Troubleshooting the power supply basically means isolating the supply as the cause of problems within a system and, if necessary, replacing it.
Caution: It is rarely recommended that an inexperienced user open a power supply to make repairs because of the dangerous high voltages present. Even when unplugged, power supplies can retain dangerous voltage and must be discharged (like a monitor) before service. Such internal repairs are beyond the scope of this book and are specifically not recommended unless the technician knows what she is doing.
Many symptoms lead me to suspect that the power supply in a system is failing. This can sometimes be difficult for an inexperienced technician to see because at times little connection seems to exist between the symptom and the cause: the power supply.
For example, in many cases a parity check error message can indicate a problem with the power supply. This might seem strange because the parity check message specifically refers to memory that has failed. The connection is that the power supply powers the memory, and memory with inadequate power fails.
It takes some experience to know when this type of failure is power related and not caused by the memory. One clue is the repeatability of the problem. If the parity check message (or other problem) appears frequently and identifies the same memory location each time, I would suspect that defective memory is the problem. However, if the problem seems random, or if the memory location the error message cites as having failed seems random, I would suspect improper power as the culprit. The following is a list of PC problems that often are related to the power supply:
Any power-on or system startup failures or lockups
Spontaneous rebooting or intermittent lockups during normal operation
Intermittent parity check or other memory-type errors
Hard disk and fan simultaneously failing to spin (no +12 V)
Overheating due to fan failure
Small brownouts that cause the system to reset
Electric shocks felt on the system case or connectors
Slight static discharges that disrupt system operation
Erratic recognition of bus-powered USB peripherals
In fact, just about any intermittent system problem can be caused by the power supply. I always suspect the supply when flaky system operation is a symptom. Of course, the following fairly obvious symptoms point right to the power supply as a possible cause:
System that is completely dead (no fan, no cursor)
Smoke
Blown circuit breakers
If you suspect a power supply problem, some of the simple measurements and the more sophisticated tests outlined in this section can help you determine whether the power supply is at fault. Because these measurements might not detect some intermittent failures, you might have to use a spare power supply for a long-term evaluation. If the symptoms and problems disappear when a known-good spare unit is installed, you have found the source of your problem.
Following is a simple flowchart to help you zero in on common power supply–related problems:
Check the AC power input. Make sure the cord is firmly seated in the wall socket and in the power supply socket. Try a different cord.
Check the DC power connections. Make sure the motherboard and disk drive power connectors are firmly seated and making good contact. Check for loose screws.
Check the DC power output. Use a digital multimeter to check for proper voltages. If it’s below spec, replace the power supply.
Check the installed peripherals. Remove all boards and drives and retest the system. If it works, add items back in one at a time until the system fails again. The last item added before the failure returns is likely defective.
Many types of symptoms can indicate problems with the power supply. Because the power supply literally powers everything else in the system, everything from disk drive problems to memory problems to motherboard problems can often be traced back to the power supply as the root cause.
Overloaded Power Supplies
A weak or inadequate power supply can put a damper on your ideas for system expansion. Some systems are designed with beefy power supplies, as if to anticipate a great deal of system add-ons and expansion components. Most desktop or tower systems are built in this manner. Some systems have inadequate power supplies from the start, however, and can’t adequately service the power-hungry options you might want to add.
The wattage rating can sometimes be misleading. Not all 500-watt supplies are created the same. People familiar with high-end audio systems know that some watts are better than others. This is true for power supplies, too. Cheap power supplies might in fact put out the rated power, but at what temperature? Many cheap power supplies are rated at ridiculously low temperatures that will never be encountered in actual use. As the temperature goes up, the power output capability goes down, meaning that in some cases these supplies will only be capable of 50% less than their rating under normal use.
Also, what about noise and distortion? Some of the supplies are under-engineered to just barely meet their specifications, whereas others might greatly exceed their specifications. Many of the cheaper supplies provide noisy or unstable power, which can cause numerous problems with the system. Another problem with under-engineered power supplies is that they can run hot and force the system to do so as well. The repeated heating and cooling of solid-state components eventually causes a computer system to fail, and engineering principles dictate that the hotter a PC’s temperature, the shorter its life. Many people recommend replacing the original supply in a system with a heavier-duty model, which solves the problem. Because power supplies come in common form factors, finding a heavy-duty replacement for most systems is easy, as is the installation process.
Inadequate Cooling
Some replacement power supplies have higher-capacity cooling fans, which can minimize overheating problems—especially for hotter-running processors. If system noise is a problem, models with special fans can run more quietly than the standard models. These power supplies often use larger-diameter fans that spin more slowly, so they run more quietly but move the same amount of air as the smaller fans. There are even fanless power supplies, although these are more expensive and are generally available only in lower output ratings.
