Technical The Internal Components The Power Supply Part 4

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The Power Supply - part 4

Obtaining Replacement Units

There may be times when it is simply easier, safer, or less expensive (considering time and materials) to replace the power supply rather than repair it. As mentioned earlier, replacement power supplies are available from many manufacturers. Before you can shop for a supplier, however, you should consider other purchasing factors.

Deciding on a Power Supply

When looking at getting a new power supply, you should take several things into account. First, consider the power supply's shape, or form factor. For example, the power supply used in the IBM AT differs physically from the one used in the PC or XT. Therefore, AT and PC/XT supplies are not interchangeable.

Differences exist in the size, shape, screw-hole positions, connector type, number of connectors, and switch position in these and other power supplies. Systems that use the same form factor supply can easily interchange. The compatible manufacturers realized this and most began designing systems that mimicked the shape of IBM's AT with regard to motherboard and power supply configuration and mounting. As the clone market evolved, four standard form factors for power supplies became popular: AT/Tower, Baby- AT, Slimline, and PC/XT. You can easily interchange any supply with another one of the same form factor. Page one gave complete descriptions of these form factors. When ordering a replacement supply, you need to know which form factor your system requires.

Many systems use proprietary-designed power supplies, which makes replacement difficult. IBM uses a number of designs for the PS/2 systems, and little interchangeability exists between different systems. Some of the supplies do interchange, especially between any that have the same or similar cases, such as the Model 60, 65, and 80. Several different output level power supplies are available for these systems, including 207-, 225-, 242-, and 250-watt versions. The most powerful 250-watt unit was supplied originally for the Model 65 SX and later version Model 80 systems, although it fits perfectly in any Model 60, 65, or 80 system.

One risk with some of the non-standard compatibles is that they might not use one of the industry-standard form factor supplies. If a system uses one of the common form factor power supplies, replacement units 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 pay through the nose for the unit. Although you can find slim-style units for as little as £30, the proprietary units from some manufacturers run as high as £100 or more. PC buyers often overlook this and discover too late the consequences of having nonstandard components in a system.

An example of IBM-compatible systems with proprietary power supply designs are those from Compaq. None of its systems use the same form factor supply as the IBM systems, which means that Compaq usually is the only place from which you can get a replacement. If the power supply in your Compaq Deskpro system "goes south," you can expect to pay £100 + for a replacement, and the replacement unit will be no better or quieter than the one you are replacing. You have little choice in the matter because almost no one offers Compaq form factor power supplies except Compaq. One exception is that PC Power and Cooling offers excellent replacement power supplies for the earlier Compaq Portable systems and for the Deskpro series. These replacement power supplies have higher-output power levels than the original supplies from Compaq and cost much less.

Sources for Replacement Power Supplies

Because one of the most failure-prone items in PC systems is the power supply, I am often called on to recommend a replacement. Literally hundreds of companies manufacture PC power supplies, and I certainly have not tested them all. I can, however, recommend some companies whose products I have come to know and trust.

Although other high-quality manufacturers are out there, at this time I recommend power supplies from either Astec Standard Power or PC Power and Cooling.

Astec makes the power supplies used in most of the high-end systems by IBM, Hewlett-Packard, Apple, and many other name brand systems. They have power supplies available in a number of standard form factors (AT/Tower, Baby-AT, and Slimline) and a variety of output levels. They have power supplies with ratings of up to 450 watts and power supplies especially designed for "green" PCs that meet the EPA Energy Star requirements for low power consumption. Their "green" power supplies are specifically designed to achieve high efficiency at low load conditions. Be aware that high output supplies from other manufacturers may have problems with very low loads. Astec also makes a number of power supplies for laptop and notebook PC systems and has numerous non-PC type supplies.

PC Power and Cooling has the most complete line of power supplies for PC systems. They make supplies in all the standard PC form factors used today (AT/Tower, Baby-AT, PC/XT, and Slimline). Versions are available in a variety of different quality and output levels, from inexpensive replacements to very high-quality high-output models with ratings up to 450 watts. They even have versions with built-in battery backup systems and a series of special models with high-volume low-speed (quiet) fan assemblies. Their quiet models are especially welcome to people who cannot take the fan noise that some power supplies emanate.

PC Power and Cooling also has units available to fit some of Compaq's proprietary designs. This can be a real boon if you have to service or repair Compaq systems because the PC Power and Cooling units are available in higher output ratings than Compaq's own. They also cost much less than Compaq and bolt in as a direct replacement.

