SMPS & UPS

SMPS & UPS

The power supply is a critical component in a PC, as it supplies electrical power to every component in the system. In our experiences, it is also one of the most failure-prone components in any computer system. Because of its importance to proper and reliable system operation, you should understand both the function and limitations of a power supply, as well as its potential problems and their solutions.

Power Supply Function and Operation

The basic function of the power supply is to convert the type of electrical power available at the wall socket to that which is usable by the computer circuitry. The power supply in a conventional desktop system is designed to convert the 120-volt, 60Hz; AC current into something the computer can use specifically, +5 and +12v DC current, and +3.3v as well on some systems. Usually, the digital electronic components and circuits in the system (motherboard, adapter cards, and disk drive logic boards) use the 3.3v or +5v power, and the motors (disk drive motors and any fans) use the +12v power. The power supply must ensure a good, steady supply of DC current so that the system can operate properly.

If you look at a specification sheet for a typical PC power supply, you see that the supply generates not only +5v and +12v, but also -5v and -12v. Because it would seem that the +5v and +12v signals power everything in the system (logic and motors), what are the negative voltages used for? The answer is not much! In fact, these additional negative voltages are not used at all in many modern systems, although they are still required for backward compatibility.

Although -5v and -12v are supplied to the motherboard via the power supply connectors, the motherboard itself uses only the +5v. The -5v signal is simply routed to the ISA bus on pin B5 and is not used in any way by the motherboard. It was originally used by the analog data separator circuits found in older floppy controllers, which is why it was supplied to the bus. Because modern controllers do not need the -5v, it is no longer used but is still required because it is part of the ISA Bus standard.

NOTE: Power supplies in systems with a Micro Channel Architecture (MCA) Bus do not have -5v. This power signal was never needed in these systems, as they always used a more modern floppy controller design.

Both the +12v and -12v signals also are not used by the motherboard logic, and instead are simply routed to pins B9 and B7 of the ISA bus (respectively). Any adapter card on the bus can use these voltages, but most notably serial port driver/receiver circuits use them. If the motherboard has serial ports built in, the +12v and -12v signals can sometimes be used for those ports.

NOTE: The load placed on these voltages by a serial port would be very small. For example, the PS/2 Dual Async adapter uses only 35mA of +12v and 35mA of -12v (0.035 amps each) to operate two ports.

Most newer serial port circuits no longer use 12v driver/receiver circuits, but instead now use circuits that run on only 5v or even 3.3v. If you have one of these modern design ports in your system, the -12v signal from your power supply is likely to be totally unused by anything in the system.

The main function of the +12v power is to run disk drive motors. Usually a large amount of current is available, especially in systems with a large number of drives, such as in a tower configuration. Besides disk drive motors, the +12v supply is used by any cooling fans in the system, which, of course, should always be running. A single cooling fan can draw between 100mA to 250mA (0.1 to 0.25 amps); however, most newer ones use the lower 100mA figure. Note that although most fans in desktop systems run on +12v, most portable systems use fans that run on +5v or even 3.3v instead.

In addition to supplying power to run the system, the power supply also ensures that the system does not run unless the power being supplied is sufficient to operate the system properly. In other words, the power supply actually prevents the computer from starting up or operating until all the correct power levels are present.

Each power supply completes internal checks and tests before allowing the system to start. The power supply sends to the motherboard a special signal, called Power_Good. If this signal is not present, the computer does not run. The effect of this setup is that when the AC voltage dips and the power supply becomes over-stressed or overheated, the Power_Good signal goes down and forces a system reset or complete shutdown. If your system has ever seemed dead when the power switch is on and the fan and hard disks are running, you know the effects of losing the Power_Good signal.

IBM originally used this conservative design with the view that if the power goes low or the supply is overheated or over-stressed, causing output power to fluctuate, the computer should not be allowed to operate. You even can use the Power_Good feature as a method of designing and implementing a reset switch for the PC. The Power_Good line is wired to the clock generator circuit (an 8284 or 82284 chip in the original PC/XT and AT systems), which controls the clock and reset lines to the microprocessor. When you ground the Power_Good line with a switch, the chip and related circuitry stop the processor by killing the clock signal and then reset the processor when the Power_Good signal appears after you release the switch. The result is a full hardware reset of the system. Instructions for installing such a switch in a system not already equipped can be found later in this chapter.

