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Upgrading & Repairing PCs, Eighth Edition

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Author: Scott Mueller
Retail Price: $49.99
Publisher: Que
ISBN: 0789712954
Publication Date: 9/16/97
Pages: 1168


Chapter 8 - The Power Supply

Scott Mueller shows you the function and limitations of a power supply, as well as potential problems and solutions

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

Power Supply Function and Operation

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

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

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 Baby AT style

AT/Desk style Slim style

AT/Tower 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 modern 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 have since begun producing 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/Desk 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, and to this day still represents the basis for most IBM- compatible designs. The power supply used in these systems is called the AT/Desktop style power supply (see Figure 8.2). Hundreds of manufacturers now make motherboards, power supplies, cases, and so on that are physically interchangeable with the original IBM AT. If you are buying a compatible system, I recommend those that have form factors that are compatible with the IBM AT, because you will have numerous motherboards and power supplies from which to choose.

AT/Tower Style

The compatible market has come up with a couple of other variations on the AT theme that are popular today. Besides the standard AT/Desktop type power supply, we also have the AT/Tower configuration, which is basically a full-sized AT-style desktop system running on its side. The power supply and motherboard form factors are basically the same in the Tower system as in the Desktop. The tower configuration is not new; in fact, even IBM's original AT had a specially mounted logo that could be rotated when you ran the system on its side in the tower configuration. The type of power supply used in a tower system is identical to that used in a desktop system, except for the power switch location. Most AT/Desktop systems required that the power switch be located right on the power supply itself, while most AT/Tower systems use an external switch attached to the power supply through a short 4-wire cable. A full sized AT power supply with a remote switch is now called an AT/Tower form-factor supply (see Figure 8.3).

FIG. 8.2  AT/Desktop form factor power supply.

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.

Slim Style

The fifth type of form factor that has developed is the Slimline (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. The slimline power supply is by far the most popular power supply design in use today. Despite how it might sound, even most full-sized AT Desktop and Tower cases today are designed to accept the slimline form factor power supply.

FIG. 8.5  Slimline/Low Profile form factor power supply.

ATX Style

The newest standard on the market today 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 that have unfortunately become common today.

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

FIG. 8.6  ATX form factor power supply.

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

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

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

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

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

Power Supply Connectors

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 currently available AT/Tower type power supplies have four drive connectors. If you are adding drives and need additional disk drive power connectors, "Y" splitter cables are available from many electronics supply houses (including Radio Shack) that can adapt a single power connector to serve two drives. As a precaution, make sure that your total power supply output is capable of supplying the additional power.

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 Key (No connect)
P8-3 +12v +12v
P8-4 -12v -12v
P8-5 Ground (0) Ground (0)
P8-6 Ground (0) Ground (0)
P9-1 Ground (0) Ground (0)
P9-2 Ground (0) Ground (0)
P9-3 -5v -5v
P9-4 +5v +5v
P9-5 +5v +5v
P9-6 +5v +5v
P10-1 +12v +12v
P10-2 Ground (0) Ground (0)
P10-3 Ground (0) Ground (0)
P10-4 +5v +5v
P11-1 +12v +12v
P11-2 Ground (0) Ground (0)
P11-3 Ground (0) Ground (0)
P11-4 +5v +5v
P12-1 +12v --
P12-2 Ground (0) --
P12-3 Ground (0) --
P12-4 +5v --
P13-1 +12v --
P13-2 Ground (0) --
P13-3 Ground (0) --
P13-4 +5v --

Notice that the Baby-AT and Slimline power supplies also use the AT/Desktop or Tower pin configuration. The only other type of industry standard power supply connector is found on the new 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

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

* = Optional signal


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

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

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

Table 8.4  Physical Power Connectors

Connector Description Female (on Power Cable) Male (on Component)
Motherboard P8/P9 Burndy GTC6P-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 these raw connectors through electronics supply houses (Allied, Newark, Digi-Key, and so on) found in the vendor list. You also can get complete cable assemblies including drive adapters from the large to small connectors, disk drive "Y" splitter cables, and motherboard power extension cables from a number of the cable and miscellaneous supply houses such as Cables To Go, the Cable Connection, Ci Design, and Key Power.

The Power_Good Signal

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

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

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

In most systems, the Power_Good connection is made via connector P8-1 (P8 Pin 1) from the power supply to the motherboard.

A well-designed power supply delays the arrival of the Power_Good signal until all voltages stabilize after you turn the system on. Badly designed power supplies, which are found in many low-cost compatibles, often do not delay the Power_Good signal properly and enable the processor to start too soon. The normal Power_Good delay is from 0.1 to 0.5 seconds. Improper Power_Good timing also causes CMOS memory corruption in some systems. If you find that a system does not boot up properly the first time you turn on the switch but subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with Power_Good. Thi is 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 I knew 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! I managed to grab a couple at the time, which is how I know the type of resistor they used.

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

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

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

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 104 90 90 90 90
Maximum Input Voltage 127 137 137 137 137
110/220v Switching No Yes No Auto Yes
Output Current (amps):+5v 7.0 11.2 15.0 20.0 19.8
-5v 0.3 0.3 0.3 0.3 0.3
+12v 2.0 4.4 4.2 4.2 7.3
-12v 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. To compile the table, I referred to the specification sheets for supplies from Astec Standard Power and PC Power and Cooling. 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
Output Current (amps):+5v 10.0 15.0 20.0 25.0 32.0 35.0 45.0
-5v 0.3 0.3 0.3 0.5 1.0 0.5 0.5
+12v 3.5 5.5 8.0 10.0 10.0 13.0 15.0
-12v 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 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 current 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 foreign 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. I have had many systems over the years. My experience has been 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. I would not give $5 for many of the cheap, junky power supplies that come in some of the low-end clone systems.

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. The vendors recommended in this chapter produce supplies that meet or exceed 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 mother-boards 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, I usually 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 modern PC system. Most standard desktop or slimline PC systems today 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. Most systems have 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