![[Bytes Link logo]](../../../cgif/bytelnk2.gif)
Uninterruptible Power Supply (UPS) Technology
by By Herbert Wong, Jr.— NOCCC Hardware SIG Leader - June 02, 2001 at 15:11:31:
Electricity is the lifeblood of computers. Cut off the electrical power source for a fraction of a second and the computer will reboot. If you’re lucky, unsaved data is all that will be lost. You could easily avoid all this by using an uninterruptible power supply (UPS).In primary functionality, an uninterruptible power supply will, within a fraction of a second (typically three to four milliseconds), temporarily replace utility company electric power during an outage. At that time, a manual or an automated process can save vital unsaved application data, shut down the operating system, and turn off the computer.
In secondary functionality, a UPS may optionally include line filtering and line conditioning capabilities. Utility company electric power is subject to a wide variety of electrical noise, interference, low voltage and high voltage conditions.
General Terminology—You’ll need to have a basic understanding of a few electrical terms. The basic concepts and relationships are what is important, not the technical accuracy.
Direct current (DC) is an electric current that flows in one direction only and is substantially constant in value. Direct current is most commonly found in batteries.
Alternating current (AC) is an electric current that reverses its direction (polarity) at regularly recurring intervals. Alternating current is most commonly found in electric utility company power. In North America, power alternates between 120-volts positive and 120-volts negative sixty times per second (cycles per second or hertz)
Voltage is electromotive force (EMF). One volt is equal to the EMF required to force one ampere of current through one ohm of resistance. Think of a volt as the potential to move electricity against a resistance.
Ampere is “the practical meter-kilogram-second unit of electric current that is equivalent to a flow of one coulomb per second or to the steady current produced by one volt applied across a resistance of one ohm” (Merriam-Webster dictionary). Think of an ampere as a quantity of electricity obtained under specific conditions.
Watt is the power produced by a current of one ampere across a potential difference of one volt. In the United States, one horsepower is 746 watts. Think of a watt as the ability to do a specific amount of work.
Volt-ampere is simply the product of volts times amperes. For direct current, one volt times one ampere equals one watt of power. However, for alternating current (as provided by the electric power utility) it has a different implication, as you’ll see later.
Switchover occurs when a UPS begins to run completely from battery power.
Surge Suppressor is a power surge is a sudden increase in electrical voltage. Its cause might be a major appliance (refrigerator, air conditioner, etc.) turning off, resulting in a rebounding of the electricity. The ultimate power surge is caused by a nearby lightning strike.
The initial protective electrical component built into any device is the surge suppressor. A surge suppressor might have any number of incarnations, however, the MOV, or metal oxide varistor (variable-resistor), is the most common due to its low price, small size, and ease of implementation. The MOV simply redirects any unwanted voltage surges. Unfortunately, MOVs are damaged by their very operation. In addition, MOVs tend to have a limited chronological lifespan. They become ineffective after a few years or less, be it on a shelf or in usage.
Power strips commonly include MOVs, capacitors, and rod-core inductors for suppressing surges and spikes.
Filter and Conditioner A line conditioner may consist of a transformer that attempts to smooth out fluctuations in input voltage to provide near uniform output voltage or voltage waveform.
Fluorescent lights and motors are two of many sources of electromagnetic and electric power line pollution. They impart changes to the characteristic sine waveform of the electricity that come from the electric utility.
Filters and line conditioners are built into many types of power conditioning devices. While computers benefit from the reduction and elimination of such line noise, other devices are virtually transformed by filtering and line conditioning. High-end audiophiles have recently discovered these benefits. Many companies now sell line conditioners for several thousand dollars.
Uninterruptible Power Supply (UPS) can provide usable emergency electric power for a short interval during a utility power failure. By storing electricity in batteries during normal power conditions, a UPS is prepared for one power failure. It must recharge before a significant reoccurrence of a power failure.
Suppose you want to run a normal (alternating current) appliance without using a utility company’s electricity. You could use one or more 12-volt batteries (a direct current electricity storage container) to power an inverter, which converts direct current into alternating current. The batteries’ 12-volts (direct current) will also have to be transformed into 120-volts (alternating current). Output power will continue to be produced until the batteries are depleted.
This is the device you’ll want during a power failure. To be useful for computer applications, this inverter will have to be switched over to before the computer’s power supply is affected by the power shortage. The adverse affects might begin in as soon as a few hundredths of a second. UPS switchover circuits respond much more quickly (typically within three to four milliseconds), so the computer doesn’t notice the power outage.
Next, you’ll have to add a means of charging the batteries. The electric utility company’s power (120-volt alternating current) must be transformed into 12-volt battery power. A rectifier converts alternating current into direct current.
Essentially, an uninterruptible power supply (UPS) is composed of a rectifier’s AC to DC (alternating current to direct current) circuit, storage batteries, an inverter’s DC to AC (direct current to alternating current), and switchover circuitry. Almost every UPS also adds surge suppression and power filtering circuitry of some sort.
A standby UPS simply waits for the electricity to fail and then switches over to battery power. A trickle charging circuit tops off the battery so that its maximum capacity will be available when the power fails.
An online UPS continuously provides power (to its output) that has undergone AC to DC conversion and then DC to AC conversion. The result is power that is absolutely consistent in waveform (sine wave) and voltage. Under normal operation, the output power is identical whether or not there is normal electricity as input. This is the preferred type of UPS. Of course, it is the most expensive variety and consumes more electricity (less efficient).
Line interactive is a term used by some manufacturers to mean a circuit that adjusts for steady but nonstandard voltage conditions. A steady over voltage (power surge) causes a switch to one compensating circuit, a steady under voltage (brownout) causes a switch to a voltage boosting circuit, and a normal voltage uses the standard circuit.