Ventilation in a system is also important. In most prebuilt systems, this is not much of a concern because most reputable manufacturers ensure that their systems have adequate ventilation to avoid overheating. If you are building or upgrading a system your own system, then the responsibility for proper cooling falls on you. In that situation it’s critical that your processor is cooled by an active heatsink and that the case include one or more cooling fans for additional ventilation. If you have free expansion slots, I recommend spacing out any expansion cards in the system to permit airflow between them. Place the hottest-running boards nearest the fan or the ventilation holes in the system. Make sure that adequate airflow exists around the hard disk drives, especially for those that spin at high rates of speed. Some hard disks can generate quite a bit of heat during operation. If the hard disks overheat, data can be lost.
Always be sure you run your computer with the case cover on, especially if you have an older, loaded system using passive heatsinks. Removing the cover in that situation can actually cause the system to overheat. With the cover off, the power supply and chassis fans no longer draw air through the system. Instead, the fans end up cooling only the supply, and the rest of the system must be cooled by simple convection. Systems that use an active heatsink on the processor aren’t as prone to this type of problem; in fact, the cooler air from outside the normally closed chassis can help them to run cooler.
In addition, be sure that any empty slot positions have the filler brackets installed. If you leave these brackets off after removing a card, the resultant hole in the case disrupts the internal airflow and can cause higher internal temperatures.
Finally, the location of the system can have an effect on cooling. I don’t recommend placing a system on a carpeted floor, as most chassis are designed to draw in air at the bottom of the front bezel, which can easily be blocked or become clogged with carpet fibers. Another problem is that a system sitting directly on a floor will ingest a large amount of dust and debris, even more so if the floor is carpeted. If you must place a system on the floor, whether it is carpeted or not I recommend elevating it at least an inch or so via some sort of platform.
If you experience intermittent problems that you suspect are related to overheating, upgraded chassis fans and/or a higher-capacity replacement power supply are usually the best cures.
Using Digital Multimeters
One simple test you can perform on a power supply is to check the output voltages. This shows whether a power supply is operating correctly and whether the output voltages are within the correct tolerance range. Note that you must measure all voltages with the power supply connected to a proper load, which usually means testing while the power supply is still installed in the system and connected to the motherboard and peripheral devices.
Selecting a Meter
You need a simple digital multimeter (DMM) or digital volt-ohm meter (DVOM) to perform voltage and resistance checks on electronic circuits (see below). Only use a DMM instead of the older needle-type multimeters because the older meters work by injecting 9 V into the circuit when measuring resistance, which damages most computer circuits.
A typical DMM.A typical DMM.
A DMM uses a much lower voltage (usually 1.5 V) when making resistance measurements, which is safe for electronic equipment. You can get a good DMM with many features from several sources. I prefer the small, pocket-size meters for computer work because they are easy to carry ar
Some features to look for in a good DMM are as follows:
Pocket size—This is self-explanatory, but small meters that have many, if not all, of the features of larger ones are available. The elaborate features found on some of the larger meters are not really necessary for computer work.
Overload protection—If you plug the meter into a voltage or current beyond the meter’s capability to measure, the meter protects itself from damage. Cheaper meters lack this protection and can be easily damaged by reading current or voltage values that are too high.
Autoranging—The meter automatically selects the proper voltage or resistance range when making measurements. This is preferable to the manual range selection; however, really good meters offer both autoranging capability and a manual range override.
Detachable probe leads—The leads can be damaged easily, and sometimes a variety of differently shaped probes are required for different tests. Cheaper meters have the leads permanently attached, which means you can’t easily replace them. Look for a meter with detachable leads that plug into the meter.
Audible continuity test—Although you can use the ohm scale for testing continuity (0 ohms indicates continuity), a continuity test function causes the meter to produce a beep noise when continuity exists between the meter test leads. By using the sound, you quickly can test cable assemblies and other items for continuity. After you use this feature, you will never want to use the ohms display for this purpose again.
Automatic power-off—These meters run on batteries, and the batteries can easily be worn down if the meter is accidentally left on. Good meters have an automatic shutoff that turns off the unit when it senses no readings for a predetermined period of time.
Automatic display hold—This feature enables you to hold the last stable reading on the display even after the reading is taken. This is especially useful if you are trying to work in a difficult-to-reach area single-handedly.
Minimum and maximum trap—This feature enables the meter to trap the lowest and highest readings in memory and hold them for later display, which is especially useful if you have readings that are fluctuating too quickly to see on the display.
Although you can get a basic pocket DMM for as little as $20, one with all these features is priced closer to $100, and some can be much higher. RadioShack carries some nice inexpensive units, and you can purchase the high-end models from electronics supply houses, such as Newark or Digi-Key.
Measuring Voltage
To measure voltages on a system that is operating, you must use a technique called back probing on the connectors. You can’t disconnect any of the connectors while the system is running, so you must measure with everything connected. Nearly all the connectors you need to probe have openings in the back where the wires enter the connector. The meter probes are narrow enough to fit into the connector alongside the wire and make contact with the metal terminal inside. The technique is called back probing because you are probing the connector from the back. You must use this back-probing technique to perform virtually all the following measurements.
To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on AT, Baby-AT, and LPX supplies; pin eight on the ATX-type connector) for +3 V to +6 V of power. If the measurement is not within this range, the system never sees the Power_Good signal and therefore does not start or run properly. In most cases, the power supply is bad and must be replaced.
Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors. If you are measuring voltages for testing purposes, any reading within 10% of the specified voltage is considered acceptable, although most manufacturers of high-quality power supplies specify a tighter 5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of the rating, except for the 3.3 V current, which must be within 4%. The table below shows the voltage ranges within these tolerances.
Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth noting that any and all power-supply tests and measurements must be made with the power supply properly loaded, which usually means it must be installed in a system and the system must be running.
Specialized Test Equipment
You can use several types of specialized test gear to test power supplies more effectively. Because the power supply is one of the most failure-prone items in PCs today, you should have these specialized items if you service many PC systems.
Digital Infrared Thermometer
One of the greatest additions to my toolbox is a digital infrared thermometer. This is also are called a noncontact thermometer because it measures by sensing infrared energy without having to touch the item it is reading. This enables me to make instant spot checks of the temperature of a chip, a board, or the system chassis. They are available from companies such as Raytek (www.raytek.com) for less than $100. To use these handheld items, you point at an object and then pull the trigger. Within seconds, the display shows a temperature readout accurate to +/–3°F (2°C). These devices are invaluable in checking to ensure the components in your system are adequately cooled.
Variable Voltage Transformer
When you’re testing power supplies, it is sometimes desirable to simulate different AC voltage conditions at the wall socket to observe how the supply reacts. A variable voltage transformer is a useful test device for checking power supplies because it enables you to exercise control over the AC line voltage used as input for the power supply. This device consists of a large transformer mounted in a housing with a dial indicator that controls the output voltage. You plug the line cord from the transformer into the wall socket and plug the PC power cord into the socket provided on the transformer. The knob on the transformer can be used to adjust the AC line voltage the PC receives.
A variable voltage transformer.A variable voltage transformer.
Most variable transformers can adjust their AC outputs from 0 V to 140 V no matter what the AC input (wall socket) voltage is. Some can cover a range from 0 V to 280 V as well. You can use the transformer to simulate brownout conditions, enabling you to observe the PC’s response. Thus, you can check a power supply for proper Power_Good signal operation, among other things.
By running the PC and dropping the voltage until the PC shuts down, you can see how much reserve is in the power supply for handling a brownout or other voltage fluctuations. If your transformer can output voltages in the 200 V range, you can test the capability of the power supply to run on foreign voltage levels. A properly functioning supply should operate between 90 V and 135 V but should shut down cleanly if the voltage is outside that range.
One indication of a problem is seeing parity check-type error messages when you drop the voltage to 80 V. This indicates that the Power_Good signal is not being withdrawn before the power supply output to the PC fails. The PC should simply stop operating as the Power_Good signal is withdrawn, causing the system to enter a continuous reset loop.
Variable voltage transformers are sold by a number of electronic parts supply houses, such as Newark and Digi-Key.
When you are shopping for a new power supply, take several factors into account. First, consider the power supply’s shape, or form factor. Power supply form factors can differ in their physical sizes, shapes, screw-hole positions, connector types, and fan locations. When ordering a replacement supply, you need to know which form factor your system requires.
Some systems use proprietary power supply designs, which makes replacement more difficult. If a system uses one of the industry-standard form factor power supplies, replacement units with a variety of output levels and performance are available from hundreds of vendors. An unfortunate user of a system with a nonstandard form factor supply does not have this kind of choice and must get a replacement from the original manufacturer of the system—and usually must pay a much higher price for the unit. PC buyers often overlook this and discover too late the consequences of having nonstandard components in a system.
Name-brand systems on both the low and high end of the price scale are notorious for using proprietary form factor power supplies. For example, Dell has used proprietary supplies in many of its systems. Be sure you consider this if you intend to own or use these types of systems out of warranty or plan significant upgrades during the life of the system. Where possible, I always insist on systems that use industry-standard power supplies, such as the ATX12V form factor supply found in most systems today.
With backward compatibility ensuring that the new 24-pin ATX power connector will plug into older 20-pin motherboard sockets, when purchasing a new power supply, I now recommend only those units that include 24-pin main power connectors, which are usually sold as ATX12V 2.x, EPS12V, or “PCI Express” models. For the most flexible and future-proof supply, also ensure that the power supply includes two or more PCI Express graphics connectors as well as multiple integrated SATA drive power connectors. Choosing a power supply with these features provides flexibility that allows it to work not only in newer systems, but also in virtually all older ATX systems—and with no adapters required.
As a guide, here are some of the features I recommend looking for in a PSU:
Adequate power connectors (24-pin main, 4/8-pin +12 V CPU, 6/8-pin PCIe Graphics, SATA, and so on) for the intended system
Adequate power output (watts) for the intended system
80 PLUS certification
Active Power Factor Correction (required with 80 PLUS)
SLI and/or Crossfire certification
Single +12 V rail design
There are other variables to consider, depending on your specific needs or desires. One feature that many people like is modular cables, which minimize the clutter in a system. Another feature to consider is noise, which is mostly related to cooling. The type and arrangement of cooling fans has a great effect on how quiet (or noisy) the unit will be. There are even some fanless units that are completely silent, but these usually come at a premium price and with a lower overall power output capability.
