Editing Uninterruptible power supply (section)

===Line-interactive===
[[Image:Line-Interactive UPS Diagram SVG.svg|thumb|500px|Line-interactive UPS: The green line illustrates the flow of electric power. Typical protection time: 5–30 minutes. Capacity expansion: several hours.]]

The line-interactive UPS is similar in operation to a standby UPS but with the addition of a multi-tap variable-voltage [[autotransformer]]. This is a special type of [[transformer]] that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer. This may also be performed by a ''[[buck–boost transformer]]'' which is distinct from an autotransformer, since the former may be wired to provide [[galvanic isolation]].

This type of UPS is able to tolerate continuous undervoltage [[Brownout (electricity)|brownouts]] and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption,<ref>{{cite web|url=http://www.stacoenergy.com/ups/on-line-uninterruptible-power-supply.htm |title=UPS On-Line Uninterruptible Power Supply Backup Power Source |url-status=dead |archive-url=https://web.archive.org/web/20131004222451/http://www.stacoenergy.com/ups/on-line-uninterruptible-power-supply.htm |archive-date=October 4, 2013 }}</ref> which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.

This has become popular even in the cheapest UPSes because it takes advantage of components already included. The main 50/60&nbsp;Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: One to convert the battery output voltage (typically a multiple of 12&nbsp;V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14&nbsp;V). The difference between the two voltages is because charging a battery requires a delta voltage (up to 13–14&nbsp;V for charging a 12&nbsp;V battery). Furthermore, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.

To gain the ''buck/boost'' feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer's primary windings as an autotransformer. The battery can still be charged while "bucking" an overvoltage, but while "boosting" an undervoltage, the transformer output is too low to charge the batteries.

Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, as well as the expense of the UPS.<!-- No significant change to size or weight; it's still one transformer.--> It is common for the autotransformer to cover a range only from about 90&nbsp;V to 140&nbsp;V for 120&nbsp;V power, and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal, so it may need a higher current circuit than a normal device. For example, to power a 1000&nbsp;W device at 120&nbsp;V, the UPS will draw 8.33&nbsp;A. If a brownout occurs and the voltage drops to 100&nbsp;V, the UPS will draw 10&nbsp;A to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need less current.