CIS 107a: Introduction to Hardware Concepts

Chapter 3: Form Factors and Power Supplies



This chapter introduces the student to computer case, motherboard, and power concepts. The objectives important to this chapter are:

  1. Form factors: computer cases, motherboads, and power supplies
  2. How electricity is measured
  3. How a computer system can be protected from damaging changes in electrical power
  4. About Energy Star
  5. How to detect and correct power supply problems

The chapter begins with a discussion of form factors for cases, power supplies, and motherboards. A form factor is defined as a size, shape, and set of features. The author observes that, when building a computer, you should pick a form factor for the motherboard first, which will limit or determine your choices for the case and the power supply. Consider these basic requirements:

  • the motherboard must be small enough to fit in the case, and the holes in it must match the attachment points in the case
  • the ports (jacks) on the motherboard must match holes in the case
  • the power supply must fit in the case, and must have power cords that fit the motherboard and provide the correct amount of power

Seven types of system board designs are listed: AT, Baby AT, ATX, BTX, LPX, NLX, and Backplane. Each has variations.

  • AT system boards, also called full AT boards, measure 12 inches by 13.8 inches.
  • They have two main connectors for power: P8 and P9.
  • AT system boards have connections for -5, +5, -12, and +12 volt lines from the power supply.
  • AT boards are larger than the other styles listed here. It may be recognized by its size, and the placement of the processor in front of the expansion bus slots, which puts it in the way of longer cards.
  • AT power supplies blow air into the system.
Baby AT
  • The smaller version of the AT board is called the Baby AT. The board is 13 inches by 8.7 inches.
  • The processor is still in the way of the expansion slots.
  • A problem with this design is that devices mounted in the case often have to string cables all the way across the motherboard to connect to it.
  • ATX system boards measure 12 inches by 9.6 inches.
  • ATX system boards have one main connector for power: P1. The originals had 20 pins, but later models have 24. (In between, there were models that had a separate 12 volt connector just for the processor. This was incorporated intothe 24 pin design.)
  • The processor on an ATX board is beside the expansion slots, not in front of them.
  • ATX system boards have connections for +3.3 volts in addition to the voltages available on the AT board. Newer models of processors typically use less power.
  • ATX power supplies blow air out of the system.
  • ATX cases may fit Baby AT boards, but the reverse is not true.
  • ATX boards have a soft switch. Operating systems such as Windows 98, 2000, and XP can turn the power off when shutting down.
  • Smaller versions of the ATX board are called the Mini ATX, Micro ATX, and Flex ATX.
  • Balanced Technology Extended (BTX) system boards focus on better airflow through the case, eliminating the need for a fan directly on the processor.
  • BTX system boards have one main connector for power, a 24 pin P1.
  • BTX system boards are made to support Serial ATA, USB 2, and PCI Express.
  • BTX power supplied blow air out of the system. A BTX box's main characteristic that that all components are oriented to have air passing directly over them.
  • The NLX form factor is for low cost, low profile computers.
  • NLX system boards have only one expansion slot. It is used for a riser card, which may contain other expansion slots, and connectors for floppy or hard drives.
  • NLX boards will have video circuits included on the board.
LPX and Mini-LPX
  • LPX boards use a riser card, like the NLX board.
  • LPX boards are low end boards, unsuited for newer processors due to heat and size.
  • LPX boards are frequently changed by a manufacturer to make them proprietary. This means that parts can only be obtained from that manufacturer.
Backplane Systems
  • A backplane is not a motherboard. It typically only holds expansion slots, one of which will be used for a mothercard.
  • Active backplanes have some slots, buffers, and driver circuits.
  • Passive backplanes have no circuits, just a slot for the mothercard.
  • These systems are not for personal computers, but are used in rack systems.

Three varieties of case types are listed:

  • Desktop - typically have four drive bays, about six expansion slots, and were meant to sit horizontally on a desk. The text puts compact cases (low profile cases) in this category. They are typically smaller, and meant for low cost, less powerful computers.
  • Tower - typically sits vertically on a desk or on the floor (bad idea: static from the carpet). These come in a variety of sizes, the larger ones generally for more powerful computers and servers.
  • Notebook/Laptop - used for portable computers. These vary in thickness and weight, number of slots and ports, and processor power. The size of the case may require that the power supply be external, and in some cases that peripheral devices are external as well.

