kern
07-21-2006, 01:45 PM
Uninterruptible Power Supply (UPS) aka "Online" Power Supply is a device that sits between a power supply (wall outlet) and a device (pc, printer, etc) to prevent undesired features of the power source (outages, sags, surges, etc) from adversely affecting the performance of the device.
Standby Power Supply (SPS) aka "Offline" Power Supply (often called a UPS) usually derives power directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available.
A true UPS generally provides the best isolation from power line problems, since they operate continuously from an inverter, with no switchover time. Note that a true UPS has a higher cost, consumes more power, and generates more heat than an SPS.
Most UPS's sold at retail computer stores (which are the focus of this article) are not intended for mission-critical equipment, and should be considered "consumer electronics". So if you depend on computer uptime, you need a special purpose, online, redundant, expensive true UPS system.
"Consumer" UPS quality and effectiveness vary. The time required for the inverter to come on line (switchover), varies by unit. Most UPS claim a transfer time to battery of about 4ms. (while some pc's can tolerate switchover times of 100ms or more, damage can still be done to a pc, or data, even if the pc stays up).
Most UPS provide line filtering and/or other line conditioners, and are often referred to as "line interactive" UPS. They can offer surge/spike suppression circuitry, transformers to "boost" voltage without switching to the battery if a modest voltage drop occurs, or other protection from line noise or frequency variations.
Most UPS's also support software that can automatically shutdown equipment during long power outages, as well as monitor and log the status of the power supply, display the Voltage/Current draw of the equipment, display the voltage currently on the line, provide alarms on certain error conditions, provide short circuit protection. and restart equipment after a long power outage. This software works by sending this information over a serial cable connected to a serial port on the back of the UPS and a serial port on the back of the pc.
How long a UPS keeps running after a power failure depends on how big a UPS you have and what kind of equipment it protects. The typical UPS can only provide power for 4-15 minutes, which is usually enough to ride out power company glitches or short outages. Even if the outage is longer than the battery lifetime of the UPS, this is usually enough time to shutdown equipment in a near-normal fashion, and avoid the inconvenience of an unexpected shutdown, and prevent stress on hardware from another (hard) power cycle.
Important items to consider when choosing a UPS:
~Sinusoidal power output- In general, the closer the AC output of the UPS is to a sine wave, the better it is for your equipment. Many UPS units, especially the cheaper ones, deviate a great deal from a sinusoidal output. Some of them generate square waves.
~The more information about a UPS's operation you can get from watching the unit itself, the better. How much power (% load) the equipment is drawing, how much battery life is left, and indications of the input power quality, are all very useful.
~Some newer UPS's can communicate with their monitoring software via a network connection and SNMP. This is handy if your network is on a UPS.
If you do have a UPS that does not output a sinusoidal waveform, some manufacturers urge not to put a surge protector between the UPS and the computer. The surge protector might mistake the non-sine waveform as a power surge and try to send it to ground, which could damage the UPS and/or your pc.
~How long your "typical" power outages are. For example, places where the power is almost never out for more than 5 minutes, but happens frequently, a UPS with a VA rating close to your equipment rating with no extra batteries may be suitable. Places that have longer outages (frequent severe weather) of 30 minutes or more, a UPS with multiples of the VA rating of the equipment may be suitable, since oversizing a VA rating for a UPS has the effect of lengthening the amount of time your equipment can stay up, or you can buy additional battery units for a smaller UPS.
To determine the size UPS that suits your needs, you need to evaluate your electrical load. The safest method is to use the equipment rating(s). Most pieces of computer equipment have a power rating on some back panel near where power cord enters the chassis. For example, most current PC's have an ATX power supply of at least 300 watts. A typical CRT monitor draws about 1 Amp (~120 W), an LCD monitor a bit less. Peripherals such as speakers, broadband modems and routers, draw little power. These power ratings are usually very conservative, as these numbers generally represent an estimate of total draw of the equipment when it is in its most power-hungry state. Typical device configurations may be less demanding.
Some older equipment, such as monitors, harddisks, etc, may be less efficient and draw more power than new equipment. Also keep in mind that as computing power increases, power consumption usually does as well. Laser printers, for example, draw alot of current during startup and during printing. Some UPS's are available that are specifically inteded for use with laser printers, but most don't.
As a rule, selecting a UPS whose VA rating is equal or greater than the sum of all listed electrical load ratings is pretty safe. Selecting a UPS that's too small, your equipment might not be protected. Selecting a bigger UPS, your equipment can survive a longer outage, or, will allow you to protect more equipment for future expansion.
Roughly speaking, 450 VA for a typical PC is satisfactory. A PC and broadband modem/router, etc, a 600 VA UPS is often suitable.
To avoid unnecessary problems with your UPS:
~Don't ever run on bad batteries.