When building systems with case windows, some people also like to look for PSUs with appearance-related features like colored cases.
Modular Cables
One feature often discussed in relation to PSUs is the use of modular cables. This means cables with connectors at both ends that are detachable from the power supply. Modular cables allow you to attach only the cables you need—in some cases greatly reducing the congestion inside the system.
The main argument against modular cables is that additional resistance is introduced via another set of connector contacts. This is true, but how much resistance exactly, and is it enough that it really matters? Fortunately, this can easily be calculated.
The connectors used in modern power supplies are mostly Molex Mini-Fit Jr. types, which have a contact resistance of 10 milli-ohms (0.01 ohms). Most power supply cables use 18 AWG (American Wire Gauge) copper wire, which has a resistance of about 0.0064 ohms per foot. This means that adding an extra connector at the PSU end is equal to about 1.5 feet of wire in additional resistance.
To put it another way, in a maximum load situation, each terminal normally carries a maximum of about four amps, at which point the additional resistance equals about 0.16 watts of power loss. In an eight-pin power connector, this only adds up to around a watt, a loss I consider negligible.
Finally, when you consider that a typical PSU cable already consists of 1.5 feet of wire with a connector on the end, adding another connector to make the cable modular only adds about one-third more overall resistance to what is already there, which was negligible to begin with.
If modular cables aren’t much of a problem technically, why don’t more PSU manufacturers include them? Well, besides the (negligible in my opinion) extra resistance, they do add to the cost of making a power supply, and that is reflected in a higher final price. They can also create clearance issues with other components in the system, depending on exactly where the connectors attach to the PSU. In addition, modular cables can easily become lost or misplaced. Think of opening a system to add another internal drive or upgraded video card several years after it was initially built, finding that the PSU uses modular cables, and discovering the extra cables needed are nowhere to be found. One solution to this problem is to place any unused cables inside the case when building a system. For example, you could place them in a small plastic bag and tape them inside, so that if or when you need them in the future, they are easy to find. Besides these issues, perhaps the biggest drawback to modular cables is that modular PSU cables using standard connectors are patented (www.google.com/
patents/about?id=w0ehAAAAEBAJ), and the patent is owned by Systemax (aka TigerDirect and Ultra Products). There is another patent on modular PSUs that use nonstandard connectors at the PSU end (www.google.com/patents/about?id=iOGqAAAAEBAJ). If there was no legal “baggage” against using them, I suspect we would see more modular cable equipped PSUs on the market today.
Power-protection systems do just what the name implies: They protect your equipment from the effects of power surges and power failures. In particular, power surges and spikes can damage computer equipment, and a loss of power can result in lost data. In this section, you learn about the four primary types of power-protection devices available and when you should use them.
Before considering any further levels of power protection, you should know that a quality power supply already affords you a substantial amount of protection. High-end power supplies from the vendors I recommend are designed to provide protection from higher-than-normal voltages and currents, and they provide a limited amount of power-line noise filtering. Some of the inexpensive aftermarket power supplies probably do not have this sort of protection. If you have an inexpensive computer, further protecting your system might be wise.
Caution: All the power-protection features in this chapter and the protection features in the power supply inside your computer require that the computer’s AC power cable be connected to a ground.
Many older homes do not have three-prong (grounded) outlets to accommodate grounded devices.
Do not use a three-pronged adapter (that bypasses the three-prong requirement and enables you to connect to a two-prong socket) to plug a surge suppressor, computer, or UPS into a two-pronged outlet. They often don’t provide a good ground and can inhibit the capabilities of your power-protection devices.
You also should test your power sockets to ensure they are grounded. Sometimes outlets, despite having three-prong sockets, are not connected to a ground wire; an inexpensive socket tester (available at most hardware stores) can detect this condition.
Of course, the easiest form of protection is to turn off and unplug your computer equipment (including your modem) when a thunderstorm is imminent. However, when this is not possible, other alternatives are available.
Power supplies should stay within operating specifications and continue to run a system even if any of these power line disturbances occur:
Voltage drop to 80 V for up to 2 seconds
Voltage drop to 70 V for up to .5 seconds
Voltage surge of up to 143 V for up to 1 second
Most high-quality power supplies (or the attached systems) will not be damaged by the following occurrences:
Full power outage
Any voltage drop (brownout)
A spike of up to 2500 V
To verify the levels of protection built into the existing power supply in a computer system, an independent laboratory subjected several unprotected PC systems to various spikes and surges of up to 6000 V—considered the maximum level of surge that can be transmitted to a system through an electrical outlet. Any higher voltage would cause the power to arc to the ground within the outlet. None of the systems sustained permanent damage in these tests. The worst thing that happened was that some of the systems rebooted or shut down when the surge was more than 2000 V. Each system restarted when the power switch was toggled after a shutdown.