The chapter continues with a discussion of how electricity works. It introduces some basic electrical terms that are used in the text such as:

  • volt - a measure of the difference in electrical potential between two points. For instance, in order to feel a discharge of static electricity, there must be a difference of 3000 or more volts between you and some object. Abbreviation: V
  • ampere - a measure of electrical current. Also called an amp, it is a measure of how much electricity is flowing through a system. Amps are given by the formula volts divided by ohms. Abbreviation: A
  • ohm - a unit of electrical resistance. Wires are rated as by their electrical resistance. For instance, coaxial cable used in networking is usually required to be 50 ohm cable. Abbreviation: Ω (Greek letter Omega)
  • watt - a measure of electrical power. Volts times amps equals watts (V * A = W). Power supplies are rated as being able to supply a certain number of watts. Abbreviation: W

Your text uses these terms in a general way. For more details on electrical terms, see Marshall Brain's response to a question about electricity. While we are at it, Mr. Brain's article on Computer Power Supplies is quite good, too.

The text explains that Alternating Current (AC) travels one way through a circuit, then changes to flowing the other way. In the United States, this oscillation takes place about 60 times per second. Direct Current (DC) does not oscillate. Power supplies are meant to produce direct current (DC) power for most components. Since they are meant to be plugged into a source of alternating current (AC), the power supply must include one or more rectifiers which are AC to DC converters. The power supply also includes a transformer, which changes the voltage from 110 (or 220) to several voltages needed by the computer: +3.3, -3.3, +5, -5, +12, or -12 volts. (110 and 220 volt alternating current are US standards. Voltages and other standards vary from country to country.)

The text explains some common electrical standards for wiring, relating to the color of the insulation on the wire. These standards may vary.
Common Wiring Standards in Homes and Buildings
  Green or non-insulated wires are often used for ground.
  Black wires are often used for hot (power from source) in 110 and 220 volt circuits.
  White wires are often used for neutral (return to source) in 110 and 220 volt circuits.
  Red wires are often used as the second hot line in 220 volt circuits.

Wiring in Computers
  Inside computers, black wires are often used for ground.
  Inside computers, red wires are often used as the only hot line.

The text offers more terminology. You should be aware of the meaning of the following terms:

  • conductor - a material that electricity flows through easily, like copper wire
  • insulator - a material that resists the flow of electricity, like plastic or rubber
  • semiconductor - a material that conducts electricity under certain conditions, such as under certain voltages or certain currents.
  • transistor - an electronic switch, can be changed in state to on or off. Made of three layers of different kinds of semiconductors.
  • capacitor - a device that stores electricity and releases it over a short time. It is meant to serve as a buffer for circuits that need an even flow of current.
  • diode - a diode is like a check valve in a plumbing system: it only lets electricity flow in one direction. Diodes are used to construct a rectifier, which converts AC to DC.
  • resistor - a device that limits the flow of current.

The text offers a list of safety advice about working on computer. Some items in the list are to keep you from damaging the equipment, some to keep you from damaging yourself. All are important to the A+ certification test:

  • Make notes. Every day I am amazed by another professional who did not make any notes about what he did, and who cannot undo a mistake or tell anyone what troubleshooting steps he has tried.
  • Cellophane, packing tape, plastic foam, and other items hold static electricity. Remove them from your work area as soon as you open the packages that they come in.
  • Keep components away from hair and clothing. Why? Hair and clothing can carry electric charge, can snag on equipment and cause malfunctions, and worst case, can pull you into contact with hot, sharp, or crushing devices.
  • Keep screws, spacers, and small parts where you can find them. The book suggests a tray, or other container. An empty egg carton can work, if you can find the paper kind. (No plastic foam around the work area, remember?)
  • Do not stack boards, cards, or other devices, because you can break pieces off when you move them.
  • Regarding cards and motherboards: do not touch the chips or the solder points. Hold them by the edges. The main thing is avoiding static discharge from you to the component.
  • Do not touch chips with magnetic tools. In general, do not work on computers with magnetic tools.
  • Do not use graphite pencils to change DIP switches. Graphite conducts electricity, and it can break or rub off in the switch. (If you need something to perform this task, the orangewood sticks sold in any cosmetics department are ideal. They are strong, hold a point, and they do not conduct electricity.)
  • In a classroom, have your work checked by your instructor before closing the case or applying power. This translates to having a partner check in the real world. (In class, it is much easier to get credit for the lab this way. Hint, hint.)
  • Turn off computers before moving them. This is to protect the hard drive, based on the idea that it is more likely to be damaged if the computer is moved while turned on.
  • Keep magnetic disks away from magnetic fields, heat, and cold. Do not touch the magnetic medium on disks.