Newer systems with ATX or LPX form factor motherboards include a special signal called PS_ON which can be used to turn the power supply (and thus the system) off via software; this is sometimes called the soft-off feature. This is most evident in Windows 95 when you select the Shut down the Computer option. If the power supply soft-offs, Windows will automatically shut down the computer rather than display a message that it's safe to shut down the computer.

AT/Tower Style

The compatible market has come up with a couple of other variations on the AT themes that are popular today. Besides the standard AT/Desktop type power supply, we also have the AT/Tower configuration, which is basically a full-sized AT-style desktop system running on its side. The power supply and motherboard form factors are basically the same in the Tower system as in the Desktop. The tower configuration is not new; in fact, even IBM's original AT had a specially mounted logo that could be rotated when you ran the system on its side in the tower configuration. The type of power supply used in a tower system is identical to that used in a desktop system, except for the power switch location. Most AT/Desktop systems required that the power switch be located right on the power supply itself, while most AT/Tower systems use an external switch attached to the power supply through a short 4-wire cable. A full sized AT power supply with a remote switch is now called an AT/Tower form-factor supply

Baby-AT Style

Another type of AT-based form factor that has been developed is the so called Baby-AT, which is simply a shortened version of the full-sized AT system. The power supply in these systems is shortened on one dimension; however, it matches the AT design in all other respects. These Baby-AT style power supplies can be used in both Baby-AT chassis and the larger AT-style chassis; however, the full size AT/Tower power supply does not fit in the Baby-AT chassis.

ATX Style

The newest standard on the market today is the ATX form factor. This describes a new motherboard shape, as well as a new case and power supply form factor. The ATX supply is based on the slimline or low-profile design, but has several differences worth noting. One difference is that the fan is now mounted along the inner side of the supply, blowing air across the motherboard and drawing it in from the outside at the rear. This flow is the opposite of most standard supplies, which blow air out the back of the supply and also have the fan positioned at the back. The reverse flow cooling used in the ATX supply forces air over the hottest components of the board, such as the CPU, SIMMs, and expansion slots. This eliminates the need for the notoriously unreliable CPU fans that have unfortunately become common today.

Another benefit of the reverse flow cooling is that the system will remain cleaner and free from dust and dirt. The case is essentially pressurized, so air will push out of the cracks in the case, the opposite of what happens in non-ATX systems. For example, if you held a lit cigarette in front of your floppy drive on a normal system, the smoke would be inhaled through the front of the drive and contaminate the heads! On an ATX system with reverse flow cooling, the smoke would be blown away from the drive because the only air intake is the single fan vent on the power supply at the rear. Those who use systems that operate in extremely harsh environments could add a filter to the fan intake vent, which would ensure even further that all air entering the system is clean and dust free.

The ATX system format was designed by Intel in 1995, but became popular in the new Pentium Pro-based PCs in 1996. The ATX form factor takes care of several problems with the Baby-AT or Slimline form factors. Where the power supply is concerned, this covers two main problems. One problem is that the traditional PC power supply has two connectors that plug into the motherboard. The problem is that if you insert these connectors backwards or out of their normal sequence, you will fry the motherboard! Most responsible system manufacturers will have the motherboard and power supply connectors keyed so they cannot be installed backwards or out of sequence, but many of the cheaper system vendors do not feature this keying on the boards or supplies they use.

To solve the potential for disaster that awaits those who might plug in their power supply connectors incorrectly, the ATX form factor includes a new power plug for the motherboard. This new connector features 20 pins, and is a single-keyed connector. It is virtually impossible to plug it in backwards, and because there is only one connector instead of two nearly identical ones, it is impossible to plug them in out of sequence. The new connector also can optionally supply 3.3v, eliminating the need for voltage regulators on the motherboard to power the CPU and other 3.3v circuits. Although the 3.3v signals are labeled as optional in the ATX specification, they should be considered mandatory in any ATX form factor power supply you purchase. Many systems will require this in the future.

