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Primary Function and Operation

The basic function of the power supply is to convert the type of electrical power available at the wall socket to the type the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 115-volt (nominal) 60Hz AC (alternating current) or 230v (nominal) 50Hz AC power into +3.3v, +5v, and +12v DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch.

Positive DC Voltages

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. Table 3.1 lists these devices and their power consumptions.

Table 3.1 Power Consumption Ratings for PC Devices


Devices Powered


Chipsets, DIMMs, PCI/AGP cards, miscellaneous chips


Disk drive logic, SIMMs, PCI/AGP cards, ISA cards, voltage regulators, miscellaneous chips


Motors, voltage regulators (high output)

The power supply must deliver a good, steady supply of DC power so that the system can operate properly. Devices that run on voltages other than these must be powered by onboard voltage regulators. For example, RIMMs run on 2.5v that is supplied by an onboard regulator, and processors are supplied by a voltage regulator module (VRM) that normally is built into the motherboard as well.


When Intel began releasing processors that required a +3.3v power source, power supplies that supplied the additional output voltage were not yet available. As a result, motherboard manufacturers began adding voltage regulators to their boards, which converted +5v current to +3.3v for the processor. When other chips began using 3.3v as well, Intel created the ATX power supply specification, which supplied 3.3v to the motherboard. Dual In-line Memory Modules (DIMMs) also run on +3.3v as supplied by the power supply. You would think that having 3.3v direct from the power supply would have eliminated the need for onboard voltage regulators, but by that time, processors began running on a wide variety of voltages lower than 3.3v. Motherboard manufacturers then included adaptable regulator circuits called Voltage Regulator Modules (VRMs) to accommodate the widely varying processor voltage requirements.

Negative DC Voltages

If you look at a specification sheet for a typical PC power supply, you can see that the supply generates not only +3.3v, +5v, and +12v, but also –5v and –12v. The positive voltages seemingly power everything in the system (logic and motors), so what are the negative voltages used for? The answer is, not much! Some of the power supply designs, such as the small form factor (SFX) design, no longer include the –5v output for that reason. The only reason it has remained in most power supply designs is that –5v is required on the Industry Standard Architecture (ISA) bus for full backward-compatibility.

Although –5v and –12v are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3v, +5v, and +12v. The –5v is simply routed to the ISA bus on pin B5 so any ISA cards can use it. Today, though, not many do. However, as an example, the analog data separator circuits found in older floppy controllers do use –5v.

The motherboard logic normally doesn't use –12v either; however, it might be used in some board designs for serial port or LAN circuits.


The load placed on the –12v output by an integrated LAN adapter is very small. For example, the integrated 10/100 Ethernet adapter in the Intel D815EEAL motherboard uses only 10mA of +12v and 10mA of –12v (0.01 amps each) to operate.

Although older serial port circuits used +/–12v outputs, today most run on only +3.3v or +5v.

The main function of the +12v power is to run disk drive motors as well as the higher-output processor voltage regulators in some of the newer boards. Usually, a large amount of +12v current is available from the power supply, especially in those designed for systems with a large number of drive bays (such as in a tower configuration). Besides disk drive motors and newer CPU voltage regulators, 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 and 250mA (0.1–0.25 amps); however, most newer fans use the lower 100mA figure. Note that although most fans in desktop systems run on +12v, portable systems can use fans that run on +5v, or even +3.3v.

Most systems with newer motherboard form factors, such as the ATX, micro-ATX, or NLX, include another special signal. This feature, called PS_ON, can be used to turn the power supply (and thus the system) on or off via software. It is sometimes known as the soft-power feature. PS_ON is most evident when you use it with an operating system, such as Windows 9x, that supports the Advanced Power Management (APM) or Advanced Configuration and Power Interface (ACPI) specification. When you select the Shut Down the Computer option from the Start menu, Windows automatically turns off the computer after it completes the OS shutdown sequence. A system without this feature only displays a message that it's safe to shut down the computer.

The Power_Good Signal

In addition to supplying electrical power to run the system, the power supply also ensures that the system does not run unless the power 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 power supply voltages are within the proper ranges.

The power supply completes internal checks and tests before allowing the system to start. If the tests are successful, the power supply sends a special signal to the motherboard, called Power_Good. This signal must be continuously present for the system to run. Therefore, when the AC voltage dips and the power supply cannot maintain outputs within regulation tolerance, the Power_Good signal is withdrawn (goes low) and forces the system to reset. The system will not restart until the Power_Good signal returns.

The Power_Good signal (sometimes called Power_OK or PWR_OK) is a +5v (nominal) active high signal (with variation from +2.4v through +6.0v generally being considered acceptable) that is supplied to the motherboard when the power supply has passed its internal self tests and the output voltages have stabilized. This normally takes place anywhere from 100ms to 500ms (0.1–0.5 seconds) after you turn on the power supply switch. The power supply then sends the Power_Good signal to the motherboard, where the processor timer chip that controls the reset line to the processor receives it.

In the absence of Power_Good, the timer chip holds the reset line on the processor, which prevents the system from running under bad or unstable power conditions. When the timer chip receives the Power_Good signal, it releases the reset, 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 the power output returns to its proper levels, the power supply regenerates the Power_Good signal and the system again begins operation (as if you had just powered on). By withdrawing Power_Good before the output voltages fall out of regulation, the system never sees the bad power because it is stopped quickly (reset) rather than being allowed to operate using unstable or improper power levels, which can cause memory parity errors and other problems.


You can use the Power_Good feature as a method of implementing a reset switch for the PC. The Power_Good line is wired to the clock generator circuit, which controls the clock and reset lines to the microprocessor. When you ground the Power_Good line with a switch, the timer chip and related circuitry reset the processor. The result is a full hardware reset of the system. Upgrading and Repairing PCs, 6th Edition, which is located on this book's CD, contains instructions for making and installing a reset switch.

On pre-ATX systems, the Power_Good connection is made via connector P8-1 (P8 Pin 1) from the power supply to the motherboard. ATX and later systems use pin 8 of the 20-pin connector, which is normally a gray wire.

A well-designed power supply delays the arrival of the Power_Good signal until all the voltages stabilize after you turn on the system. Badly designed power supplies, which are found in many low-cost systems, often do not delay the Power_Good signal properly and enable the processor to start too soon. (The normal Power_Good delay is 0.1–0.5 seconds.) Improper Power_Good timing also causes CMOS memory corruption in some systems.


If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem.

Some cheaper power supplies do not have proper Power_Good circuitry and might 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 the result of improper Power_Good signal timing. A common example is when you replace a motherboard in a system and then find that the system intermittently fails to start properly when you turn on the power. This can be very difficult to diagnose, especially for the inexperienced technician, because the problem appears to be caused by the new motherboard. Although it seems as though the new motherboard is defective, it usually turns out that the power supply is poorly designed. It either cannot produce stable enough power to properly operate the new board or has an improperly wired or timed Power_Good signal (which is more likely). In these situations, replacing the supply with a higher-quality unit, in addition to the new motherboard, is the proper solution.

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