~Don't run batteries to very low or drained levels ("deep cycle") any more than is necessary. Almost all UPS's use lead-acid batteries (like car batteries). Unlike NiCad batteries, lead-acid batteries do not have a "memory", and deep cycling will decrease a lead-acid batteries life. Battery(ies) will periodically need to be replaced (approx 2-4 years), especially the more frequently (and deeper) the batteries are cycled. Figure that under a normal workload the batteries will be good till the end of the UPS warranty.
~Change the batteries periodically (3-5 years or as per vendor recommendations).
~Periodically test the UPS and their failure modes by simulating an outage to see how the transition goes. The best method is to install a GFI** (Ground Fault Interrupter) on your UPS-covered outlet(s) to avoid having to throw a circuit-breaker, especially if you don't have your UPS protected machines on an isolated circuit. **GFI sockets have a red and a black button- (push black to cut power, red to restore power).
NOTE: Make sure the UPS keeps in contact with its electrical ground at all times- DO NOT test a UPS by pulling the plug from the wall. This is dangerous (creates a "floating ground" shock hazard), and its bad for electronic devices
~Don't subject the UPS to temperature or humidity extremes, water, excessive dust, or excessive static electricity.
~Keep the area around the UPS clean and dry.
~Don't overload the UPS.
~If the UPS shows signs of misbehavior or malfunction, remove it from service asap.
Common Terms:
Blackout - Complete loss of power. Generally, a voltage drop below 80V+- is considered to be a blackout since most equipment will not operate below these levels.
Sag/Brownout - Decrease in voltage levels which can last from fractions of a second to hours. Can be caused by heavy equipment coming on line such as air conditioners, stereo amplifiers, etc. Also occurs when utility companies initiate voltage drops in oder to cope with peak electricity use.
Spike - A large increase in voltage over a very short period of time often caused by a lightning strike on a power line or when power returns after a blackout.
Surge - A substantial increase in voltage lasting a small fraction of a second, often caused when high powered appliances such as air conditioners are switched off.
MOV - Metal Oxide Varistors are added to circuits in order to control spikes. These are common in Power Strips. A fairly decent Power Strip has two or more (they look similar to large disk capacitors).
Inverter - Circuitry that converts DC battery power to AC power required by most computer equipment.
Surge Protector - Circuitry consisting of MOVs, capacitors, rod-core inductors etc, for suppressing surges and spikes usually embedded in a power strip.
Line Conditioner - A transformer that attempts to smooth out fluctuations in input voltage to provide near uniform output voltage or voltage waveform.
Standby Power Supply (SPS) aka "Offline" Power Supply (often called a UPS) usually derives power directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available.
A true UPS generally provides the best isolation from power line problems, since they operate continuously from an inverter, with no switchover time. Note that a true UPS has a higher cost, consumes more power, and generates more heat than an SPS.
Most UPS's sold at retail computer stores (which are the focus of this article) are not intended for mission-critical equipment, and should be considered "consumer electronics". So if you depend on computer uptime, you need a special purpose, online, redundant, expensive true UPS system.
"Consumer" UPS quality and effectiveness vary. The time required for the inverter to come on line (switchover), varies by unit. Most UPS claim a transfer time to battery of about 4ms. (while some pc's can tolerate switchover times of 100ms or more, damage can still be done to a pc, or data, even if the pc stays up).
Most UPS provide line filtering and/or other line conditioners, and are often referred to as "line interactive" UPS. They can offer surge/spike suppression circuitry, transformers to "boost" voltage without switching to the battery if a modest voltage drop occurs, or other protection from line noise or frequency variations.
Most UPS's also support software that can automatically shutdown equipment during long power outages, as well as monitor and log the status of the power supply, display the Voltage/Current draw of the equipment, display the voltage currently on the line, provide alarms on certain error conditions, provide short circuit protection. and restart equipment after a long power outage. This software works by sending this information over a serial cable connected to a serial port on the back of the UPS and a serial port on the back of the pc.
How long a UPS keeps running after a power failure depends on how big a UPS you have and what kind of equipment it protects. The typical UPS can only provide power for 4-15 minutes, which is usually enough to ride out power company glitches or short outages. Even if the outage is longer than the battery lifetime of the UPS, this is usually enough time to shutdown equipment in a near-normal fashion, and avoid the inconvenience of an unexpected shutdown, and prevent stress on hardware from another (hard) power cycle.
Important items to consider when choosing a UPS:
~Sinusoidal power output- In general, the closer the AC output of the UPS is to a sine wave, the better it is for your equipment. Many UPS units, especially the cheaper ones, deviate a great deal from a sinusoidal output. Some of them generate square waves.
~The more information about a UPS's operation you can get from watching the unit itself, the better. How much power (% load) the equipment is drawing, how much battery life is left, and indications of the input power quality, are all very useful.