Besides the new 3.3v signals, there is one other set of signals that will be found on the ATX supply not normally seen on standard supplies. They are the Power_On and 5v_Standby signals, which are also called Soft Power. Power_On is a motherboard signal that can be used with operating systems like Windows 95 or Windows NT, which support the ability to power the system down with software. This will also allow the optional use of the keyboard to power the system back on, exactly like the Apple Macintosh systems. The 5v_Standby signal is always active, giving the motherboard a limited source of power even when off.

The other problem solved by the ATX form factor power supply is that of system cooling. Most of the high-end Pentium and Pentium Pro systems have active heat sinks on the processor, which means there is a small fan on the CPU designed to cool it. These small fans are notoriously unreliable; not to mention expensive when compared to standard passive heat sinks. In the ATX design, the CPU fan is eliminated, and the CPU is mounted in a socket right next to the ATX power supply, which has a reverse flow fan blowing onto the CPU.

You will find it easy to locate supplies that fit these industry-standard form factors. Several vendors who manufacture PC power supplies in all these form factors are listed later in this chapter. For proprietary units, you will likely have to go back to the manufacturer.

Power Supply Connectors

Some systems may have more or fewer drive connectors. For example, IBM's AT system power supplies have only three disk drive power connectors, although most of the currently available AT/Tower type power supplies have four drive connectors. If you are adding drives and need additional disk drive power connectors, "Y" splitter cables are available from many electronics supply houses (including Radio Shack) that can adapt a single power connector to serve two drives. As a precaution, make sure that your total power supply output is capable of supplying the additional power.

ATX Power Supply Connections

Signal	Pin	Pin	Signal
3.3v*	11	1	3.3v*
-12v	12	2	3.3v*
GND	13	3	GND
Pwr_On	14	4	5v
GND	15	5	GND
GND	16	6	5v
GND	17	7	GND
-5v	18	8	Power_Good
5v	19	9	5v_Standby
5v	20	10	12v

·        = Optional signal

NOTE: The ATX supply features several signals not seen before, such as the 3.3v, Power_On, and 5v_Standby signals. Because of this, it will be difficult to adapt a standard slimline or low profile form factor supply to work properly in an ATX system, although the shapes are virtually identical.

Although the PC/XT power supplies do not have any signal on pin P8-2, you can still use them on AT-type motherboards, or vice versa. The presence or absence of the +5v signal on that pin has little or no effect on system operation. If you are measuring voltages for testing purposes, anything within 10 percent is considered acceptable, although most manufacturers of high-quality power supplies specify a tighter 5 percent tolerance.

Desired Voltage	Loose Tolerance Min. (-10%)	Tight Tolerance Max. (+8%)	Min. (-5%)	Max. (+5%)
+/-5.0v	4.5v	5.4v	4.75	5.25
+/-12.0v	10.8v	12.9v	11.4	12.6

The Power_Good signal has tolerances different from the other signals, although it is nominally a +5v signal in most systems. The trigger point for Power_Good is about +2.5v, but most systems require the signal voltage to be within about 3v to 6v. A power supply should be replaced if the voltages are out of these ranges.

Power Switch Connectors.

The AT/Tower and Slimline power supplies use a remote power switch. This switch is mounted in the front of the system case and is connected to the power supply through a standard type of 4-wire cable. The ends of the cable are fitted with spade connector lugs, which plug into the spade connectors on the power switch itself. The switch is usually a part of the case, so the power supply comes with the cable and no switch. The cable from the power supply to the switch in the case contains four color-coded wires. There may also be a fifth wire supplying a ground connection to the case as well.

The four or five wires are color-coded as follows:

·        The brown and blue wires are the live and neutral feed wires from the 110v power cord to the power supply itself. These wires are always hot when the power supply is plugged in.

·        The black and white wires carry the AC feed from the switch back to the power supply itself. These leads should only be hot when the power supply is plugged in and the switch is turned on.

·        A green wire or a green wire with a yellow stripe is the ground lead. It should be connected somewhere to the PC case, and helps to ground the power supply to the case.

On the switch itself, the tabs for the leads are usually color-coded if not, they can still be easily connected. If there is no color coding on the switch, then plug the blue and brown wire onto the tabs that are parallel to each other, and the black and white wires to the tabs that are angled away from each other.

As long as the blue and brown wires are on the one set of tabs, and the black and white leads are on the other, the switch and supply will work properly. If you incorrectly mix the leads, you can create a direct short circuit, and you will likely blow the circuit breaker for the wall socket.

