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The Power Supply

The Power Supply

The power supply or PSU (Power Supply Unit) is a critical component in a PC, as it supplies electrical power to every component in the system. 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 110-volt, 60Hz, (or 220-volt, 50Hz) AC current into something the computer can use--specifically, +5v and +12v DC current, and +3.3v as well on newer 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 newer systems, although they are still required for backwards 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). These voltages can be used by any adapter card on the bus, but most notably they are used by serial port driver/receiver circuits. 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 drive bays, 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 falter, 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.

Power Supply Form Factors

The shape and general physical layout of a component is called the form factor, and items that share form factors are generally interchangeable. When a system is designed, the designers can choose to use one of the popular standard form factors, or they can "roll their own." Choosing the former means that a virtually inexhaustible supply of inexpensive replacements is available in a variety of quality and power output levels. Going the custom route means that the supply will be unique to the system and available only from the original manufacturer in only the model(s) they produce.

The form factor of the power supply that a particular system uses is based on the case design. Six popular case and power supply types can be called industry standard. The different types are:

PC/XT style
AT/Desktop style
AT/Tower style
Baby AT style
Slimline style
ATX style

Each of these supplies are available in numerous different configurations and power output levels. Of these standard types, the Slim style and ATX style are found in most newer systems, while the others are largely obsolete.

PC/XT Style

When IBM introduced the XT, it used the same basic power supply shape as the original PC, except that the new XT supply had more than double the power output capability (see Figure 8.1). Because they were identical in both external appearance and the type of connectors used, you could easily install the better XT supply as an upgrade for a PC system. Because of the tremendous popularity of the original PC and XT design, a number of manufacturers began building systems that mimicked their shape and layout. These clones, as they have been called, could interchange virtually all components with the IBM systems, including the power supply. Numerous manufacturers did produce these components, and nearly all follow the form factor of one or more IBM systems.

FIG. 8.1  PC/XT form factor power supply.

AT/Desktop Style

When IBM later introduced the AT desktop system, it created a larger power supply that had a form factor different from the original PC/XT. This system was rapidly cloned as well. The power supply used in these systems is called the AT/Desktop style power supply (see Figure 8.2). Hundreds of manufacturers have made motherboards, power supplies, cases, and so on that are physically interchangeable with the original IBM AT.

FIG. 8.2  AT/Desktop form factor power supply.

AT/Tower Style

The compatible market came up with a couple of other variations on the AT theme that became popular. 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 was 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 called an AT/Tower style power supply (see Figure 8.3).

FIG. 8.3  AT/Tower form factor power 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 (see Figure 8.4).

FIG. 8.4  Baby-AT form factor power supply.

Slimline Style

The fifth type of form factor that has developed is the Slimline or Low Profile form factor (see Figure 8.5). These systems use a different motherboard configuration that mounts the slots on a "riser" card that plugs into the motherboard. The expansion cards plug into this riser and are mounted sideways in the system. These types of systems are very low in height, hence the name Slimline. A new power supply was specifically developed for these systems and allows interchangeability between different manufacturers' systems. Some problems with motherboard interchanges occur because of the riser cards, but the Slimline power supply has become a standard in its own right. Despite how it might sound, even most full-sized AT Desktop and Tower cases are designed to accept the Slimline form factor power supply.

FIG. 8.5  Slimline form factor power supply.

ATX Style

A newer standard is the ATX form factor (see Figure 8.6). 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.

FIG. 8.6  ATX form factor power supply.

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 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 did not feature this keying on the boards or supplies they used.

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 ATX 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. Many systems will require this signal.

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 newer 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. For proprietary units, you will likely have to go back to the manufacturer.

Power Supply Connectors

Table 8.1 shows the pinouts for most standard AT or PC/XT-compatible systems. 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 newer 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 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.