The support offered by PC Power and Cooling is excellent also, and they have been in business a long time, which is sometimes rare in this industry. Besides just power supplies, they have an excellent line of cases as well.

A high-quality power supply from either of these vendors is one of the best cures for intermittent system problems and goes a long way toward ensuring trouble-free operation in the future.

Using Power-Protection Systems

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 under what circumstances you should use them.

Before considering any further levels of power protection, you should know that the power supply in your system (if your system is well-made) 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 provide a limited amount of power-line noise filtering. Some of the inexpensive aftermarket power supplies probably do not have this sort of protection; be careful if you have an inexpensive clone system. In those cases, further protecting your system might be wise.

CAUTION: All of the power protection features in this chapter and the protection features in the power supply inside your computer expect and require that a ground be connected.

Many older homes do not have three-prong (grounded) outlets to accommodate grounded devices.

Do not use a three-prong adapter with a two-prong outlet to plug in a surge suppresser to a computer or UPS. They don't necessarily provide a good ground and can hamper the capability for the power protection devices to protect your system.

Power supplies should stay within operating specifications and continue to run a system if any of these power line disturbances occur:

• Voltage drop to 180v for up to 2 seconds
  
  
• Voltage drop to 170v for up to .5 seconds
  
  
• Voltage surge of up to 273v for up to 1 second

IBM also states that neither their power supplies nor systems will be damaged by the following occurrences:

• Full power outage
  
  
• Any voltage drop (brownout)
  
  
• A spike of up to 2,500v

For example, because of the high-quality power supply design that IBM uses, they state in their documentation that external surge suppressers are not needed for PS/2 systems. Most other high-quality name brand manufacturers also use high-quality power supply designs. Companies like Astec, PC Power and Cooling, and others make very high-quality units.

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 6,000v, considered the maximum level of surge that can be transmitted to a system by an electrical outlet. Any higher voltage would cause the power to arc to ground within the outlet itself. Note that 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 if the surge was more than 2,000v. Each system restarted when the power switch was toggled after a shutdown.

I do not use any real form of power protection on my systems, and they have survived near-direct lightning strikes and powerful surges. The most recent incident, only 50 feet from my office, was a direct lightning strike to a brick chimney that blew the top of the chimney apart. None of my systems (which were running at the time) were damaged in any way from this incident; they just shut themselves down. I was able to restart each system by toggling the power switches. An alarm system located in the same office, however, was destroyed by this strike. I am not saying that lightning strikes or even much milder spikes and surges cannot damage computer systems. Another nearby lightning strike did destroy a modem and serial adapter installed in one of my systems. I was just lucky that the destruction did not include the motherboard.

This discussion points out an important oversight in some power-protection strategies: You may elect to protect your systems from electrical power disturbances, but do not forget to provide similar protection also from spikes and surges on the phone line.

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, such as those in most of the PS/2 systems, 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 three to six seconds and then resets itself and powers the system back up. Because no manual switch resetting is required, this feature is desirable in systems functioning as a network file server or in a system in a remote location.

The first time I witnessed a large surge cause 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 suppressers
  
  
• Standby power supplies (SPS)
  
  
• Line conditioners
  
  
• Uninterruptible power supplies (UPS)

Surge Suppressers (Protectors)

The simplest form of power protection is any 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 very 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 6,000v and divert any power above 200v to ground. MOVs can handle normal surges, but powerful surges such as a direct lightning strike can blow right through them. MOVs are not designed to handle a very 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 cannot 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 if 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 suppresser 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 suppressers, called UL 1449. Any surge suppresser that meets this standard is a very good one, and definitely offers an additional line of protection beyond what the power supply in your PC already does. The only types of surge suppressers 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, and you should avoid it.

Another good feature to have in a surge suppresser is a built-in circuit breaker that can be reset rather than a fuse. The breaker protects your system if it or a peripheral develops a short. These better surge suppressers usually cost about £40.

Phone Line Surge Protectors

In addition to protecting the power lines, it is critical to provide protection to your systems from any phone lines that are connected. If you are using a modem or fax board that is plugged into the phone system, any surges or spikes that travel the phone line can potentially damage your system. In many areas, the phone lines are especially susceptible to lightning strikes, which is the largest cause of fried modems and any computer equipment attached to them.

Several companies manufacture or sell simple surge protectors that plug between your modem and the phone line. These inexpensive devices can be purchased from most electronics supply houses. Most of the cable and communication products vendors sell these phone line surge protectors.