The UPS’ 120-volt alternating current output has a characteristic waveform. Ideally, it will perfectly match the pure sine wave shape that should be provided by the electric utility company. It is more expensive to do this than to produce other waveforms.
To imagine a sine wave for the less technically minded, imagine a line with a series of consecutive hills drawn with the first above the line, the next upside-down below the line, the next above the line, the next upside-down below the line, etc.
On the worst end, the voltage output looks like square waves. Square waves provide more voltage (than sine waves) for much of the cycle. This is bad for many devices that might overheat and fail.
To draw a square wave for the slightly technically minded, start drawing a voltage-time graph line at zero volts. Instantly, the line rises to 120 volts for one-half of the cycle. Instantly, the line drops to minus 120 volts for the second half of the cycle. This repeats. Each cycle is one-sixtieth of a second. For the less technically minded, think of a line with a series of consecutive rectangles drawn with one above the line, the next below, the next above the line, the next below…
Somewhere between the two, is a stepped waveform. Think of it as a series of up and down steps (or rectangles) that are arranged to fit under the sine wave (hill shapes). This matches a sine wave more closely than a square wave. It is a reasonable compromise in results and cost. However, new electronics make pure sine waves more affordable.
Automatic Shutdown—While you were away… You never knew what hit you…
Since a UPS has a relatively short span of time for which it can provide battery power, more advanced UPS devices provide a communications port and monitoring software to automatically shutdown applications and the operating system. The communications port is used to communicate with the software that runs on your computer. During a power failure, the software will communicate with the operating system to notify network users, and initiate a shutdown of applications, the operating system, and ultimately the UPS itself.
If an application is properly written, it will save its work and shutdown in a timely manner. It is common, however, for ill behaved applications to open a dialog box and ask if you want to save your work before it closes. This may prevent subsequent applications from receiving the shutdown command and their data will also be lost.
The communications port was traditionally a DB-9 serial port (probably with special pin outs). Newer UPS communications ports can be USB (universal serial bus) or even Ethernet.
Depending upon the included functionality in the UPS itself and the software, many different types of data might be logged by the software. It might track line voltage, power outages, system shutdown commands, battery charge, battery load, battery temperature, battery discharge history, etc.
Choosing a UPS—To determine the power draw or power load of an individual piece of equipment, calculate the volt-ampere rating. If the load specification is in amperes, simply multiply the amperes times the nominal voltage (120 volts for North America, 220 volts for Europe, etc.) to yield volt-amperes. If the specification is in watts, multiply watts by the conversion factor of 1.4 to yield volt-amperes.
Add the total volt-ampere load for every piece of equipment that you plan to run off of the UPS. That is the minimum capacity rating for the UPS that you can use. Do not use a smaller capacity UPS since it will be overloaded, trip its circuit breaker, and produce no power output (rather than power for a much shorter interval).
By now you have figured out that you can’t afford to run all of your computer equipment for an extended period of time. You’d require a commercial class UPS in order to easily outlast a one-hour California rolling blackout.
Primarily, you’ll want to run your computer and monitor off of the UPS. Obviously, the computer must be powered during an “event” to protect data until it is saved. The monitor is powered so that you can see your applications to save data and shutdown everything possible in the few minutes that you will have until the UPS batteries fully discharge.
The computer’s UPS should not power a laser printer. Firstly, such devices draw too much power to be squandered on protecting a few pages that are being printed. Secondly, some devices may interact with the UPS and cause damage.
You might consider adding a few other low power loads to your UPS. A modem might be an option. An inkjet printer might also be reasonable if expensive paper and a great deal of ink was in jeopardy. However, unless you have a significant amount of surplus battery capacity, a long print job or modem download may never complete anyway.
A UPS that is rated for twice the actual volt-ampere load (computer and monitor) might last for three times the estimated full load time. For example, one manufacturer’s estimate for their 650 VA UPS estimates a run time of twenty-one minutes with a 300 VA (195-watts) load and a run time of six minutes with a 600 VA (395-watts) load. Be careful, depending upon the battery’s condition, these times may vary quite dramatically.
Consider what this means. The same manufacturer estimates that under a 450 VA (292.5-watts) load, a 650 VA UPS will last for eleven minutes while a 500 VA UPS will last for only five minutes. The VA rating increase of thirty percent (500 VA to 650 VA) results in a doubling of run time under a common load condition.
There is one tremendously important point to remember. After a UPS is discharged through an “event” its batteries must be recharged. This is a very time consuming process. The operating manual will tell you how long it will take. Expect to see recharge times of as much as twelve to sixteen hours. If the power goes off and the UPS is fully discharged, the UPS will not protect against any outage, no matter how brief, for many hours.
Surge suppressors should be used on almost every residential electronic component. Do not arbitrarily daisy chain surge suppressors, line conditioners, and UPS devices because potentially harmful interactions can result.
Conclusions—Uninterruptible power supplies are cheaper and more efficient than ever. An on-line UPS will make your electronic components last longer and help you protect your valuable data in the event of a power failure. However, all UPS units exact the toll of higher power consumption under normal operating conditions.
The benefits of getting a 650 VA or larger UPS are significant as compared to 500 VA or smaller units. These units are demonstrating commodity pricing and should be part of every computer system that must protect unsaved data.
The latest version of this article will eventually be posted at http://www.SingularityTechnology.com/articles/UPStechnology.html.
You can contact me at ocug@SingularityTechnology.com.
![[------STRIPE-----]](../../../cgif/str1.gif){kind=small}
Site Disclaimer Suggestions? E-Mail to webmaster@noccc.org