Table 8.1  Typical PC/XT and AT Power Supply Connections

Connector AT Type PC/XT Type
P8-1 Power_Good (+5v) Power_Good (+5v)
P8-2 +5v Access key
P8-3 +12v +12v
P8-4 -12v -12v
P8-5 Ground Ground
P8-6 Ground Ground
P9-1 Ground Ground
P9-2 Ground Ground
P9-3 -5v -5v
P9-4 +5v +5v
P9-5 +5v +5v
P9-6 +5v +5v
P10-1 +12v +12v
P10-2 Ground Ground
P10-3 Ground Ground
P10-4 +5v +5v
P11-1 +12v +12v
P11-2 Ground Ground
P11-3 Ground Ground
P11-4 +5v +5v
P12-1 +12v --
P12-2 Ground --
P12-3 Ground --
P12-4 +5v --
P13-1 +12v --
P13-2 Ground --
P13-3 Ground --
P13-4 +5v --

Notice that the Baby-AT and Slimline power supplies also use the AT/Desktop or AT/Tower pin configuration. The only other type of industry standard power supply connector is found on the ATX form factor power supply. This is a 20-pin keyed connector with pins configured as shown in Table 8.2.

Table 8.2  ATX Power Supply Connections

Pin Signal Pin Signal
11 +3.3v * 1 +3.3v *
12 -12v 2 +3.3v *
13 Ground 3 Ground
14 Pwr_On 4 +5v
15 Ground 5 Ground
16 Ground 6 +5v
17 Ground 7 Ground
18 -5v 8 Power_Good
19 +5v 9 5v_Standby
20 +5v 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. It's recommended to go by the 5 percent tolerance, which is a tougher test to pass.

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.


CAUTION: The remote power switch leads carry 110v (or 220v) AC current at all times. You could be electrocuted if you touch the ends of these wires with the power supply plugged in! Always make sure the power supply is unplugged before connecting or disconnecting the remote power switch.

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 (or 220v) 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 wires 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. See Figure 8.7 for a guide.

FIG. 8.7  Power supply remote switch connections.

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 8.3 shows the standard disk drive power connector pinout and wire colors.

Table 8.3  Disk Drive Power Connector Pinout

Pin Wire Color Signal
1 Yellow +12v
2 Black Ground
3 Black Ground
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 8.8 shows the keying with respect to pin numbers on the larger drive power connector.

FIG. 8.8  A disk drive female power supply cable 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.

Physical Connector Part Numbers

The physical connectors used in industry-standard PC power supplies were originally specified by IBM for the supplies used in the original PC/XT/AT systems. They used a specific type of connector between the power supply and the motherboard (the P8 and P9 connectors), as well as specific connectors for the disk drives. The motherboard connectors used in all the industry-standard power supplies (except the ATX form factor power supplies) have not changed since 1981 when the IBM PC appeared. With the advent of 3 1/2-inch floppy drives in 1986, however, a new smaller type of drive power connector appeared on the scene for these drives. Table 8.4 lists the standard connectors used for motherboard and disk drive power.

Table 8.4  Physical Power Connectors

Connector Description Female (on Power Cable) Male (on Component)
Motherboard P8/P9 Burndy GTC 6P-1 Burndy GTC 6RI
Disk Drive (large style) AMP 1-480424-0 AMP 1-480426-0
Disk Drive (small style) AMP 171822-4 AMP 171826-4

You can get complete cable assemblies including drive adapters from the large to small connectors, disk drive "Y" splitter cables, and motherboard power extension cables.

The Power_Good Signal

The Power_Good signal is a +5v signal (+3.0v through +6.0v 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 older systems, the Power_Good connection is made via connector P8-1 (P8 Pin 1) from the power supply to the motherboard, ATX systems use Pin 8 of the ATX moetherboard connector.

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. Intermittent startup problems are often caused by improper Power_Good signal timing. 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. Several computer stores in the mid-1980s would order the diskless AT and install their own 20M or 30M drives, which they could get more cheaply from sources other than IBM. They were throwing away the load resistors by the hundreds!

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 was missing, the system would intermittently fail to start up or operate properly. The motherboard draws +5v at all times, but +12v is normally used only by motors, and the floppy drive motors are off most of the time.

Most of the newer 200-watt power supplies 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.