~Some newer UPS's can communicate with their monitoring software via a network connection and SNMP. This is handy if your network is on a UPS.
If you do have a UPS that does not output a sinusoidal waveform, some manufacturers urge not to put a surge protector between the UPS and the computer. The surge protector might mistake the non-sine waveform as a power surge and try to send it to ground, which could damage the UPS and/or your pc.
~How long your "typical" power outages are. For example, places where the power is almost never out for more than 5 minutes, but happens frequently, a UPS with a VA rating close to your equipment rating with no extra batteries may be suitable. Places that have longer outages (frequent severe weather) of 30 minutes or more, a UPS with multiples of the VA rating of the equipment may be suitable, since oversizing a VA rating for a UPS has the effect of lengthening the amount of time your equipment can stay up, or you can buy additional battery units for a smaller UPS.
To determine the size UPS that suits your needs, you need to evaluate your electrical load. The safest method is to use the equipment rating(s). Most pieces of computer equipment have a power rating on some back panel near where power cord enters the chassis. For example, most current PC's have an ATX power supply of at least 300 watts. A typical CRT monitor draws about 1 Amp (~120 W), an LCD monitor a bit less. Peripherals such as speakers, broadband modems and routers, draw little power. These power ratings are usually very conservative, as these numbers generally represent an estimate of total draw of the equipment when it is in its most power-hungry state. Typical device configurations may be less demanding.
Some older equipment, such as monitors, harddisks, etc, may be less efficient and draw more power than new equipment. Also keep in mind that as computing power increases, power consumption usually does as well. Laser printers, for example, draw alot of current during startup and during printing. Some UPS's are available that are specifically inteded for use with laser printers, but most don't.
As a rule, selecting a UPS whose VA rating is equal or greater than the sum of all listed electrical load ratings is pretty safe. Selecting a UPS that's too small, your equipment might not be protected. Selecting a bigger UPS, your equipment can survive a longer outage, or, will allow you to protect more equipment for future expansion.
Roughly speaking, 450 VA for a typical PC is satisfactory. A PC and broadband modem/router, etc, a 600 VA UPS is often suitable.
To avoid unnecessary problems with your UPS:
~Don't ever run on bad batteries.
~Don't run batteries to very low or drained levels ("deep cycle") any more than is necessary. Almost all UPS's use lead-acid batteries (like car batteries). Unlike NiCad batteries, lead-acid batteries do not have a "memory", and deep cycling will decrease a lead-acid batteries life. Battery(ies) will periodically need to be replaced (approx 2-4 years), especially the more frequently (and deeper) the batteries are cycled. Figure that under a normal workload the batteries will be good till the end of the UPS warranty.
~Change the batteries periodically (3-5 years or as per vendor recommendations).
~Periodically test the UPS and their failure modes by simulating an outage to see how the transition goes. The best method is to install a GFI** (Ground Fault Interrupter) on your UPS-covered outlet(s) to avoid having to throw a circuit-breaker, especially if you don't have your UPS protected machines on an isolated circuit. **GFI sockets have a red and a black button- (push black to cut power, red to restore power).
NOTE: Make sure the UPS keeps in contact with its electrical ground at all times- DO NOT test a UPS by pulling the plug from the wall. This is dangerous (creates a "floating ground" shock hazard), and its bad for electronic devices
~Don't subject the UPS to temperature or humidity extremes, water, excessive dust, or excessive static electricity.
~Keep the area around the UPS clean and dry.
~Don't overload the UPS.
~If the UPS shows signs of misbehavior or malfunction, remove it from service asap.
Common Terms:
Blackout - Complete loss of power. Generally, a voltage drop below 80V+- is considered to be a blackout since most equipment will not operate below these levels.
Sag/Brownout - Decrease in voltage levels which can last from fractions of a second to hours. Can be caused by heavy equipment coming on line such as air conditioners, stereo amplifiers, etc. Also occurs when utility companies initiate voltage drops in oder to cope with peak electricity use.
Spike - A large increase in voltage over a very short period of time often caused by a lightning strike on a power line or when power returns after a blackout.
Surge - A substantial increase in voltage lasting a small fraction of a second, often caused when high powered appliances such as air conditioners are switched off.
MOV - Metal Oxide Varistors are added to circuits in order to control spikes. These are common in Power Strips. A fairly decent Power Strip has two or more (they look similar to large disk capacitors).
Inverter - Circuitry that converts DC battery power to AC power required by most computer equipment.
Surge Protector - Circuitry consisting of MOVs, capacitors, rod-core inductors etc, for suppressing surges and spikes usually embedded in a power strip.
Line Conditioner - A transformer that attempts to smooth out fluctuations in input voltage to provide near uniform output voltage or voltage waveform.