by Wayne M. Krakau - Chicago Computer Guide, February 1993

Continuing with last month's subject, the highest category of power protection, the Uninterruptible Power Supply (UPS), is the most varied. It is an absolute requirement for file servers. It can also be used for critical workstations, wiring hubs, and even phone systems.

In spite of some vendors' snide remarks to the contrary, the term UPS does cover devices that do not draw power continuously from a battery. The basic standard (set prior to the development of the IBM PC) involves saving the computer from damage and avoiding data loss, but does not specify the exact method. The standard does allow for minute power losses, as long as they can be absorbed by the built-in capacitance (energy holding capability) of the computer's power supply. Given the generosity of this definition, you are still better off maximizing the protection within practical and monetary limits. The current pricing of the different divisions of this category permits the selection of devices that greatly exceed this bare-minimum standard without breaking your budget.

At the top of the UPS food chain is the true online UPS. All power is supplied continuously through its battery, with the battery constantly recharged in turn. Many of these (especially the room-sized ones found in mainframe and minicomputer installations) were designed based on the linear power supplies found in pre-PC computers and other electronic devices. They output power with square or other unnaturally shaped waves. Since PCs use switching power supplies that sip power only off the very top and bottom of the power curve, square or other oddly shaped waves are radically inappropriate for PCs.

This restriction applies to all the different types of UPSs, not just true online ones. Look for a UPS with true sine wave output. While you can make do with a very tight pseudo-sine UPS, current pricing for all types of UPSs have eliminated any price difference between sine and pseudo-sine so don't bother.

True online UPSs don't really provide extra benefits for PCs, given the design of the PC's power supply and the advances in other types of UPSs. They also have several disadvantages. The battery is constantly in use and therefore wears out fast. Converting electricity from AC (from the socket) to DC (for the battery) and then back to AC (for the PC) is not 100% efficient. There is a major loss of power due to this inherent inefficiency. That means higher electrical bills. The lost power is converted to heat. This means more noise (and even more wasted energy) due to the required large, fast fan. The extra heat also demands more air conditioning, wasting even more energy. Finally, true online UPSs are the most expensive type of UPS.

SPSs (Standby Power Supplies) wait for a major power drop (a blackout or very severe brownout) to kick in with battery power. As long as they are fast enough, they meet the theoretical minimum limits for protecting a PC. Generally, 4ms (milliseconds) is considered fast enough to switch to battery power. The problem is that this doesn't take into effect the time it takes to recognize the power outage. Power doesn't usually sharply drop to zero. It gradually drops randomly, getting more erratic as it falls. Only when it reaches a predesignated (by the manufacturer) limit will the switching circuits start. While power drops, the PC can be supplied with wildly varying and potentially damaging power. Also, the simplest SPSs don't even include surge suppression. They feed raw AC power straight through to the PC.

More advanced SPSs have added circuits to provide surge suppression and to improve detection of uneven power losses. They also synchronize better with the waveform upon both the switch to the battery and the eventual switch back to line power. Unsynchronized switching stresses the PC.

Hybrid UPSs are a combination of an SPS and a power conditioner in one box. The power conditioner portion provides regulation and suppression while the SPS portion protects against outages. The transformer allows the device to stay on line power much longer before being forced to switch to the battery. A regular SPS must switch even for a brownout. The transformer has one extra feature. Its inherent capacitance causes it to keep providing power for a short period even after line power has failed. This period is just long enough to smooth out the transition to battery, providing better quality power right up to the switch.

The latest incarnation of the UPS is the line interactive UPS. This uses advanced circuitry combined with a clever design trick to match the features of the hybrid and adds sophisticated monitoring and self-diagnostic features without bothering with a ferro-resonant transformer. The clever trick is to connect the invertor (the mechanism that converts DC power to AC power and vice versa) directly to the output line. This is my UPS of choice.

In choosing a UPS you must select a power rating in Volt-Amps (volts times amps), rather than the more commonly known watts. The formula for conversion is:

Watts = Volt-Amps X Power Factor

where the Power Factor for PCs is 0.6 (that's six-tenths).

Keeping in mind that as a systems integrator I can sell any brand, my current favorite brand of both surge suppressors (for the last year) and line interactive UPSs (for the last two and a half years) is American Power Conversion (APC). Their combination of quality products, great warranty ($25,000 worth of protection for damaged equipment), and outstanding tech support have kept me in their fold.

There is one final warning about this column. For the sake of brevity, it contains many oversimplifications and gross generalities about a very complicated subject. While engineers (including the professor who originally taught me wave propagation theory in Energy Engineering class) might nitpick, I believe that the basics are accurately presented. Now, where did I put that Jolt Cola?

                                    1993, Wayne M. Krakau