Technical The Internal Components The Power Supply

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The Power Supply - part 1

by Scott Mueller - 16 September 1997

• Power Supply Function and Operation - part 1
  • Power Supply Form Factors
  • Power Supply Connectors
  • The Power_Good Signal - part 2
  • Power Supply Loading
  • Power Supply Ratings
  • Power Supply Specifications
  • Power-Use Calculations
• Leave It On or Turn It Off? - part 3
• Energy Star Systems
• Power Supply Problems
• Power Supply Troubleshooting
  • Digital Multi-Meters
  • Specialised Test Equipment
• Repairing the Power Supply
• Obtaining Replacement Units - part 4
  • Deciding on a Power Supply
  • Sources for Replacement Power Supplies
• Using Power-Protection Systems
  • Surge Suppressers (Protectors)
  • Phone Line Surge Protectors
  • Line Conditioners
  • Backup Power
• RTC/NVRAM Batteries

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 240-volt, 50Hz, 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 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 9x 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 and 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. 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.

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. 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.

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.

Slim Style

The fifth type of form factor that has developed is the Slimline. 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.

ATX Style

The newest standard on the market today is the ATX form factor. This describes a new motherboard shape, as well as a new case and power supply form factor. The ATX supply is based on the slimline or low-profile design, but has several differences worth noting. One difference is that the fan is now mounted along the inner side of the supply, blowing air across the motherboard and drawing it in from the outside at the rear. This flow is the opposite of most standard supplies, which blow air out the back of the supply and also have the fan positioned at the back. The reverse flow cooling used in the ATX supply forces air over the hottest components of the board, such as the CPU, SIMMs, and expansion slots. This still doesn't eliminate 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 pressurised, so air will push out of the cracks in the case, the opposite of what happens in non-ATX systems. For example, if you held a lit cigarette in front of your floppy drive on a normal system, the smoke would be inhaled through the front of the drive and contaminate the heads! On an ATX system with reverse flow cooling, the smoke would be blown away from the drive because the only air intake is the single fan vent on the power supply at the rear. Those who use systems that operate in extremely harsh environments could add a filter to the fan intake vent, which would ensure even further that all air entering the system is clean and dust free.

The ATX system format was designed by Intel in 1995, but became popular in the new Pentium Pro-based PCs in 1996. The ATX form factor takes care of several problems with the Baby-AT or Slimline form factors. Where the power supply is concerned, this covers two main problems. One problem is that the traditional PC power supply has two connectors that plug into the motherboard. The problem is that if you insert these connectors backwards or out of their normal sequence, you could 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 labelled 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 9x 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 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 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 RS Components) 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 1 Typical PC/XT and AT Power Supply Connections

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 2.

Table 2 ATX Power Supply Connections

* = 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.

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.

Disks - The Basics

Other Co-processors