The automatic shutdown of a computer during power disturbances is a built-in function of most high-quality power supplies. You can reset the power supply by flipping the power switch from on to off and back on again. Some power supplies even have an auto-restart function. This type of power supply acts the same as others in a massive surge or spike situation: It shuts down the system. The difference is that after normal power resumes, the power supply waits for a specified delay of 3–6 seconds and then resets itself and powers the system back up. Because no manual switch resetting is required, this feature might be desirable in systems functioning as network servers or in those found in other unattended locations.
The first time I witnessed a large surge that caused an immediate shutdown of all my systems, I was extremely surprised. All the systems were silent, but the monitor and modem lights were still on. My first thought was that everything was blown, but a simple toggle of each system-unit power switch caused the power supplies to reset, and the units powered up with no problem. Since that first time, this type of shutdown has happened to me several times, always without further problems.
The following types of power-protection devices are explained in the sections that follow:
- Surge suppressors
- Phone-line surge protectors
- Line conditioners
- Standby power supplies (SPS)
- Uninterruptible power supplies (UPS)
Surge Suppressors (Protectors)
The simplest form of power protection is any one of the commercially available surge protectors—that is, devices inserted between the system and the power line. These devices, which cost between $20 and $200, can absorb the high-voltage transients produced by nearby lightning strikes and power equipment. Some surge protectors can be effective for certain types of power problems, but they offer only limited protection.
Surge protectors use several devices, usually metal-oxide varistors (MOVs), that can clamp and shunt away all voltages above a certain level. MOVs are designed to accept voltages as high as 6000 V and divert any power above 200 V to ground. MOVs can handle normal surges, but powerful surges such as direct lightning strikes can blow right through them. MOVs are not designed to handle a high level of power and self-destruct while shunting a large surge. These devices therefore cease to function after either a single large surge or a series of smaller ones. The real problem is that you can’t easily tell when they no longer are functional. The only way to test them is to subject the MOVs to a surge, which destroys them. Therefore, you never really know whether your so-called surge protector is protecting your system.
Some surge protectors have status lights that let you know when a surge large enough to blow the MOVs has occurred. A surge suppressor without this status indicator light is useless because you never know when it has stopped protecting.
Underwriters Laboratories has produced an excellent standard that governs surge suppressors, called UL 1449. Any surge suppressor that meets this standard is a good one and definitely offers a line of protection beyond what the power supply in your PC already offers. The only types of surge suppressors worth buying, therefore, should have two features:
Conformance to the UL 1449 standard
A status light indicating when the MOVs are blown
Units that meet the UL 1449 specification say so on the packaging or directly on the unit. If this standard is not mentioned, it does not conform. Therefore, you should avoid it.
Another good feature to have in a surge suppressor is a built-in circuit breaker that can be manually reset rather than a fuse. The breaker protects your system if it or a peripheral develops a short.
Network and Phone Line Surge Protectors
A far bigger problem than powerline surges are surges through network and/or phone cabling. I’ve personally experienced surges resulting from nearby lightning strikes damage multiple computers and other equipment via ethernet and telephone lines, while virtually nothing was damaged through the power lines. In systems with separate network cards the damage was often limited to just the card, while in systems with the network interface built-in to the motherboard, the motherboard itself was damaged. In many areas, the cable and phone lines are above ground, making them especially susceptible to lightning strikes.
Several companies manufacture or sell simple surge protectors that plug in between your modem and the network or phone lines. These inexpensive devices can be purchased from most electronics supply houses. Some of the standard power line surge protectors include connectors for network and/or phone line protection as well.
Line Conditioners
In addition to high-voltage and current conditions, other problems can occur with incoming power. The voltage might dip below the level needed to run the system, resulting in a brownout. Forms of electrical noise other than simple voltage surges or spikes might travel through the power line, such as radio-frequency interference or electrical noise caused by motors or other inductive loads.
Remember two things when you wire together digital devices (such as computers and their peripherals):
Any wire can act as an antenna and have voltage induced in it by nearby electromagnetic fields, which can come from other wires, telephones, CRTs, motors, fluorescent fixtures, static discharge, and, of course, radio transmitters.
Digital circuitry responds with surprising efficiency to noise of even a volt or two, making those induced voltages particularly troublesome. The electrical wiring in your building can act as an antenna, picking up all kinds of noise and disturbances.
A line conditioner can handle many of these types of problems. It filters the power, bridges brownouts, suppresses high-voltage and current conditions, and generally acts as a buffer between the power line and the system. A line conditioner does the job of a surge suppressor, and much more. It is more of an active device, functioning continuously, rather than a passive device that activates only when a surge is present. A line conditioner provides true power conditioning and can handle myriad problems. It contains transformers, capacitors, and other circuitry that can temporarily bridge a brownout or low-voltage situation. These units usually cost $100–$300, depending on the power-
handling capacity of the unit.
Backup Power
The next level of power protection includes backup power-protection devices. These units can provide power in case of a complete blackout, thereby providing the time necessary for an orderly system shutdown. Two types are available: the SPS and the uninterruptible power supply (UPS). The UPS is a special device because it does much more than just provide backup power; it is also the best kind of line conditioner you can buy.
