A UPS (uninterruptible power supply) is also referred to as a battery backup and standby power device. However, UPS devices can also provide, depending on the model and how much you pay for it, surge suppression and even line conditioning. it is essentially a large battery and a battery charger. It provides a PC protection against short-term power outages, surges, spikes, and brownouts. it helps provide a constant, reliable, and non fluctuating stream of AC power by monitoring the AC power line and providing voltage from its battery whenever the voltage of the AC line is below a certain level. it also help buffer spikes and surges by storing off any voltage above a certain level as well. The power stored in it’s battery is passed through an inverter that creates an AC supply for the PC to convert to DC power.
Better uninterruptible power supply devices supply AC power to a PC that is usually better than the AC power from the wall. A less expensive UPS may not provide a smooth power wave and may actually damage equipment plugged into it. an uninterruptible power supply is designed to provide a number of power-related services to the devices connected to it:
Power source :- The UPS unit is placed between the devices you wish to protect from blackouts, and other power line events and the electrical outlet from the normal AC electrical service.
Line conditioning :- Virtually all but the very least expensive UPS units provide line conditioning to filter line noise from the electrical supply.
Surge suppression:- uninterruptible power supply units provide protection from power surges and spikes on the electrical line.
Brownout and sag protection:- UPS units fill in the power loss during a power sag or a brownout. Most uninterruptible power supply units cannot make upthe power loss of a brownout indefinitely, but unless the brownout is severe they can replace most of the power loss for a short period.
Backup power :- The primary purpose of it is to provide backup electricity to the devices plugged into it for a certain amount of time.
Alarm system Most of the quality UPS units now available include a means to connect the UPS unit to some means of notifying you of an electrical event serious enough to invoke the UPS. On most units, this is a network connection, but some also offer a telephone line connection used to send an e-mail, a paging call, or the like to notify you of the event. Some also include software that is installed on the protected PC that will initiate shutdown procedures when notified by the UPS over a serial line connection of a serious electricity supply system event.
that detects the incoming power level, a battery that is constantly being recharged for use should the power source fail, and an outlet to which a device, such as a PC can connect with its power cord. One thing that is common to all UPS technologies is that the incoming AC power must be converted into DC power for use inside the UPS and then converted back into AC power for use by the PC (which will convert it back to DC for use by its internal components). All of this power conversion may seem redundant, but keep in mind that the UPS is there to simulate the normal AC power source.
There are three basic UPS technologies, along with several hybrids, used to protect against or solve different types of power issues: standby power supply (SPS), line-interactive UPSs, and on-line UPSs.
Standby Power Supply (SPS):- A standby power supply (SPS), also known as an off-line power supply, is a pass-through unit that is inactive until the power fails. It shares the incoming power with its devices to charge its batteries and, as long as the electrical power source is available, power is passed through the unit to its outlets and the devices plugged into them. Figure 23-4, in the preceding section, illustrates the general configuration of an SPS unit. When a brownout or blackout occurs, the unit switches over to the battery to provide power to its outlets. Because of the time involved to switch its modes, which varies by manufacturer and model, SPS units are not good for dealing with power sags because the reaction time of the unit is usually longer than the duration of these events.
This type of UPS is an inexpensive solution for stand-alone, noncritical PCs and peripherals and is generally not suitable for servers. However, because of their lower cost, they are often used to protect desktop workstations. Standby UPS technology is typically very reliable and switches modes fast enough to prevent serious problems when the power source blacks out. The key specification when considering a standby UPS is the voltage range that the UPS accepts as its normal operating range. Whenever the voltage level of the incoming power is outside this range, the UPS begins drawing on its battery.
You want a standby power supply to have a wide operating range, but not too wide. If the voltage range is too wide, your PC may be running on low or high voltage for extended periods. However, you want the range wide enough to minimize the number of times the UPS switches and draws on its battery. Each time the standby UPS switches to its battery backup, it shortens the battery’s life. Most standby UPS units use an operating range of 103 volts AC to 132 volts AC, which means that whenever the power sags below 103 volts or surges above 132 volts the unit goes “on-battery.”
Line-Interactive UPS:- A line-interactive UPS unit is especially well-suited to environments where there are few brownouts and blackouts but where surges, spikes, or sags are common. When the power supply is available, the line-interactive UPS provides line conditioning and produces a steady level of output voltage from a fluctuating input voltage level. This type of UPS also provides good protection from EMI, RFI, and other forms of line noise. As shown in Figure 23-5, this type of UPS adds line conditioning to the battery backup capability of the UPS. In terms of providing battery backup, a line-interactive UPS works just like a standby power supply and is able to switch to its battery backup faster than any connected equipment can detect the power loss.
Remember that the PC’s power supply has a small amount of reserve power on which it can draw when the power suddenly drops, and most UPS units switch faster than this reserve can be exhausted. The benefit of a line-interactive UPS over a standard SPS is that because it also filters the incoming power, it can reduce the amount of times the UPS switches to its battery. Use the same criteria used for an SPS unit to judge a line-interactive UPS. However, remember that a blackout is normally preceded by a swarm of surges and spikes before the power is lost completely. Before purchasing a line-interactive UPS, investigate how much over-voltage the unit can withstand before it needs to switch to its battery because a UPS draws on its battery to neutralize over-voltage events.