Power Supply Ratings

Most system manufacturers will provide you with the technical specifications of each of their system-unit power supplies. This type of information is usually found in the system's technical reference manual and also on stickers attached directly to the power supply. Power supply manufacturers can supply this data, which is preferable if you can identify the manufacturer and contact them directly.

Tables 8.5 and 8.6 list power supply specifications for several of IBM's units, from which most of the compatibles are derived. The PC-system power supplies are the original units that most compatible power supplies have duplicated. The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. IBM reports output wattage level as "specified output wattage". If your manufacturer does not list the total wattage, you can convert amperage to wattage by using the following simple formula:

Wattage = Voltage x Amperage

By multiplying the voltage by the amperage available at each output and then adding them up, you can calculate the total capable output wattage of the supply.

Table 8.5  Power Supply Output Ratings for IBM "Classic" Systems

PC Port-PC XT XT-286 AT
Minimum Input Voltage (at 110v setting) 104 90 90 90 90
Maximum Input Voltage (at 110v setting) 127 137 137 137 137
110/220v Switching No Yes No Auto Yes
+5v Output Current (amps): 7.0 11.2 15.0 20.0 19.8
-5v Output Current (amps): 0.3 0.3 0.3 0.3 0.3
+12v Output Current (amps): 2.0 4.4 4.2 4.2 7.3
-12v Output Current (amps): 0.25 0.25 0.25 0.25 0.3
Calculated Output Wattage 63.5 113.3 129.9 154.9 191.7
Specified Output Wattage 63.5 114.0 130.0 157.0 192.0

Table 8.6 shows the standard power supply output levels available in industry-standard form factors. Most manufacturers that offer power have supplies with different ratings. Supplies are available with ratings from 100 watts to 450 watts or more. Table 8.6 shows the rated outputs at each of the voltage levels for supplies with different manufacturer-specified output ratings. As you can see, although most of the ratings are accurate, they are somewhat misleading for the higher wattage units.

Table 8.6  Typical Compatible Power Supply Output Ratings

Specified Output Wattage 100W 150W 200W 250W 300W 375W 450W
+5v Output Current (amps): 10.0 15.0 20.0 25.0 32.0 35.0 45.0
-5v Output Current (amps): 0.3 0.3 0.3 0.5 1.0 0.5 0.5
+12v Output Current (amps): 3.5 5.5 8.0 10.0 10.0 13.0 15.0
-12v Output Current (amps): 0.3 0.3 0.3 0.5 1.0 0.5 1.0
Calculated Output Wattage 97.1 146.1 201.1 253.5 297.0 339.5 419.5

Most compatible power supplies have ratings between 150 to 250 watts output. Although lesser ratings are not usually desirable, it is possible to purchase heavy-duty power supplies for most compatibles that have outputs as high as 500 watts.

The 300-watt and larger units are excellent for enthusiasts who are building a fully optioned desktop or tower system. These supplies run any combination of motherboard and expansion card, as well as a large number of disk drives. In most cases, you cannot exceed the ratings on these power supplies--the system will be out of room for additional items first!

Most power supplies are considered to be universal, or worldwide. That is, they also run on the 220v, 50-cycle current used in Europe and many other parts of the world. Most power supplies that can switch to 220v input are automatic, but a few require that you set a switch on the back of the power supply to indicate which type of power you will access. (The automatic units sense the voltage and switch automatically.)

If your supply does not autoswitch, make sure the voltage setting is correct. If you plug the power supply into a 110v outlet while set in the 220v setting, there will be no damage, but it will certainly not operate properly until you correct the setting. On the other hand, if you are in a country with a 220v outlet and have the switch set for 110v, you may cause some damage.

Power Supply Specifications

In addition to power output, many other specifications and features go into making a high-quality power supply. You will see that if a brownout occurs in a room with several systems running, the systems with higher-quality power supplies and higher output ratings always make it over power disturbances, whereas others choke.

High-quality power supplies also help to protect your systems. A power supply from a vendor like Astec or PC Power and Cooling will not be damaged if any of the following conditions occur:

  • A 100 percent power outage of any duration

  • A brownout of any kind

  • A spike of up to 2,500v applied directly to the AC input (for example, a lightning strike or a lightning simulation test)

Decent power supplies have an extremely low current leakage to ground of less than 500 microamps. This safety feature is important if your outlet has a missing or improperly wired ground line.