Standby Power Supplies
A standby power supply is known as an offline device: It functions only when normal power is disrupted. An SPS system uses a special circuit that can sense the AC line current. If the sensor detects a loss of power on the line, the system quickly switches over to a standby battery and power inverter. The power inverter converts the battery power to 120 V AC power, which is then supplied to the system.
For a stand-by (switching) type UPS to work, the hold-up time of the power supply has to be longer than the switching time of the UPS. For example, I have a TrippLite SMART1000LCD, which is a stand-by type UPS with a 4ms switching time. Because this is well below the 16ms hold-up time called for in the official Power Supply Design Guide, it should switch well before any properly designed and functioning power supply allows the system to reset. Unfortunately if the power supply in a PC is poorly designed or overloaded, it may be disrupted by even a 4ms switch, causing the system to shut down or reset anyway, and all unsaved work to be lost.
A truly outstanding SPS adds to the circuit a ferroresonant transformer, which is a large transformer with the capability to store a small amount of power and deliver it during the switch time. This device functions as a buffer on the power line, giving the SPS almost uninterruptible capability.
Tip: Look for SPS systems with a switch-over time of less than 10 milliseconds (ms). This is shorter than the hold-up time of typical power supplies.
SPS units also might have internal line conditioning of their own. Under normal circumstances, most cheaper units place your system directly on the regular power line and offer no conditioning. The addition of a ferroresonant transformer to an SPS gives it extra regulation and protection capabilities because of the buffer effect of the transformer. SPS devices without the ferroresonant transformer still require the use of a line conditioner for full protection. SPS systems usually cost between a hundred and several thousand dollars, depending on the quality and power-output capacity.
UPSs
Perhaps the best overall solution to any power problem is to provide a power source that is conditioned and that can’t be interrupted—which is the definition of an uninterruptible power supply. UPSs are known as online systems because they continuously function and supply power to your computer systems. Because some companies advertise ferroresonant SPS devices as though they were UPS devices, many now use the term true UPS to describe a truly online system. A true UPS system is constructed in much the same way as an SPS system; however, because the computer is always operating from the battery, there is no switching circuit.
In a true UPS, your system always operates from the battery. A voltage inverter converts from +12 V DC to 120 V AC. You essentially have your own private power system that generates power independently of the AC line. A battery charger connected to the line or wall current keeps the battery charged at a rate equal to or greater than the rate at which power is consumed.
When the AC current supplying the battery charger fails, a true UPS continues functioning undisturbed because the battery-charging function is all that is lost. Because the computer was already running off the battery, no switch takes place and no power disruption is possible. The battery begins discharging at a rate dictated by the amount of load your system places on the unit, which (based on the size of the battery) gives you plenty of time to execute an orderly system shutdown. Based on an appropriately scaled storage battery, the UPS functions continuously, generating power and preventing unpleasant surprises. When the line power returns, the battery charger begins recharging the battery, again with no interruption.
Note: Occasionally, a UPS can accumulate too much storage and not enough discharge. When this occurs, the UPS emits a loud alarm, alerting you that it’s full. Simply unplugging the unit from the AC power source for a while can discharge the excess storage (as it powers your computer) and drain the UPS of the excess.
Many SPS systems are advertised as though they are true UPS systems. You can tell a Standby Power Supply from a true UPS by the unit’s switch time. If a specification for switch time exists, the unit can’t be a true UPS because UPS units never switch. However, true UPS systems are very expensive, and a good SPS with a ferroresonant transformer can virtually equal the performance of a true UPS at a much lower cost.
Note: Many UPSs and SPSs today come equipped with a cable and software that enables the protected computer to shut down in an orderly manner on receipt of a signal from the UPS. This way, the system can shut down properly even if the computer is unattended. Some OSs designed for server environments contain their own UPS software components.
UPS cost is a direct function of both the length of time it can continue to provide power after a line current failure and how much power it can provide. You therefore should purchase a UPS that provides enough power to run your system and peripherals and enough time to close files and provide an orderly shutdown. Remember, however, to manually perform a system shutdown procedure during a power outage. You will probably need your monitor plugged into the UPS and the computer. Be sure the UPS you purchase can provide sufficient power for all the devices you must connect to it.
Because of a true UPS’s almost total isolation from the line current, it is unmatched as a line conditioner and surge suppressor. The best UPS systems add a ferroresonant transformer for even greater power conditioning and protection capability. This type of UPS is the best form of power protection available. The price, however, can be high. To find out just how much power your computer system requires, look at the UL sticker on the back of the unit. This sticker lists the maximum power draw in watts, or sometimes in just volts and amperes. If only voltage and amperage are listed, multiply the two figures to calculate the wattage.
As an example, if the documentation for a system indicates that the computer can require as much as 120 V at a maximum current draw of five amps, the maximum power the system can draw is about 550 watts. The system should never draw any more power than that; if it does, a five-amp fuse in the power supply will blow. This type of system usually draws an average of 200 to 300 watts. However, to be safe when you make calculations for UPS capacity, be conservative; use the 550-watt figure. Adding an LCD monitor that draws 50 watts brings the total to 600 watts or more. Therefore, to run two fully loaded systems (including monitors), you’d need a 1200-watt UPS. A UPS of that capacity or greater normally costs several hundred dollars. Unfortunately, that is what the best level of protection costs. Most companies can justify this type of expense only for critical-use PCs, such as network servers.