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 and result in a brownout. Other forms of electrical noise other than simple voltage surges or spikes might be on 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 will 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 also 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 and pick up all kinds of noise and disturbances.

A line conditioner can handle many of these types of problems. A line conditioner is designed to remedy a variety 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 suppresser, 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 a myriad of problems. It contains transformers, capacitors, and other circuitry that temporarily can bridge a brownout or low-voltage situation. These units usually cost a few hundred pounds, 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, which provides the time needed for an orderly system shutdown. Two types are available: the standby power supply (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 (SPS)

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 240-volt AC power, which then is supplied to the system.

SPS systems do work, but sometimes a problem occurs with the switch to battery power. If the switch is not fast enough, the computer system unit shuts down or reboots anyway, which defeats the purpose of having the backup power supply. A truly outstanding SPS adds to the circuit a ferroresonant transformer, a large transformer with the capability to store a small amount of power and deliver it during the switch time. Having this device is similar to having on the power line a buffer that you add to an SPS to give it almost truly uninterruptible capability.

SPS units also may or may not have internal line conditioning of their own; most cheaper units place your system directly on the regular power line under normal circumstances and offer no conditioning. The addition of a ferroresonant transformer to an SPS gives it additional regulation and protection capabilities due to 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 from £200 to several thousands of pounds, depending on the quality and power-output capacity.

Uninterruptible Power Supplies (UPS)

Perhaps the best overall solution to any power problem is to provide a power source that is both conditioned and that also cannot be interrupted, which describes 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 much the same as an SPS system; however, because you always are operating from the battery, there is no switching circuit.

In a true UPS, your system always operates from the battery, with a voltage inverter to convert from 12v DC to 240v 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 power is disconnected, the true UPS continues functioning undisturbed because the battery-charging function is all that is lost. Because you already were running off the battery, no switch takes place, and no power disruption is possible. The battery then 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.

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; therefore, purchasing a UPS that gives you enough power to run your system and peripherals as well as enough time to close files and provide an orderly shutdown would be sufficient. In most PC applications, this solution is the most cost-effective because the batteries and charger portion of the system must be much larger than the SPS type of device, and will be more costly.

Many SPS systems are advertised as though they were true UPS systems. The giveaway is the unit's switch time. If a specification for switch time exists, the unit cannot be a true UPS because UPS units never switch. Understand, however, that a good SPS with a ferroresonant transformer can virtually equal the performance of a true UPS at a lower cost.

Because of a UPS's almost total isolation from the line current, it is unmatched as a line conditioner and surge suppresser. 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 very high. A true UPS costs from £1 to £2 per watt of power supplied. To find out just how much power your 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 a wattage figure.

As an example, the back of an IBM PC AT Model 339 indicates that the system can require as much as 220v at a maximum current draw of 5 amps. The maximum power this AT can draw is about 550 watts. This wattage is for a system with every slot full, two hard disks, and one floppy, in other words, the maximum possible level of expansion. The system should never draw any more power than that; if it does, a 5-ampere fuse in the power supply blows. This type of system normally draws an average 300 watts; to be safe when you make calculations for UPS capacity, however, be conservative and use the 550-watt figure. Adding a monitor that draws 100 watts brings the total to 650 watts or more. To run two fully loaded AT systems, you need an 1100-watt UPS. Don't forget two monitors, each drawing 100 watts; the total, therefore, is 1,300 watts. Using the £1 to £2 per watt figure, a UPS of at least that capacity or greater will cost from £1,300 to £2,600 - expensive, but unfortunately what the best level of protection costs. Most companies can justify this type of expense for only a critical-use PC, such as a network file server.

NOTE: 1,400 watts is about the highest size UPS that will be sold for a conventional 15-amp outlet. Any higher, and 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 differentiate one UPS from another. The addition of a ferroresonant transformer improves a unit's power conditioning and buffering capabilities. Good units have also an inverter that produces a true sine wave output; the cheaper ones may 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 signal are not compatible with some computer equipment power supplies. Be sure that the UPS you purchase produces a signal 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, though: Most UPS systems are not designed for you to sit and compute for hours through an electrical blackout. They are designed to provide power to whatever is needed, 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.

There are many sources of power protection equipment, but several include APC, Best Power, Tripp Lite, Liebert, and others. These companies sell a variety of UPS, SPS, line, and surge protectors.

CAUTION: Don't connect a laser printer to any SPS or UPS unit. They are both "electrically noisy" and have widely varying current draws. This can be hard on the inverter in a SPS or UPS and frequently causes the inverter to fail, or detect an overload and shut down. Either case means that your system will lose power, too.

