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Introduction
Epox EP-8KHA+ with Socket 462 Mainboard

The Power Supply Unit also known as the P.S.U provides a steady flow of power to the computer neccessary for it to function. Just like a car has an engine to motor around, the PC has a PSU to control the power of its internal engines.

The PSU can be easily identified in a computer system. Take a look at the back of your computer and you should see a medium sized silver or gray box with an opening containing the PSU fan and electrical circuitry, as well as an electrical voltage switch and one or more cages for ventillation purposes. Inside these ventillation cages are fans that the PSU uses to dissipate hot air throughout an entire computer.

The responsibility of the power supply unit is to supply power to all devices within the system through the use of connectors. A connector is simply a cable extending outwards to reach devices in the system. The power supply must be attached to some electrical outlet usually located in a wall in your home or office.

The PSU has a central fan that circulates air through a system to prevent peripheral devices from over heating. The fan is dependant on a good supply of air to operate efficiently because the fan pulls air through the power supply through the vent at the front and back sides of a case, and it circulates this air throughout the entire system and then sends air back out through the main supply fan vent.

Computer power supplies are rated by wattage, or the amount of power they can produced. Typical power supplies come in 220 watt configurations, but some may go as high as 500 watts.

The older Baby AT style power supplies were the most common type of power supply suitable for powering AT boards only. They lacked support for ATX motherboards and as a result were quickly phased out by the newer, better ATX technology.

The ATX motherboard requires a special ATX power supply connector which contains special circuitry that allows software to control the power signal of the supply.

The pages contained within this section will provide a fundamental understanding of how the power supply works as well as explain the main characterisitics that seperate the good power supplies from the bad.

 
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Epox EP-8KHA+ with Socket 462 Mainboard
Introduction What's On Board? How It Works PSU Wires
Connectors Illustrated Replacement Procedures Upgrade Procedures PSU Voltage And Frequency Levels
How It Works
How The Power Supply Operates

The power supply in your computer converts the standard 120 volt power from your wall, into a format that a computer can work with. The power supply then distributes power accordingly to all devices in the system based on how much power the system actually needs. Because the power inside a wall is an AC current, the power supply must be converted into a DC current which can be recognized by the PC. The motherboard, expansion cards, and drives typically use either 5V or 12V as their power supply connector. The disk motors and cooling fans receive about 12V of power each.

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Connectors Illustrated

4 Pin Connector (MOLEX)

Standard ATX Molex ConnectorA power connector such as the one depicted here is referred to as a Molex 4 Pin Connector and is used to power a hard disk drive or CD-ROM drive. The yellow wire supplies 12V of power, the black is a ground wire, and the red supplies 5V of power.

4 Pin Mini Plug Connector (BERG)

Standard ATX Molex ConnectorA power connector such as the one depicted here is referred to as a Mini Plug connector normally used to power floppy disk drives, internal zip drives, and even some PCI-Express video cards.
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How To Replace A Dead Power Supply Unit

The first step in the replacement of your power supply is choosing an appropriate supply that will fit your case. Replacing the power supply is a fairly straightforward task. This installation is assuming you are removing a dysfunctional power supply.

  1. Shut down the computer and power off the machine. This will involve turning off the power button on the front of the computer, and if you have a newer ATX power supply there will be an additional switch you have to turn off.
  2. Disconnect all power cables and connectors from all devices in the system.
  3. Remove any screws attaching the power supply to the unit casing.
  4. Before removing the power supply it may be helpful to document any connectors in the system and the devices they are connected. This will be helpful when reconnecting the cables on the new power supply.
  5. Gently slide out the power supply unit which typically involves you pulling the unit out from either the front or back side.
  6. After you have removed the old power supply, you are ready to install the one. It may be helpful in the future to record the make, model, and total wattage of your new power supply.
  7. Insert the power supply into the metal bracket and secure it with the provided screws.
  8. Reconnect all power cables and connectors. These include the P8 and P9 motherboard power connectors, any hard and floppy disk cables and connectors, and any other peripheral devices you may need to attach a power connector controller cable to.
  9. If all cables are securely attached to all required devices within the system, the installation is complete. If you cannot boot up your system with the new power supply, then you may have attached a cable incorrectly, or it may be possible that your new power supply is defective.
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What's On Board?
What Does It Look Like?
Standard ATX Horizental Desktop Case
Graphical representation of a standard Baby AT Power Supply. This type of power supply is normally not seen in any newer style personal computers but can still have a practical use in Pentium II or higher equipped machines.
Hardware Architecture

The power supply unit consists of six part types as described in the table below:

Part Type Description
Power Switch Used to manually power on and off a computer system. This is useful if you are troubleshooting or diagnosing a PC or if you are building a system entirely from scratch. You can also use this power switch in case a mishap occurs, and your system requires immediate shutdown because of a hardware failure.
Voltage Selector Switch Used to control the amount of wattage running through the system. It's important to set this standard at an appropriate level based on the electrical system in your home as setting this switch to higher then what it is supposed to run can damage components and cause system failure.
Air Vents The air vents are neccessary to help a continuous supply of air flow through the system to keep it running at a reasonable temperature. Without air vents the power supply can become overheated and affect the rest of the peripherals in the machine. Air vents are to peripheral devices like oxygen is to human beings. Without an efficient airflow system and a method for disapating heat in a system, components are likely to malfunction after only a short period of time. Air vents are also the most predominat way dust gets into a system so it is highly recommended to check that these elements remain dust free every few months. Cleaning these elements can be done with a can of compressed air.
Fan Exhaust The Fan Exhaust element of a power supply is always used to keep the core temperature of the power supply unit to a bare minimum. It also provides cooling to other peripherals in a system.
Power Inlet (Female) The Power Inlet (Female) connector can normally be used to power an additional peripheral device such as a monitor or other device that uses an appropriate cable or connector. This connector is more common on older power supply units.
Power Inlet (Male) The power inlet (male) connector is where the power cable plugs in. The other end of the cable plugs in directly to an electrical wall outlet.

 

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PSU Voltage And Frequency Levels
A power supply's usage is measured in voltages, wattages, and frequencies. This reflects the actual amount of power that the PSU directs to each component in the PC. The table below summarizes the most common peripheral devices and the required power wattages each element requires to function properly:
 
Component PowerVoltage Required
CPU 3.5 - 6 volts
Motherboard 15-20 watts
Hard Drive 5-50 watts
Floppy Drive 3-20 watts
Expansion Card 5-10 watts
CPU Fan
Memory Chips 3 or 5 volts
CD-Rom Drive 10-30 watts
Power Voltages of Various Devices in the Personal Computer
 
As you can see from the table listed above, not all devices use the same number of watts or voltage levels so it is important for you to make sure all connectors are in there proper places. Typically, most power connectors will fit into only one place within a system, but there may be a slim possibility that you can plug in connector to the wrong device or in the wrong direction. This is particularly crucial with memory chips. If you set the voltage level for memory chip to high, there is a possibility high possibility the chips could be damaged.
 
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Power Supply Wires

Common Power Supply Wires And Associated Purposes

Standard ATX Horizental Desktop Case

There are several connectors associated with power supplies. There are normally four color coded wires on a connector and each is explained in the following table:

Wire Color Purpose
Brown and Blue These wires are the feed wires from the 110V supply. They bring the full 110 volts to the power supply switch.
Black and White These wires carr the AC current from the switch to the power supply. These wires only heat up when the system is active because the switch actually closes the circuit.
Green These wires are used as a ground wires.
Red  
Common power supply wires and their associated purposes.
It is important to note that when connecting the brown and blue wire, you must make sure they are parallel and adjacent to each other. You place the black and white wires on the tabs that are angled and located next to each other. The black and brown wires must always be next to each other.
Wire Colors, Pin Sizes, And Signal Frequencies
The connectors on a power supply remain generic except in terms of size. The following table explains the types, sizes, and voltages of power connectors:
Pin Wire Color Signal
1 Yellow + 12 V
2 Black Ground
3 Black Ground
4 Red +5 V
Types and Wattage Of Wire Cables in a Power Supply Connector
It is important to note that when connecting the brown and blue wire, you must make sure they are parallel and adjacent to each other. You place the black and white wires on the tabs that are angled and located next to each other. The black and brown wires must always be next to each other.

 

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