Note: The highest-capacity UPS sold for use with a conventional 15-amp outlet is about 1400 watts. If it’s any higher, you risk tripping a 15-amp circuit when the battery is charging heavily and the inverter is drawing maximum current.
In addition to the total available output power (wattage), several other factors can distinguish one UPS from another. The addition of a ferroresonant transformer improves a unit’s power conditioning and buffering capabilities. Good units also have an inverter that produces a true sine wave output; the cheaper ones might generate a square wave. A square wave is an approximation of a sine wave with abrupt up-and-down voltage transitions. The abrupt transitions of a square wave are not compatible with some computer equipment power supplies. Be sure that the UPS you purchase produces power that is compatible with your computer equipment. Every unit has a specification for how long it can sustain output at the rated level. If your systems draw less than the rated level, you have some additional time.
Caution: Be careful! Most UPS systems are not designed for you to sit and compute for hours through an electrical blackout. They are designed to provide power only to essential components and to remain operating long enough to allow for an orderly shutdown. You pay a large amount for units that provide power for more than 15 minutes or so. At some point, it becomes more cost-effective to buy a generator than to keep investing in extended life for a UPS.
Some of the many sources of power protection equipment include American Power Conversion (APC) and Tripp Lite. These companies sell a variety of UPS, SPS, line protector, and surge protector products.
Caution: Don’t connect a laser printer to a backed-up socket in any SPS or UPS unit. Such printers are electrically noisy and have widely varying current draws. This can be hard on the inverter in an SPS or a UPS and frequently cause the inverter to fail or detect an overload and shut down. Either case means that your system will lose power, too.
Printers are normally noncritical because whatever is being printed can be reprinted. Don’t connect them to a UPS unless there’s a good business need to do so.
Some UPSs and SPSs have sockets that are conditioned but not backed up—that is, they do not draw power from the battery. In cases such as this, you can safely plug printers and other peripherals into these sockets.
Most PCs have a special type of chip in them that combines a real-time clock (RTC) with at least 64 bytes (including 14 bytes of clock data) of nonvolatile RAM (NVRAM) memory. This chip is officially called the RTC/NVRAM chip, but it is often referred to as the CMOS or CMOS RAM chip because the type of chip used is produced using a CMOS Complementary Metal-Oxide Semiconductor (CMOS) process. CMOS design chips are known for low power consumption. This special RTC/NVRAM chip is designed to run off a battery for several years.
The original chip of this type was the Motorola MC146818, which was used in the IBM AT dating from August 1984. Although the chips used today have different manufacturers and part numbers, they all are designed to be compatible with this original Motorola part. Most modern motherboards have the RTC/NVRAM integrated in the motherboard chipset South Bridge or I/O Controller Hub (ICH) component, meaning no separate chip is required.
The clock enables software to read the date and time and preserves the date and time data even when the system is powered off or unplugged. The NVRAM portion of the chip has another function: It is designed to store the basic system configuration, including the amount of memory installed, types of disk drives installed, PnP device configuration, power-on passwords, and other information. Although some chips have been used that store up to 4KB or more of NVRAM, most motherboard chipsets with integrated RTC/NVRAM incorporate 256 bytes of NVRAM, of which the clock uses 14bytes. The system reads this information every time you power it on.
Modern CMOS Batteries
Motherboard NVRAM (CMOS RAM) batteries come in many forms. Most are of a lithium design because they last 2–5 years or more. I have seen systems with conventional alkaline batteries mounted in a holder; these are much less desirable because they fail more frequently and do not last as long. Also, they are prone to leak, and if a battery leaks on the motherboard, the motherboard can be severely damaged. By far, the most commonly used battery for motherboards today is the coin cell, mounted in a holder that is part of the motherboard. Two main types of coin cells are used, differing in their chemistry. Most use a manganese dioxide (Mn02) cathode, designated by a CR prefix in the part number; others use a carbon monoflouride (CF) cathode, designated by a BR prefix in the part number. The CR types are more plentiful (and thus easier to get) and offer slightly higher capacity. The BR types are useful for higher-temperature operation (above 60°C or 140°F).
Because the CR series is cheaper and easier to obtain, it is generally what you will find in a PC. The other digits in the battery part number indicate the physical size of the battery. For example, the most common type of lithium coin cell used in PCs is the CR2032, which is 20 mm in diameter (about the size of a quarter) and 3.2 mm thick and uses a manganese dioxide cathode. These are readily available at electronics supply stores, camera shops, and even drugstores. The following figure shows a cutaway view of a CR2032 lithium coin cell battery.