As you can see, these specifications are fairly tough and are certainly representative of a high-quality power supply. Make sure that your supply can meet these specifications.

Power-Use Calculations

One way to see whether your system is capable of expansion is to calculate the levels of power drain in the different system components and deduct the total from the maximum power supplied. This calculation might help you decide when to upgrade the power supply to a more capable unit. Unfortunately, these calculations can be difficult to make because many manufacturers do not publish power consumption data for their products.

It is difficult to get power consumption data for most +5v devices, including motherboards and adapter cards. Motherboards can consume different power levels, depending on numerous factors. Most 486DX2 motherboards consume about 5 amps or so, but if you can get data on the one you are using, so much the better. For adapter cards, if you can find the actual specifications for the card, use those figures. To be on the conservative side, however, you should go by the maximum available power levels as set forth in the respective bus standards.

For example, consider the typical power consumption figures for components in a typical PC system. Most standard desktop or slimline PC systems come with a 200-watt power supply rated for 20 amps at +5v and 8 amps at +12v. The ISA specification calls for a maximum of 2.0 amps of +5v and 0.175 amps of +12v power for each slot in the system. For example, your system has eight slots, and you can assume that four of them are filled for the purposes of calculating power draw. The following calculation shows what happens when you subtract the amount of power necessary to run the different system components:

5v Power: 20.0 amps
Less: Motherboard -5.0
4 slots filled at 2.0 each -8.0
3 1/2 and 5 1/4-inch
floppy drives -1.5
3 1/2-inch hard disk drive -0.5
CD-ROM drive -1.0
Remaining Power: 4.0 amps
12v Power: 8.0 amps
Less: 4 slots filled at 0.175 each -0.7
3 1/2-inch hard disk drive -1.0
3 1/2 and 5 1/4-inch
Floppy drives -1.0
Cooling fan -0.1
CD-ROM drive -1.0
Remaining Power: 4.2 amps

In the preceding example, everything seems alright for now. With half the slots filled, two floppy drives, and one hard disk, the system still has room for more. There might be trouble if this system were expanded to the extreme. With every slot filled and two or more hard disks, there definitely will be problems with the +5v. However, the +12v does seem to have room to spare. You could add a CD-ROM drive or a second hard disk without worrying too much about the +12v power, but the +5v power will be strained.

If you anticipate loading up a system to the extreme, you may want to invest in the insurance of a higher output supply. For example, a 250-watt supply usually has 25 amps of +5v and 10 amps of +5v current, whereas a 300-watt unit has 32 amps of +5v power. These supplies would permit a fully expanded system and are likely to be found in full-sized desktop or tower case configurations in which this capability can be fully used.

Motherboards can draw anywhere from 4 to 15 amps or more of +5v power to run. In fact, a single Pentium 66MHz CPU draws up to 3.2 amps of +5v power all by itself. Most 486DX2 motherboards draw approximately 5 to 7 amps of +5v. Bus slots are allotted maximum power in amps, as shown in Table 8.7.

Table 8.7  Maximum Power Consumption in Amps per Bus Slot

Bus Type +5v Power +12v Power +3.3v Power
ISA 2.0 0.175 N/A
EISA 4.5 1.5 N/A
VL-Bus 2.0 N/A N/A
16-Bit MCA 1.6 0.175 N/A
32-Bit MCA 2.0 0.175 N/A
PCI 5 0.5 7.6

As you can see from the table, ISA slots are allotted 2.0 amps of +5v and 0.175 amps of +12v power. Note that these are maximum figures and not all cards draw this much power. If the slot has a VL-Bus extension connector, an additional 2.0 amps of +5v power is allowed for the VL-Bus.