Disk Drive Power Connectors.

The disk drive connectors are fairly universal with regard to pin configuration and even wire color. Table shows the standard disk drives power connector pinout and wire colors.

Disk Drive Power Connector Pinout

Pin	Wire Color	Signal
1	Yellow	+12v
2	Black	Gnd
3	Black	Gnd
4	Red	+5v

This information applies whether the drive connector is the larger Molex version or the smaller mini-version used on most 3 1/2-inch floppy drives. In each case, the pinouts and wire colors are the same. To determine the location of pin 1, look at the connector carefully. It is usually embossed in the plastic connector body; however, it is often tiny and difficult to read. Fortunately, these connectors are keyed and therefore are difficult to insert incorrectly. Figure shows the keying with respect to pin numbers on the larger drive power connector.

Notice that some drive connectors may supply only two wires usually the +5v and a single ground (pins 3 and 4) because the floppy drives in most newer systems run on only +5v and do not use the +12v at all.

The Power Good Signal

The Power Good signal is a +5v signal (+3.0 through +6.0 is generally considered acceptable) generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes anywhere from 0.1 to 0.5 seconds after you turn on the power supply switch. This signal is sent to the motherboard, where it is received by the processor timer chip, which controls the reset line to the processor.

In the absence of Power_Good, the timer chip continuously resets the processor, which prevents the system from running under bad or unstable power conditions. When the timer chip sees Power_Good, it stops resetting the processor and the processor begins executing whatever code is at address FFFF: 0000 (usually the ROM BIOS).

If the power supply cannot maintain proper outputs (such as when a brownout occurs), the Power_Good signal is withdrawn, and the processor is automatically reset. When proper output is restored, the Power_Good signal is regenerated and the system again begins operation (as if you just powered on). By withdrawing Power_Good, the system never "sees" the bad power because it is "stopped" quickly (reset) rather than allowed to operate on unstable or improper power levels, which can cause parity errors and other problems.

In most systems, the Power_Good connection is made via connector P8-1 (P8 Pin 1) from the power supply to the motherboard. A well-designed power supply delays the arrival of the Power_Good signal until all voltages stabilize after you turn the system on. Badly designed power supplies, which are found in many low-cost compatibles, often do not delay the Power_Good signal properly and enable the processor to start too soon. The normal Power_Good delay is from 0.1 to 0.5 seconds. Improper Power_Good timing also causes CMOS memory corruption in some systems. If you find that a system does not boot up properly the first time you turn on the switch but subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with Power_Good. This happens because the Power_Good signal is tied to the timer chip that generates the reset signal to the processor. What you must do in these cases is find a new high-quality power supply and see whether it solves the problem.

Many cheaper power supplies do not have proper Power_Good circuitry and often just tie any +5v line to that signal. Some motherboards are more sensitive to an improperly designed or improperly functioning Power_Good signal than others. Improper Power_Good signal timing often causes intermittent startup problems. A common example occurs when somebody replaces a motherboard in a system and then finds that the system intermittently fails to start properly when the power is turned on. This ends up being very difficult to diagnose, especially for the inexperienced technician, because the problem appears to be caused by the new motherboard. Although it seems that the new motherboard might be defective, it usually turns out to be that the original power supply is poorly designed and either cannot produce stable enough power to properly operate the new board, or more likely has an improperly wired or timed Power_Good signal. In these situations, replacing the supply with a high-quality unit is the proper solution.

Power Supply Loading

PC power supplies are of a switching rather than a linear design. The switching type of design uses a high-speed oscillator circuit to generate different output voltages, and is very efficient in size, weight, and energy compared to the standard linear design, which uses a large internal transformer to generate different outputs.

One characteristic of all switching type power supplies is that they do not run without a load. This means that you must have the supply plugged into something drawing +5v and +12v or the supply does not work. If you simply have the supply on a bench with nothing plugged into it, the supply burns up or protection circuitry shuts it down. Most power supplies are protected from no-load operation and will shut down. Some of the cheap clone supplies, however, lack the protection circuit and relay and are destroyed after a few seconds of no-load operation. A few power supplies have their own built-in load resistors, so that they can run even though no normal load is plugged in.