Estimated battery life can be calculated by dividing the battery capacity by the average current required. For example, a typical CR2032 battery is rated 220 mAh (milliamp hours), and the RTC/NVRAM circuit in most current motherboard chipsets draws 5 µA (microamps) with the power off. Battery life can therefore be calculated as follows:
220 000 µAh ÷ 5 µA = 44 000 hours = 5 years
If a thinner (and lower-capacity) battery such as the CR2025 is used, battery life will be shorter:
165 000 µAh ÷ 5 µA = 33 000 hours = 3.7 years
Battery life starts when the system is first assembled, which can be several months or more before you purchase the system, even if it is new. Also, the battery might be partially discharged before it is installed in the system; higher temperatures both in storage and in the system can contribute to shorter battery life. All these reasons and more can cause battery life to be less than what might be indicated by calculation.
As the battery drains, output voltage drops somewhat. Lower battery voltage can impair the accuracy of the RTC. Most lithium coin cell batteries are rated at 3 V; however, actual readings on a new battery are usually higher. If your system clock seems inaccurate (it runs slow, for example), check the voltage on the CMOS battery. The highest accuracy is obtained if the battery voltage is maintained at 3.0 V or higher. Lithium batteries normally maintain a fairly steady voltage until they are nearly fully discharged, whereupon the voltage quickly drops. If you check the battery voltage and find it is below 3.0 V, consider replacing the battery, even if it is before the intended replacement time.
Obsolete or Unique CMOS Batteries
Although most modern systems use 3.0 V coin cells, older systems have used a variety of battery types and voltages over the years. For example, some older systems have used 3.6 V, 4.5 V, and 6 V types as well. If you are replacing the battery in an older machine, be sure your replacement is the same voltage as the one you removed from the system. Some motherboards can use batteries of several voltages, and you use a jumper or switch to select the various settings. If you suspect your motherboard has this capability, consult the documentation for instructions on changing the settings. Of course, the easiest thing to do is to replace the existing battery with another of the same type.
Some systems over the years have used a special type of chip that actually has the battery embedded within it. These are made by several companies, including Dallas Semiconductor and Benchmarq. These chips are notable for their long lives. Under normal conditions, the integral battery lasts for 10 years—which is, of course, longer than the useful life of the system. If your system uses one of the Dallas or Benchmarq modules, the battery and chip must be replaced as a unit because they are integrated. Most of the time, these chip/battery combinations are installed in a socket on the motherboard just in case a problem requires an early replacement. You can get new modules directly from the manufacturers for $18 or less, which is much more expensive than the coin-type lithium battery found in most modern systems. In fact, due to their expense and the fact that most motherboard chipset manufacturers have integrated the RTC/NVRAM functionality into the motherboard chipset, few if any modern PCs use these chip/battery modules.
Some systems do not use a battery. Hewlett-Packard, for example, includes a special capacitor in some of its systems that is automatically recharged anytime the system is plugged in. The system does not have to be running for the capacitor to charge; it only has to be plugged in. If the system is unplugged, the capacitor powers the RTC/NVRAM chip for up to a week or more. If the system remains unplugged for longer than that, the NVRAM information is lost. In that case, these systems can reload the NVRAM from a backup kept in a special flash ROM chip contained on the motherboard. The only pieces of information that are actually missing when you repower the system are the date and time, which you have to re-enter. By using the capacitor combined with an NVRAM backup in flash ROM, these systems have a reliable solution that lasts indefinitely.
Many older systems use a separate battery that plugs in via a cable or that can even be directly soldered into the motherboard (mostly older, obsolete systems). For those older systems with the battery soldered in, a spare battery connector usually exists on the motherboard where you can insert a conventional plug-in battery if the original ever fails.
CMOS Battery Troubleshooting
Symptoms that indicate that the battery is about to fail include having to reset the clock on your PC every time you shut down the system (especially after moving it) and problems during the system’s POST, such as drive-detection difficulties. If you experience problems such as these, you should make note of your system’s CMOS settings and replace the battery as soon as possible.
Caution: When you replace a PC battery, be sure you get the polarity correct; otherwise, you will damage the RTC/NVRAM (CMOS) chip, which is normally integrated into the motherboard chipset. Because the chip is soldered onto most motherboards, this can be an expensive mistake! The coin cell battery holder on the motherboard is normally designed so that the positive of the battery should be facing up. Older motherboards may use a plug-in battery, the connections for which are normally keyed.
When you replace a battery, in most cases the existing data stored in the NVRAM is lost. Sometimes, however, the data remains intact for several minutes (I have observed NVRAM retain information with no power for an hour or more), so if you make the battery swap quickly, the information in the NVRAM might be retained. Just to be sure, I recommend that you record all the system configuration settings stored in the NVRAM by your system Setup program. In most cases, you should run the BIOS Setup program and copy or print out all the screens showing the various settings. Some Setup programs offer the capability to save the NVRAM data to a file for later restoration if necessary.
Tip: If your system BIOS is password-protected and you forget the password, one possible way to bypass the block is to remove the battery for a few minutes and then replace it. This resets the BIOS to its default settings, removing the password protection.
After replacing a battery, power up the system and use the Setup program to check the date and time setting and any other data that was stored in the NVRAM.
0 comments: