Thread: Voltages????
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Old 3rd June, 2003, 04:28 AM
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Southern Man Southern Man is offline
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I thought I would ad this to the thread.

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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.
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Though the course may change,
rivers will always reach the sea.
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Last edited by Southern Man; 3rd June, 2003 at 04:31 AM.
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