Electrostatic Discharge (ESD) is the flow of static, or stored, electricity. Electric charge will flow between two objects if they have different charges. This flow can damage electronic devices even if you cannot feel the flow. A static discharge has to be at least 3,000 volts to be felt, at least 6,000 volts to be heard, and at least 8,000 volts to be seen. Damage can be done with as little as 20 to 30 volts.

Grounding yourself while working on most computer components is recommended. Several methods are recommended:

  • grounding strap - Attaches to you, and to the computer case, a ground mat, or the ground wire in an electrical outlet.
  • ground mat - You can attach your ground strap to a ground mat, you can place metal parts on it to ground out any static charge in them. This only works if the ground mat is connected to a good neutral ground.
  • leaving the power cord plugged in while working on the computer - The text does not recommend this, due to the potential hazard of touching an exposed part of the power switch.

The author mentions that components are frequently shipped in static shielding bags. Such bags are usually silver or gray, due to a metal layer sandwiched between two plastic layers. (Follow this link to a page that explains how a Faraday Cage works. It is named for Michael Faraday.)

Exception to grounding advice: It is important to remember that power supplies contain capacitors that store large electrical charges, making it dangerous to work on them. You should never be grounded when working on a power supply. You should not be grounded when working on a monitor, for the same reason, nor when working on any high voltage equipment.

Electromagnetic Interference (EMI) can be a problem around electrical flows. In general, the flow of electricity, especially in a coil, produces a magnetic field. (Conversely, the movement of a magnetic field can produce an electric current.) Unintended flows of electricity or magnetism can cause problems for computers. Cables should be shielded when possible, and computers should have plates covering open drive bays and open expansion slots. An old trick to find a source of EMI is to get a cheap AM radio that you can carry to various points in the work area. Tune the radio to the low end of the dial, away from all stations, and listen for static. You will hear it near the sources of EMI.

Electrical companies do not necessarily supply clean, error free flow of power to customers. Several devices are described in the text that improve on the service you may be receiving from your provider.

  • surge suppressor - These provide protection from electrical spikes, unintended increases in electrical voltage. The number of volts that a surge suppressor allows to pass is called the let-through voltage. This may also be called the clamping voltage, because a suppressor clamps down on the circuit and lets no more through.
  • data line protector - Telephone lines can be a source of voltage spikes, although it is less common. A data line protector is a surge suppressor for a telephone line.
  • power conditioner - A power conditioner can protect against surges, and it may also store a charge to boost voltage in case of a partial voltage drop on the power line. They do not protect against total power loss.
  • uninterruptible power supply (UPS) - These devices are available in two main types. A standby UPS keeps a charge on a battery which it uses to supply power in case of a total loss. An inline UPS also has a battery, but it constantly provides power from it, while continuously charging it from the standard electrical power. The standby model has a short lag time in the event of a power loss before the battery circuit starts working. The inline model does not have this lag time.
  • smart UPS - This is a UPS that connects to a computer by a serial port, and includes a microprocessor that allows it to send messages about power losses, to schedule tests of the system, and to automatically shut down protected equipment in case of a power loss.

Some advice is given about buying a UPS. Like other components to a computer system, do not buy "just enough". An inline UPS that is running at its capacity will run hot, and will wear out sooner. The text recommends running a UPS only up to 25% below capacity.

Energy Star

Energy Star systems have components whose performance meets U.S. Envronmental Protection Agency requirements. They have power management feature that use less power when idle. The text lists several acronyms that have to do with power management:

  • Advanced Power Management (APM)
  • AT Attachment (ATA) - for IDE hard drives
  • Display Power Management Signaling (DPMS) - for video cards and monitors
  • Advanced Configuration and Power Interface (ACPI) - for Windows 98, 2000, and XP
Detecting and Correcting Power Supply Problems

Adding new devices may exceed the capacity of a computer's power supply. We used to think of 200 watts as plenty, but 350 to 400 watts is a more recommendable level for a power supply for an average computer. (This comment was written in 2004. It is still true in 2007.) To test whether you need more power:

  1. Test the power supply after installation of new devices. What is its output?
  2. Run new devices at same time. Do they all work? Does the system crash?
  3. Look for errors when multiple devices are working at same time.
  4. Power supplies are rated to supply a certain number of watts. The actual load on a system can be calculated:
    1. Determine the number of volts drawn by a component.
    2. Multiply the volts times the amperes drawn by the component. This gives watts.
    3. Add the watts together for all system components.

Do not add so many components to a system that you exceed the wattage available from the power supply. In fact, the text recommends that you should try to run the system at (or under) 60% of the capacity of the power supply. Also, do not exceed the power supply capacity of the outlets you are using. It is tempting to plug everything you have into a large UPS or surge protector, forgetting that this will cause all the devices plugged into it to draw power from the same circuit.


A common problem is that technicians can waste a lot of time troubleshooting the wrong part of a computer. Before making a plan for troubleshooting a problem, gather information to make that plan:

  • Get a description of the problem, including all present characteristics.
  • Find out when the problem started, and what else happened then or just before.
  • What was running when the problem happened? What was installed or changed about that time?
  • What physical changes took place? Was the computer moved? Is there a potential problem with power?
  • Was there an electrical event recently? Thunderstorm? Power spike or outage?
  • If the system is still running, cna the problem be reproduced?

The text offers a general decision map about what to check based on a series of questions.

  1. Does the PC boot properly?
    1. If yes, troubleshoot the subsystem that has problems.
    2. If no, is the screen blank?
      1. If no, is there an error message on the screen?
        1. If yes, troubleshoot based on the message.
        2. If no, eliminate all devices and software you can from the boot process, and add them one at a time until the error is found.
      2. If yes, is the hard drive running, the fan running, or are lights on?
        1. If no, troubleshoot the power supply.
        2. If yes, does the computer beep just once during the boot? (One beep is normal.)
          1. If yes, troubleshoot the video system.
          2. If no, troubleshoot the motherboard. (Beep codes differ from one BIOS to another, but several usually mean a motherboard problem.)

Power related problems are not always complex. Sometimes it is as simple as something being unplugged. If the user moved or rearranged components, check to see if they are all plugged in and turned on. This extends to the electrical service to the room where the computer is located. Check the circuit breakers, fuses, etc. by testing the power outlet used for the computer.

If the user has installed new components, and has an ATX motherboard, check whether the user may have disconnected a wire that should run from the main power switch to a connector on the motherboard. The wire may be labeled REMOTE SW, and the connection on the motherboard may be labeled PWR.SW. If this wire is not connected on an ATX motherboard, the system will not boot.

Power supplies can be damaged by overheating. It is a good maintenance procedure to periodically clear a computer of accumulated dust and debris that may be pulled into it by a fan. The recommended method for clearing dust out of computer components is to use compressed air. Some people use small vacuums, but this can cause problems with electrostatic discharge from the vacuum itself.

If you smell something hot or burnt, suspect overheating. A basic fact about computer cases is that air is meant to flow through them in a particular way. The airflow will be impaired if the user removes a component, and leaves a part of the case open that was not meant to be open. This may seem counter-intuitive, since the computer can get more air now, but it may mean that some components are not having any air move across them, resulting in improper cooling and overheating. If there are plates missing from the front or back of the computer, replace them.

Although it may be difficult to do, you can try to improve cooling in the computer case by bundling cables together. If you understand the intended airflow in the case, check to see if it is blocked by ribbon cables that might be turned or moved. Also, keep the PCI slot next to an AGP card vacant. This means that you have a slot you can't use, but it will allow the AGP card to cool more efficiently, and will prevent having a PCI card overheated by the AGP card.