Floppy drives can vary in power consumption, but most of the newer 3 1/2-inch drives have motors that run off +5v in addition to the logic circuits. These drives usually draw 1.0 amp of +5v power and use no +12v at all. Most 5 1/4-inch drives use standard +12v motors that draw about 1.0 amp. These drives also require about 0.5 amps of +5v for the logic circuits. Most cooling fans draw about 0.1 amps of +12v power, which is negligible.

Typical 3 1/2-inch hard disks draw about 1 amp of +12v power to run the motors and only about 0.5 amps of +5v power for the logic. The 5 1/4-inch hard disks, especially those that are full-height, draw much more power. A typical full-height hard drive draws 2.0 amps of +12v power and 1.0 amps of +5v power.

Another problem with hard disks is that they require much more power during the spinup phase of operation than during normal operation. In most cases, the drive draws double the +12v power during spinup, which can be 4.0 amps or more for the full-height drives. This tapers off to normal after the drive is spinning.

The figures most manufacturers report for maximum power supply output are full duty-cycle figures, which means that these levels of power can be supplied continuously. You usually can expect a unit that continuously supplies some level of power to supply more power for some noncontinuous amount of time. A supply usually can offer 50 percent greater output than the continuous figure indicates for as long as one minute. This cushion is often used to supply the necessary power to start spinning a hard disk. After the drive has spun to full speed, the power draw drops to a value within the system's continuous supply capabilities. Drawing anything over the rated continuous figure for any long length of time causes the power supply to run hot and fail early, and it can prompt several nasty symptoms in the system.


TIP: If you are using internal SCSI hard drives, you can ease the startup load on your power supply. The key is to enable the "Start on Command" (delayed start) option on the SCSI drive, which causes the drive to start spinning only when it receives a startup command over the SCSI bus. The effect is such that the drive remains stationary (drawing very little power) until the very end of the POST and spins up right when the SCSI portion of the POST is begun.

If you have multiple SCSI drives, they all spin up sequentially based on their SCSI ID setting. This is designed so that only one drive is spinning up at any one time, and that no drives start spinning until the rest of the system has had time to start. This greatly eases the load on the power supply when you first power the system on.


The biggest causes of overload problems are filling up the slots and adding more drives. Multiple hard drives, CD-ROM drives, floppy drives, and so on can place quite a drain on the system power supply. Make sure that you have enough +12v power to run all the drives you are going to install. Tower systems can be a problem here because they have so many drive bays. Make sure that you have enough +5v power to run all your expansion cards, especially if you are using VL-Bus or EISA cards. It pays to be conservative, but remember that most cards draw less than the maximum allowed.

Many people wait until an existing unit fails before they replace it with an upgraded version. If you are on a tight budget, this "if it ain't broke, don't fix it" attitude works. Power supplies, however, often do not just fail; they can fail in an intermittent fashion or allow fluctuating power levels to reach the system, which results in an unstable operation. You might be blaming system lockups on software bugs when the culprit is an overloaded power supply. If you have been running with your original power supply for a long time, you should expect some problems.

Leave It On or Turn It Off?

A frequent question that relates to the discussion of power supplies concerns whether you should turn off a system when it is not in use. You should understand some facts about electrical components and what makes them fail. Combine this knowledge with information on power consumption and cost, not to mention safety, and perhaps you can come to your own conclusion. Because circumstances can vary, the best answer for your own situation might be different depending on your particular needs and application.

Frequently, powering a system on and off does cause deterioration and damage to the components. This seems logical, and the reason is simple but not obvious to most. Many people believe that flipping system power on and off frequently is harmful because it electrically "shocks" the system. The real problem, however, is temperature. In other words, it is not so much electrical shock as thermal shock that damages a system. As the system warms up, the components expand; and as it cools off, the components contract. This alone stresses everything. In addition, various materials in the system have different thermal expansion coefficients, which means that they expand and contract at different rates. Over time, thermal shock causes deterioration in many areas of a system.

From a pure system-reliability point, it is desirable to insulate the system from thermal shock as much as possible. When a system is turned on, the components go from ambient (room) temperature to as high as 185F (85C) within 30 minutes or less. When you turn the system off, the same thing happens in reverse, and the components cool back to ambient temperature in a short period of time. Each component expands and contracts at slightly different rates, which causes the system an enormous amount of stress.

