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(Uninterruptible Power Supply) A device that provides battery backup when the electrical power fails or drops to an unacceptable voltage level. Small UPS systems provide power for a few minutes; enough to power down the computer in an orderly manner, while larger systems have enough battery for several hours. In mission critical datacenters, UPS systems are used for just a few minutes until electrical generators take over.

UPS systems can be set up to alert file servers to shut down in an orderly manner when an outage has occurred, and the batteries are running out.

Surge Suppression and Voltage Regulation
A surge protector filters out surges and spikes, and a voltage regulator maintains uniform voltage during a brownout, but a UPS keeps a computer running when there is no electrical power. UPS systems typically provide surge suppression and may provide voltage regulation. See surge suppression.

Standby and Line Interactive
A standby UPS, also called an "offline UPS," is the most common type of UPS found in a computer or office supply store. It draws current from the AC outlet and switches to battery within a few milliseconds after detecting a power failure.

The line interactive UPS "interacts" with the AC power line to smooth out the waveforms and correct the rise and fall of the voltage.

Online UPS
The online UPS is the most advanced and most costly UPS. The inverter is continuously providing clean power from the battery, and the computer equipment is never receiving power directly from the AC outlet. However, online units contain cooling fans, which do make noise and may require some location planning for the home user or small office.

Three UPS Types
All UPS systems switch to battery when the power fails. The difference is how they handle the power under normal circumstances. Standby units provide limited attenuation whereas line interactive systems will adjust the voltage and smooth out bad harmonics. Online systems are constantly regenerating clean power. (Diagrams courtesy of MGE UPS SYSTEMS.)

Size Does Matter
When the power goes off, the unit on the right will keep more equipment running for a longer time than the one on the left, because the battery is larger. Battery size is an important criterion when selecting UPS systems.

Backing Up the Backup
In APC's Symmetra Power Array, all modules share in the total load; thus, any unit can fail and be replaced without jeopardizing the security the UPS was intended to provide in the first place. (Image courtesy of American Power Conversion Corporation.)

surge suppression

The diverting and/or diminishing of excessive current and voltage from the AC power line, which can damage sensitive electronic equipment. Power surges generally last less than 50 microseconds, but can reach as much as 6,000 volts and draw 3,000 amps when they arrive at the equipment. There are two principal types of technologies used in surge protection devices.

Shunt Mode - Divert
The most common method is the use of a metal oxide varistor (MOV), which acts like a pressure relief valve to divert the surge to the neutral and/or ground lines. However, shunt mode methods can be problematic. Diverting high voltage to ground may damage equipment because all electronic devices are interconnected via ground. Since all data lines use ground as a signal reference, excessive voltage on that line can disrupt and impair networks and communications.

In addition, MOVs can eventually stop working without warning. There are countless low-energy surges occurring all the time, even from innocuous, everyday operations such as turning a motor on and off. Each surge causes a minuscule degradation in the MOV. If the MOV is high quality, it can take decades before the countless surges render the MOV ineffective. In a poorly made device, they can add up to failure in a much shorter time.

Series Mode - Block and Absorb
Series mode surge protectors actually block high current and absorb excessive voltage. They do not divert current to ground, but limit the surge to acceptable levels that electronic equipment can handle. Another feature of series mode is the ability to suppress all excessive voltage rather than wait for a certain level. MOVs shunt the current when a preset voltage is reached, but series mode units can track the powerline voltage and activate the suppression components as soon as the voltage goes over the norm. Zero Surge, Inc., Frenchtown, NJ ( is the pioneer in series mode technologies and began shipping their products in 1989.

Surge protectors may use both shunt mode and series mode methods in some combination. For example, they may use series mode for low-energy surges and shunt mode for high-energy surges. See voltage regulator, UPS, power surge, power swell, spike and sag.

Shunt Mode and Series Mode
This American Power surge protector is a hybrid. The capacitors absorb low-energy surges, while the MOVs divert the high energy. (Image courtesy of American Power Conversion Corporation.)

All Series Mode
This diagram shows how Zero Surge protectors ( operate. The current is not diverted to ground. The Zero Surge units use coils and capacitors to block and absorb the energy and release only acceptable levels to the equipment. There are no parts that degrade with each surge encountered.

Keep Running Smoothly
An excellent safeguard is to power a computer from an uninterruptible power supply (bottom) that plugs into a series mode suppressor (top), which plugs into the wall. The high energy of a surge is brought down to low levels before it reaches the UPS. The Zero Surge suppressor and American Power UPS are high quality units.

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