According to IBM specifications for the standard 192-watt power supply used in the original AT, a minimum load of 7.0 amps was required at +5v and a minimum load of 2.5 amps was required at +12v for the supply to work properly. Because floppy drives present no +12v load unless they are spinning, systems without a hard disk drive often do not operate properly. Most power supplies have a minimum load requirement for both the +5v and +12v sides, and if you fail to meet this minimum load, the supply shuts down.

Because of this characteristic, when IBM used to ship AT systems without a hard disk, they had the hard disk drive power cable plugged into a large 5-ohm 50-watt sandbar resistor mounted in a little metal cage assembly where the drive would have been. The AT case had screw holes on top of where the hard disk would go, specifically designed to mount this resistor cage.

This resistor would be connected between pin 1 (+12v) and pin 2 (Ground) on the hard disk power connector. This placed a 2.4-amp load on the supply's 12-volt output, drawing 28.8 watts of power it would get hot thus enabling the supply to operate normally. Note that the cooling fan in most power supplies draws approximately 0.1 to 0.25 amps, bringing the total load to 2.5 amps or more. If the load resistor were missing, the system would intermittently fail to start up or operate properly. The motherboard draws +5v at all times, but only motors normally use +12v, and the floppy drive motors are off most of the time.

Most of the 200-watt power supplies in use today do not require as much of a load as the original IBM AT power supply. In most cases, a minimum load of 2.0 to 4.0 amps at +5v and a minimum load of 0.5 to 1.0 amps at +12v are considered acceptable. Most motherboards will easily draw the minimum +5v current by themselves. The standard power supply cooling fan draws only 0.1 to 0.25 amps, so the +12v minimum load may still be a problem for a diskless workstation. Generally the higher the rating on the supply, the more minimum load is required; however, there are exceptions, so this is a specification you want to check into.

Some high-quality switching power supplies, like the Astec units used by IBM in all the PS/2 systems, have built-in load resistors and can run under a no-load situation because the supply loads itself. Most of the cheaper clone supplies do not have built-in load resistors, so they must have both +5v and +12v loads to work.

If you want to bench test a power supply, make sure that loads are placed on both the +5v and +12v outputs. This is one reason why it is best to test the supply while it is installed in the system instead of separately on the bench. For impromptu bench testing, you can use a spare motherboard and hard disk drive to load the +5v and +12v outputs, respectively.

Energy Star Systems

The EPA has started a certification program for energy-efficient PCs and peripherals. To be a member of this program, the PC or display must drop to a power draw at the outlet of 30 watts or less during periods of inactivity. Systems that conform to this specification get to wear the Energy Star logo. This is a voluntary program, meaning there are no requirements to meet the specification; however, many PC manufacturers are finding that it helps to sell their systems if they can advertise them as energy-efficient.

One problem with this type of system is that the motherboard and disk drives literally can go to sleep, which means they can enter a standby or sleep mode where they draw very little power. This causes damage with some of the older power supplies because the low power draws does not provide enough of a load for them to function properly. Most of the newer supplies on the market are designed to work with these systems, and have a very low minimum load specification. I suggest that if you are purchasing a power supply upgrade for a system, ensure that the minimum load will be provided by the equipment in your system; otherwise, when the PC goes to sleep, it may take a power switch cycle to wake it up again! This problem would be most noticeable if you invest in a very high output supply and use it in a system that draws very little power to begin with.

Power Supply Problems

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-on or expansion components. Most desktop or tower systems are built in this manner. Some systems have inadequate power supplies from the start, however, and cannot accept the number and types of power-hungry options you might want to add.

In particular, portable systems often have power supply problems because they are designed to fit into a small space. Likewise, many older systems had inadequate power supply capacity for system expansion. For example, the original PC's 63.5-watt supply was inadequate for all but the most basic system. Add a graphics board, hard disk, math coprocessor (8087) chip, and 640K of memory, and you would kill the supply in no time. The total power draw of all the items in the system determines the adequacy of the power supply.

The wattage rating can sometimes be very misleading. Not all 200-watt supplies are created the same. Those who are into high-end audio systems know that some watts are better than others. Cheap power supplies may in fact put out the rated power, but what about noise and distortion? Some of the supplies are under-engineered to meet their specifications just barely, whereas others may greatly exceed their specifications. Many of the cheaper supplies output noisy or unstable power, which can cause numerous problems with the system. Another problem with under-engineered power supplies is that they 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.

