The uninterruptible power supply. Commonly called the UPS, this device is a clever threefold package--a set of batteries, an inverter that transforms the low-voltage direct current of the batteries into the standard alternating current used by your computer, and a battery charger that assures that reserve power is always available inside the batteries. In other words, it is a power storage system (the batteries) with interfaces to match it to utility power and your computer system. The UPS is designed to protect against changes, specifically a temporary loss of electrical supply. It keeps your computer and its support equipment operating without interruption--hence the name--so that you don't lose your work. As such, the UPS is meant to supply power only for as long as it takes you to save your work and shut down your computer. It might also bridge the gap between the loss of utility-supplied electricity and the acquisition of power from another source, such as a generator, which may take several seconds to start and stabilize. A general-purpose UPS is not meant to keep your computer running for more than a short while. A growing trend today, however, is the extended run UPS. Equipped with extra battery reserves, the extended run UPS is meant to keep you going for the duration of a rolling blackout, typically about two hours.
Full power protection means more than just buying a UPS. The UPS must be integrated with your computer and network via control and monitoring software. Moreover, in major installations, the UPS is part of a larger picture that may include a standby generator and special heating and cooling arrangements. In fact, some UPS makers like American Power Conversion are adding complete turn-key power services that embrace overall wiring, power systems engineering, and even HVAC (heating, ventilation, and air conditioning) design to their offerings.
Although the traditional UPS might not seem a hot spot for engineering development--after all, electrical power hasn't changed much in the last 100 years--the last few years have seen major changes in the UPS industry. Most of the effort has been concentrated on fulfilling the needs of LAN and Internet servers. Today's processor-intensive rack packaging of thin servers is driving the need for ever more compact rack-mount UPSs. Tripp Lite, for example, now packs a 3 KVA unit into a 3.5-inch high rack-mount package. Such dense packaging has focused UPS maker's efforts on high-capacity cooling. In addition, demands for high availability have led to the creation of modular and redundant UPSs, units that will keep functioning--and continue to protect your data--even if a critical component such as the inverter fails. Further, the threat of power-line pollution has focused awareness on the quality of UPS power in addition to the quantity each UPS makes available.
You'll find a wide range between inexpensive UPSs listing for around $100 and professional products targeted at the server market, which cost from $700 to more than $2000. The connection method, usually reflected in the number of electrical receptacles on the unit--determines how convenient the UPS is to hook up and use.
The general tendency in modern UPS design is to provide a combination of battery-protected and unprotected outlets. When utility power fails, only the protected outlets will provide battery power. The unprotected outlets serve only as a convenience function, giving you a place to plug in your printer and other non-critical peripherals. The largest UPSs are meant to be permanently wired in place and may connect directly to your fuse box or power distribution system.
The UPS core electronics serve two basic functions. The power circuitry handles the heavy-duty electricity that powers your computer and its peripherals. It takes care of the power conversions--from AC to DC to charge batteries, and from DC to AC to convert battery power to computer-compatible voltages. The control circuitry tells the power circuitry what to do--when to switch from line power to the battery supply, how much current to use to charge the batteries, when to limit power or otherwise protect the electronics of the UPS and your computer. In addition, the control circuitry lets you monitor the operation of the UPS. The control circuitry also links with your computer so you can remotely monitor your UPS and the UPS can warn your computer of power failures.
The next step up the UPS ladder is the line-interactive UPS. The chief change between the off-line and line-interactive designs is that the line-interactive UPS adds some degree of voltage regulation. That is, the line interactive UPS attempts to correct for voltage fluctuations on your power line and deliver electricity that is as close as possible to 120 or 240 volts (depending on what your computer uses).
The term on-line UPS is used differently by various manufacturers. Most UPS makers reserve the term for true double-conversion UPSs because they keep their inverter circuitry constantly on-line. Some manufacturers, however, also use the term for line-interactive products The double-conversion UPS is the only truly uninterruptible system. It never switches because its inverter is always connected to your computer. But the double-conversion design does more than eliminate the brief switching interruption. The battery reservoir completely isolates your computer from the vagaries of the power line. It protects against all forms of power irregularity including surges, spikes, overvoltages, sags, brownouts, and blackouts. The double-conversion UPS doesn't switch transformer taps when line voltage surges or sags, but adjusts its own solid-state voltage regulators to cope with under- and overvoltages. Consequently the double-conversion UPS maintains a closer tolerance on its output voltage. Typically, a double-conversion UPS delivers an output within 2% of its nominal rating.