Thermal expansion and contraction remains the single largest cause of component failure. Chip cases can split, allowing moisture to enter and contaminate them. Delicate internal wires and contacts can break, and circuit boards can develop stress cracks. Surface-mounted components expand and contract at different rates from the circuit board they are mounted on, which causes enormous stress at the solder joints. Solder joints can fail due to the metal hardening from the repeated stress causing cracks in the joint. Components that use heat sinks such as processors, transistors, or voltage regulators can overheat and fail because the thermal cycling causes heat sink adhesives to deteriorate, breaking the thermally conductive bond between the device and the heat sink. Thermal cycling also causes socketed devices and connections to "creep", which can cause a variety of intermittent contact failures.

Thermal expansion and contraction affects not only chips and circuit boards, but also things like hard disk drives. Most newer hard drives have sophisticated thermal compensation routines that make adjustments in head position relative to the expanding and contracting platters. Most drives perform this thermal compensation routine once every five minutes for the first 30 minutes the drive is running, and then every 30 minutes thereafter. In many drives, this procedure can be heard as a rapid "tick-tick-tick-tick" sound.

In essence, anything you can do to keep the system at a constant temperature prolongs the life of the system, and the best way to accomplish this is to leave the system either permanently on or off. Of course, if the system is never turned on in the first place, it should last a long time indeed!

Now, you could say that you should leave all systems on 24 hours a day, that is not necessarily true. A system powered on and left unattended can be a fire hazard and is a data security risk (cleaning crews, other nocturnal visitors, and so on). It also can be easily damaged if moved while running, and it simply wastes electrical energy.

A typical desktop style PC with display consumes at least 300 watts (0.3 kilowatts) of electricity (and that is a conservative estimate). Using systems certified under the new EPA Energy Star program (also called "Green" PCs) would save a lot more energy. The great thing about Energy Star systems is that the savings are even greater if the systems are left on for long periods of time because the power management routines are automatic.

Based on these facts, it's recommended that you power the systems on at the beginning of the work day, and off at the end of the work day. Do not power the systems off for lunch, breaks, or any other short duration of time. Servers and the like of course should be left on continuously. This seems to be the best compromise of system longevity with pure economics.

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 havoc with some of the older power supplies, because the low power draw does not provide enough of a load for them to function properly. Most of the newer power supplies are designed to work with these systems, and have a very low minimum load specification. 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.

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 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.

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. There are devices called fan cards, but that is probably not 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 aluminium 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, try to use the biggest passive (finned aluminium) heat sink and stay away from more fans.


TIP: If you seal the ventilation holes on the bottom of the original IBM PC chassis, starting from where the disk drive bays begin and all the way to the right side of the PC, you drop the interior temperature some 10 to 20F (5 to 10C)--not bad for that price. IBM "factory-applied" this tape on every XT and XT-286 it sold. The result is greatly improved interior aerodynamics and airflow over the heat-generating components. For other PC-compatible systems, this may not apply because their case designs may be different.

No matter what system you have, 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 will disrupt the internal airflow and may cause higher internal temperatures.


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 documentation and are specifically not recommended unless the technician knows what he or she is doing.

Many symptoms would lead you 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, you can suspect defective memory as the problem. However, if the problem seems random, or the memory location given as failed seems random or wandering, you can suspect improper power as the culprit. The following is a list of PC problems that often are power supply-related:

  • 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 fail to spin (no +12v).

  • Overheating due to fan failure.

  • Small brownouts cause the system to reset.

  • Electric shocks felt on the system case or connectors.

  • Slight static discharges disrupt system operation.

In fact, just about any intermittent system problem can be caused by the power supply. You can 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 is completely dead (no fan, no cursor)

  • Smoke

  • 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.

Selecting a Meter

You need a simple Digital Multi-Meter (DMM) or Digital Volt-Ohm Meter (DVOM) to make voltage and resistance checks in electronic circuits. You should use only a DMM rather than the older needle type multi-meters because the older meters work by injecting a 9v signal into the circuit when measuring resistance. This will damage most computer circuits. A DMM uses a much smaller voltage (usually 1.5v) when making resistance measurements, which is safe for electronic equipment. You can get a good DMM from many sources and with many different features. The small pocket-sized meters are recommended for computer work because they are easy to carry around.