On-line UPS:- To many, an on-line UPS is the true uninterruptible power supply. This type of UPS provides all of the services of a surge suppressor, a line conditioner, and a battery backup in a single package. On-line UPS units provide the best protection of all of the UPS technologies but they also cost more than the other technologies. An on-line UPS supplies power continuously from an AC to DC power inverter. There is no switchover when the power fails because the outlets on the UPS are powered from the battery at all times. The incoming power source is conditioned to protect the circuitry of the UPS, but the result is that any device connected to the on-line UPS is isolated from power problems. An on-line UPS produces the best quality power of any of the UPS technologies.
It produces a near-perfect power stream that is free of even the smallest fluctuations. Standby (off-line) and line-interactive UPS technologies reduce the severity of spikes, surges, and sags by clamping them down into the normal operating voltage range of the unit, but fluctuations within the normal range are unaffected. The on-line UPS is able to deal with over- and under-voltage events without using its battery, which can extend the battery’s life. Like the line-interactive UPS, the on-line UPS draws a small amount of the incoming power to keep its battery charged. This type of UPS technology is usually applied to mission-critical networks and high-availability devices, such as disk arrays and network access servers.
UPS Device Characteristics some of the features commonly found
Information displays:- All UPS devices will issue a warning before its battery is completely discharged, but the better devices have information displays to provide information on the charge level of the battery, the amount of power being demanded by the PC, and other information you need to decide how much time you have before the battery is dead.
Monitoring systems :- Many UPS devices include a serial cable that is attached to a serial (COM) port on the PC, which is used by software running on the PC to monitor the “heartbeat” of the UPS. it sends a signal at regular intervals over the serial cable. These signals are monitored by a software program running in background on the PC. If the UPS fails to send too many signals, the software assumes that the UPS is gone and begins to shut down the PC. The software program that monitors it is usually supplied by the manufacturer of the UPS. There are advanced monitoring systems that can display console messages, send e-mail, or dial a pager to notify the system administrator.
Line conditioners:- A line conditioner, also commonly called a power conditioner, eliminates line noise from the incoming power and keeps voltage within normal levels. Line conditioners don’t protect against blackouts, but they do smooth out any low or high-voltage conditions on the incoming power line.
Alarm systems:- Most UPS and line conditioning devices sound an alarm when the input power source drops below a certain level or if the power becomes unreliable.
Watts and Volt-amps Ratings
Most UPS devices are rated in volt-amps, but the power requirements of most PC devices, including that of the power supply, are generally stated in watts. To determine the right-sized UPS device for your system, you need to understand the difference (and similarities) of these two electrical measures:
Watts:- The real power used by an electrical device. It is the power actually taken from the AC input source.
Volt-amps (VA) The VA rating of a device is computed as the volts it uses times the amount of current in amps it draws from the circuit. The volt-amps rating is used for determining the size of wiring, circuit breakers, and UPS devices. The watts and volt-amps ratings for PCs are usually different values. In most cases, the VA rating is never less than the watt rating and is generally larger. In fact, many devices have what is called a “power factor” that indicates the percentage their watts rating is to their VA rating. The power factor is a ratio that is expressed either as a fractional number, like 0.8, or as a percentage, like 80 percent. The industry standard for UPS device power factors is around 0.6 or 60 percent.
Typically, a UPS device will list only its VA rating, but you can count on its watts rating being somewhere around 60 to 80 percent of the VA rating. The general rule of thumb for it’s sizing is that the total demand in watts should be only 60 percent of its VA rating. The worst thing that could happen if you oversize your UPS is that it will last longer than its load ratings. A UPS’VA rating indicates roughly the amount of volt-amps it can supply for about a five-minute period. A UPS with a 300VA rating can provide 300VA for about five minutes with a full load of around 180 watts (60 percent of 300VA). If the load is less, it can last longer. If the load is only 120 watts, it may be able to provide power for 15 minutes or more.
You can size a PC by using either the amount of watts you need or the number of volt-amps you need, whichever number you happen to have. The capacity of it should be enough to power your system for 15 minutes. This is ample time for you to shut down the system without losing data or programs. One thing to bear in mind is that the higher the VA rating, the more it will cost. You can most definitely find it will power your PC for an hour or more, but expect the cost to be prohibitive. Here is a formula you can use to calculate the amount of time it will support your system:
(Max. Load (Amps) x 120) + (Power (Watts) x 1.4) = Volt Amps Required
Total Volt Amps Required / Full Draw = Minimum Supply
Total Volt Amps Required / Half Draw = Nominal Supply
Maximum load in amps :-The total draw in amps of the PC and any other devices to be powered by the UPS. The “120” multiplier is the volts on the AC power source.
Power supply (in watts):- The watts demand of the power supply on the PC. The 1.4 factor converts it to a VA rating.
Full draw:- Dividing the Total VA Required number calculated above by the Total VA rating of it should be greater than 5 minutes. However, you never want to load a UPS this heavily.
Half draw Using a loading factor of 50 to 60 percent, the result will be a UPS on which you can relay to supply the emergency power you need, buy typically at least 15 to 20 minutes of standby power.
To calculate the VA requirements for your system, you can gather most of the information you need from your system’s documentation or its manufacturer’s Web site. The VA requirements for some of the components in the PC you should consider when determining this value are:
Power supply 110 to 180VA (180 to 300 watts)
Pentium processor 40VA (50 watts)
Hard disk drive 15VA (24 watts)
Motherboard 20 to 35VA (30 to 50 watts)
CD-ROM 20 to 25VA (30 to 35 watts)
Expansion cards 5 to 15VA (7 to 20 watts)
Floppy disk drive 5VA (10 watts)
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