Some of the available replacement supplies have higher capacity cooling fans than the originals, which can greatly prolong system life and minimize overheating problems, especially with some of the newer high-powered processors. If noise is a problem, models with special fans can run quieter than the standard models. These types often use larger diameter fans that spin slower, so that they run quiet while moving the same amount of air as the smaller fans. A Company called PC Power and Cooling specializes in heavy-duty and quiet supplies. Another company called Astec has several heavy-duty models as well. Astec supplies are found as original equipment in many high-end systems, such as those from IBM and Hewlett-Packard.

Ventilation in a system can be important. You must ensure adequate air flow to cool the hotter items in the system. Most processors have heat sinks today that require a steady stream of air to cool the processor. If the processor heat sink has its own fan, this is not much of a concern. If you have free slots, space out the boards in your system to allow air flow between them. Place the hottest running boards nearest the fan or ventilation holes in the system. Make sure that there is adequate air flow around the hard disk drive, especially those that spin at higher rates of speed. Some hard disks can generate quite a bit of heat during operation. If the hard disks overheat, data is lost.

Always make sure that you run with the lid on, especially if you have a loaded system. Removing the lid can actually cause a system to overheat. With the lid off, the power supply fan no longer draws air through the system. Instead, the fan ends up cooling the supply only, and the rest of the system must be cooled by simple convection. Although most systems do not immediately overheat because of this, several of my own systems, especially those that are fully expanded, have overheated within 15 to 30 minutes when run with the case lid off.

If you experience intermittent problems that you suspect are related to overheating, a higher capacity replacement power supply is usually the best cure. Specially designed supplies with additional cooling fan capacity also can help. At least one company sells a device called a fan card, but I am not convinced that it is a good idea. Unless the fan is positioned to draw air to or from outside the case, all the fan does is blow hot air around inside the system and provide a spot cooling effect for anything it is blowing on. In fact, adding fans in this manner contributes to the overall heat inside the system because each fan consumes power and generates heat.

The CPU-mounted fans are an exception to this because they are designed only for spot cooling of the CPU. Many of the newer processors run so much hotter than the other components in the system that a conventional finned aluminum heat sink cannot do the job. In this case, a small fan placed directly over the processor can provide a spot cooling effect that keeps the processor temperatures down. One drawback to these active processor-cooling fans is that if they fail, the processor overheats instantly and can even be damaged. Whenever possible, I try to use the biggest passive (finned aluminum) heat sink and stay away from more fans.

Power Supply Troubleshooting

Troubleshooting the power supply basically means isolating the supply as the cause of problems within a system. Rarely is it recommended to go inside the power supply to make repairs because of the dangerous high voltages present. Such internal repairs are beyond the scope of this book and are specifically not recommended unless the technician knows what he or she is doing.

Many symptoms would lead us 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 appears between the symptom and the cause the power supply.

For example, in many cases a "parity check" type of error message or problem indicates a problem with the supply. This may seem strange because the parity check message itself specifically refers to memory that has failed. The connection is that the power supply is what powers the memory, and memory with inadequate power fails.

It takes some experience to know when these failures are not caused by the memory and are in fact power-related. 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 suspect defective memory as the problem. However, if the problem seems random, or the memory location given as failed seems random or wandering, I suspect improper power as the culprit. The following is a list of PC problems that often are power supply-related:

Ø      Any power-on or system startup failures or lockups.

Ø      Spontaneous rebooting or intermittent lockups during normal operation.

Ø      Intermittent parity check or other memory type errors.

Ø      Hard disk and fan simultaneously fail to spin (no +12v).

Ø      Overheating due to fan failure.

Ø      Small brownouts cause the system to reset.

Ø      Electric shocks felt on the system case or connectors.

Ø      Slight static discharges disrupt system operation.

In fact, just about any intermittent system problem can be caused by the power supply. 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:

Ø      System is completely dead (no fan, no cursor)

Ø      Smoke

Ø      Blown circuit breakers

If you suspect a power supply problem, some simple measurements as well as more sophisticated tests outlined in this section can help you determine whether the power supply is at fault. Because these measurements may 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.