Some features to look for in a good DMM are:

  • Pocket size. Small meters are available that have many if not all the features of larger ones. The elaborate features found on some of the larger meters are not really needed for computer work.

  • Overload protection. This means that if you plug the meter into a voltage or current beyond the capability of the meter's measurements, 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. This means that 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 an autoranging capability and a manual range override.

  • Detachable probe leads. The leads can be easily damaged, and sometimes a variety of differently shaped probes are required for different tests. Cheaper meters have the leads permanently attached, which means that they cannot easily be replaced. Look for a meter with detachable leads.

  • Audible continuity test. Although you can use the ohm scale for testing continuity (0 ohms indicates continuity), a continuity test function causes a beep noise to be heard when continuity exists between the meter test leads. By using the sound, you can more quickly 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 meter if no readings are sensed for a predetermined period of time.

  • Automatic display hold. This feature enables the last stable reading to be held 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 lowest and highest readings to be trapped in memory and held for later display. This is especially useful if you have readings that are fluctuating too quickly to see on the display.

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, Pin 8 on an ATX connector) 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 between 3.0v and 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.

Specialized Test Equipment

You can use several types of specialized test gear to test power supplies more effectively. Because the power supply is perhaps the most failure-prone item in PCs, if you service many PC systems, it is wise to have many of these specialized items.

Load Resistors for Bench Testing a Power Supply

Bench testing a power supply requires some special setup because all PC power supplies require a load to operate.

Variable Voltage Transformer

In testing power supplies, it is desirable to simulate different 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 have 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 to control 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 seen by the PC.

Most variable transformers can adjust their AC output from 0v to 140v (or 0v to 280v) no matter what the AC input (wall socket) voltage is. You can use the transformer to simulate brownout conditions, enabling you to observe the PC's response. Thus, among other things you can check for proper Power_Good signal operation.

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 higher voltages, you can test the capability of the power supply to run on foreign voltage levels as well. A properly functioning supply should operate between 90v to 135v (or 180v to 270v) but shut down cleanly if the voltage is outside that range.

An indication of a problem is seeing "parity check" type error messages when you drop the voltage to 80v (or 160v). 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.

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 to repair 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.

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. This is not recommended.

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 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. Consider yourself warned!

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.

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.

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. You can easily interchange any supply with another one of the same form factor. Earlier, this chapter 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. 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 a lot 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.

Sources for Replacement Power Supplies

Because one of the most failure-prone items in PC systems is the power supply, you will often have to recommend a replacement. Literally hundreds of companies manufacture PC power supplies.

Although other high-quality manufacturers are out there, power supplies from either Astec Standard Power or PC Power and Cooling are recommended.

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 and a variety of output levels. 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. Versions are available in a variety of different quality and output levels, from inexpensive replacements to very high-quality high-output models. 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.

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 recommended vendors 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 to plug in a surge suppresser, 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 80v (or 160v) for up to 2 seconds

  • Voltage drop to 70v (or 140v) for up to .5 seconds

  • Voltage surge of up to 143v (or 286v) 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.

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 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 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 (or 400v) 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.

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.

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.

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 known: 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 110v (or 220v) 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.

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 (or 220v) 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. 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.


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). 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 in other systems 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. Newer motherboards use extended NVRAM chips with as much as 2K or more of space to hold this configuration information. This is especially true for 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 newer systems 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.

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.

A lot of clone systems use a rechargable NiCad (Nickel Cadmium) battery, which is directly soldered into the motherboard. The motherboard has an on-board charging circuit. These batteries usually last for only two to four years, because of the so-called memory effect of NiCad batteries. NiCad batteries can only keep their full capacity when they are fully charged and discharged every time. Because the battery is recharged every time the PC is switched on, it will never discharge enough to eliminate the memory effect.

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. There are 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 3.6v, 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, 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.

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