choosing a UPS? Off line vs. On line

by Roxanne Pierce

Choosing a UPS
Off-line vs. On-line

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Roxanne Pierce


Roxanne Pierce is the former UPS Product Manager, for the Elgar Corporation.

There are two major types of UPS: Switching ("off-line", or "line interactive") and on-line. The "online" UPS is what it sounds like; the battery is always supplying power to the device, and by definition it supplies conditioned power. Voltage highs and lows, spikes, and other nasty
electrical problems simply don't make it through the double conversion required of an online UPS. For the online UPS, the power goes from the wall, into an AC to DC converter, into the batteries, out of the batteries, through a DC to AC converter, to the receptacles. Because of the constant load on all the circuitry, these units are large, heavy, and expensive.

The switching UPS is not always supplying power from the battery. When the detector circuit detects a power outage of a low number of milliseconds, there is literally a switch that moves to take the unit off of wall power
(line mode) and onto battery power. The power goes from the wall and immediately splits, with one leg going through a switch straight to the receptacles the computer is plugged into (usually through a conditioning circuit located just before the switch). The other leg goes to an AC to DC converter, which "trickle charges" the batteries. When power loss is detected, the switch flips to the output of the battery leg, where it connects to a DC to AC converter which powers the computer system for some
small amount of time (5, 10, 20 minutes.)

In the switching UPS, the amount of electrical conditioning is purely dependent on what type of circuitry is built into the non-battery leg. When I was working in the industry, there were battery backup units (switching
UPS) that had nothing but a cheap surge suppressor -- not even a real filter -- on the line side of the circuit. The better units are built with voltage regulators and/or real filtering.

The APC units are switching UPS, as indicated in this sentence from the APC literature: "An APC Back-UPS instantly switches your computer to emergency battery backup power and allows you to work through brief power outages..." The APC Back-UPS 300 has a transfer time of "4ms typical; 8ms maximum." That unit has a surge suppressor and filtering unit on the non-battery leg;
the surge suppressor is rated at 320 Joules, and the filtering is "full time multi-pole noise filtering; 0.7% IEEE surge let-hrough; zero clamping response time; meets UL 1449."

The only point I really want to make is that affordable single-unit UPS's do not typically "condition" the power used by the computer. Filter it some, sure. But if it is imperative that a computer receives very clean, regulated
power AND be online at all times, the only answers are either to put it on a true online UPS, or to run the power to a computer through both a true line conditioner AND a switching UPS.

Roxanne Pierce is the owner/operator of

a computer consulting business
in Southern California


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