Printers are normally non-critical since whatever is being printed can be reprinted. Don't connect them to a UPS unless there's a good business need to do so.

RTC/NVRAM Batteries

All 16-bit and higher systems have a special type of chip in them that combines a Real- Time Clock (RTC) with at least 64 bytes (including the clock data) of Non-Volatile RAM (NVRAM) memory. This chip is officially called the RTC/NVRAM chip, but is often referred to as the CMOS chip or CMOS RAM, because the type of chip used is produced using a CMOS (Complimentary Metal Oxide Semiconductor) process. CMOS design chips are known for very low power consumption, and this special RTC/NVRAM chip is designed to run off of a battery for several years.

The original chip of this type used in the original IBM AT was the Motorola 146818 chip. Although the ones used today have different manufacturers and part numbers, they are all designed to be compatible with this original Motorola part.

These chips include a real-time clock, and the function there is obvious. The clock is used so that software can read the date and time, and so that the date and time will be preserved even though 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 floppy and hard disk drives, and other information as well. Some of the more modern motherboards use extended NVRAM chips with as much as 2K or more of space to hold this configuration information. This is especially true for the new breed of Plug and Play systems, where the configuration of not only the motherboard but also of adapter cards is stored. This information can then be read every time the system is powered on.

These chips are normally powered by some type of battery while the system is off to preserve the information in the NVRAM and to power the clock. Most often a lithium type battery is used, because they have a very long life, especially at the low power draw from the typical RTC/NVRAM chip.

Most of the higher-quality modern systems sold today have a new type of chip that has the battery embedded within it. These are made by several companies including Dallas Semiconductor and Benchmarq. These are notable for their long life. Under normal conditions, the battery within these chips will last 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 will be installed in a socket on the motherboard just in case there is a problem requiring an early replacement. You can get new modules for £8 or less direct from the manufacturers, which is often less than the cost of the older separate battery alone.

Some systems do not use a battery at all. Hewlett-Packard, for example includes a special capacitor in many of their systems that is automatically recharged any time the system is plugged in. Note that 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 will power the RTC/NVRAM chip for up to a week or more. If the system remains unplugged for a duration longer than that, the NVRAM information will be 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 information that will actually be missing when you re-power the system is the date and time, which will have to be re-entered. By using the capacitor combined with a NVRAM backup in Flash ROM, they have a very reliable system that will last indefinitely.

Many systems use only a conventional battery, which may be either directly soldered into the motherboard or plugged in via a battery connector. For those systems with the battery soldered in, should it ever fail, they will normally have a spare battery connector on the motherboard where a conventional plug in battery can be used. In most cases, you would never have to replace the motherboard battery, even if it were completely dead.

Conventional-type batteries come in many forms. The best are of a lithium design because they will last from two to five years or more. I have seen systems with conventional alkaline batteries mounted in a holder; these are much less desirable as they fail more frequently and do not last as long. Also, they can be prone to leak, and if a battery leaks on the motherboard, the motherboard may be severely damaged.

Besides the different battery types, there are several different voltages used. The batteries used in PCs are normally either 3v, 4.5v, or 6v. If you are replacing the battery, make sure that your replacement is the same voltage as the one you removed from the system. Some motherboards can use batteries of several different voltages, and will have a jumper or switch to select the different settings. If you suspect your motherboard has this capability, consult the documentation for instructions on how to change the settings. Of course, the easiest thing to do is to replace the existing battery with another of the same type, in which case the settings would not have to be changed.

CAUTION: When you replace a PC battery, be sure that you get the polarity correct, or you will damage the RTC/NVRAM (CMOS) chip. Because these are soldered into most motherboards, this will be an expensive mistake! The battery connector on the motherboard as well as the battery itself are normally keyed to prevent a backwards connection. The pinout of this connector should be listed in your system documentation.

When you replace a battery, in most cases the existing data stored in the NVRAM will be lost. Often the data will remain for several minutes (I have observed NVRAM retain information with no power for an hour or more), so if you make the swap quickly, the information in the NVRAM will be retained. Just to be sure, it is recommended that you record all the system configuration settings stored in the NVRAM by your system Setup program. In most cases, you would want to run the BIOS Setup program and print out all the screens showing the different settings. Some Setup programs offer the capability to save the NVRAM data to a file for later restoration if necessary. That would be a good idea if it is an option in your system.

After replacing a battery, power up the system and use the Setup program to check the date and time setting as well as any other data that was stored in the NVRAM.

Disks - The Basics

Other Co-processors