Digital Multi-Meters

A simple test that can be performed to a power supply is to check the output voltage. This shows if a power supply is operating correctly and whether the output voltages are within the correct tolerance range. Note that all voltage measurements must be made with the power supply connected to a proper load, which usually means testing while the power supply is still installed in the system.

Measuring Voltage

When making measurements on a system that is operating, you must use a technique called back probing the connectors. This is because you cannot disconnect any of the connectors while the system is running and instead 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. This technique is called back probing because you are probing the connector from the back. Virtually all the following measurements must be made using this back probing technique.

To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on most IBM-compatible supplies) for +3v to +6v. 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 supply is bad and must be replaced.

Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors:

	Loose Tolerance		Tight Tolerance
Desired Voltage	Min. (-10%)	Max. (+8%)	Min. (-5%)	Max. (+5%)
+/-5.0v	4.5v	5.4v	4.75	5.25
+/-12.0v	10.8v	12.9v	11.4	12.6

The Power_Good signal has tolerances that are different from the other signals, although it is nominally a +5v signal in most systems. The trigger point for Power_Good is about +2.5v, but most systems require the signal voltage to be within the tolerances listed:

Signal Minimum Maximum

Power_Good (+5v) 3.0v 6.0v 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.

Repairing the Power Supply

Actually repairing a power supply is rarely performed anymore, primarily because it is usually cheaper simply to replace the supply with a new one. Even high-quality power supplies are not that expensive relative to the labor required repairing them.

Defective power supplies are usually discarded unless they happen to be one of the higher quality or more expensive units. In that case, it is usually wise to send the supply out to a company that specializes in repairing power supplies and other components. These companies provide what is called depot repair, which means you send the supply to them; they repair it and return it to you. If time is of the essence, most of the depot repair companies immediately send you a functional equivalent to your defective supply and take yours in as a core charge. Depot repair is the recommended way to service many PC components such as power supplies, monitors, and printers. If you take your PC in to a conventional service outlet, they often diagnose the problem to the major component and send it out to be depot repaired. You can do that yourself and save the markup that the repair shop normally charges in such cases.

For those with experience around high voltages, it might be possible to repair a failing supply with two relatively simple operations; however, these require opening the supply.

Most manufacturers try to prevent you from entering the supply by sealing it with special tamper-proof Torx screws. These screws use the familiar Torx star driver, but also have a tamper-prevention pin in the center that prevents a standard driver from working. Most tool companies such as Jensen or Specialized sell sets of TT (tamperproof Torx) bits, which remove the tamper-resistant screws. Other manufacturers rivet the power supply case shut, which means you must drill out the rivets to gain access. Again, the manufacturers place these obstacles there for a reason to prevent entry by those who are inexperienced around high voltage.

Most power supplies have an internal fuse that is part of the overload protection. If this fuse is blown, the supply does not operate. It is possible to replace this fuse if you open the supply. Be aware that in most cases in which an internal power supply problem causes the fuse to blow, replacing it does nothing but cause it to blow again until the root cause of the problem is repaired. In this case, you are better off sending the unit to a professional depot repair company. The vendor list lists several companies that do depot repair on power supplies and other components.

PC power supplies have a voltage adjustment internal to the supply that is calibrated and set when the supply is manufactured. Over time, the values of some of the components in the supply can change, thus altering the output voltages. If this is the case, you often can access the adjustment control and tweak it to bring the voltages back to where they should be.

Several adjustable items are in the supply usually small variable resistors that can be turned with a screwdriver. You should use a nonconductive tool such as a fiberglass or plastic screwdriver designed for this purpose. If you were to drop a metal tool into an operating supply, dangerous sparks or fire could result, not to mention danger of electrocution and damage to the supply.

You also have to figure out which of the adjustments are for voltage and which ones are for each voltage signal. This requires some trial and error testing. You can mark the current positions of all the resistors, begin measuring a single voltage signal, and try moving each adjuster slightly until you see the voltage change. If you move an adjuster and nothing changes, put it back to the original position you marked. Through this process, you can locate and adjust each of the voltages to the standard 5v and 12v levels.

UNINTERRUPTED POWER SUPPLY

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

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 110v 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 function 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 an 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 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 110v 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.

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.

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.