PC Hardware Lab
you can download the PC-Lab notes from the below link:-
--> http://www.sgdmgdc.com/Genxtechno/PC_Lab.pdfyou can download the PC-Lab notes from the below link:-
POWER SUPPLY OF PC
A Short History Of PC Power Supply Voltage Rails
Let's clear up any confusion about the term "rail". A voltage "rail" refers to a single voltage provided by the PSU (short for power supply - actually it stands for power supply unit). An ATX PSU has one 3.3 volt rail. It also has one 5 volt rail. The 3.3 volt rail has its own circuitry in the PSU which generates the voltage. It also has a lot of wires and connectors to distribute the 3.3 volts to any hardware which needs it. The 5 volt rail has its own separate set of circuitry, wires, and connectors to deliver 5 volts. Modern ATX12V PSUs may have as many as four separate 12 volt rails. Each 12 volt rail has its own set of wires and connectors just like the 3.3 and 5 volt rails. The 12 volt rails just happen to be generating the same voltage as each other. If you'd like to have a look at the official ATX specifications they can be found at formfactors.org.
The original IBM PCs drew most of their power from two voltage rails: 5 volts and 12 volts. Their power supplies also provided -5 and -12 volts but those only delivered small amounts of power. They had a 5 volt rail because that was the voltage needed to power most of the standard silicon chips of the time. The 12 volt rail was used primarly to operate fans and floppy disk drive motors. The original PC PSU could deliver a maximum 63.5 watts most of which was on the 5 volt rail. As time passed, PCs included bigger, faster chips which increased the load on 5 volts. People also added new-fangled devices like hard disks and eventually CD-ROM drives so the 12 volt rail had to deliver more power too. But the 5 volt rail still continued to deliver most of the power because most of the power is consumed by the chips.
Chip technology improves by cramming larger numbers of smaller transistors onto the chips. As transistors shrink they need to operate on lower voltages. When the newer ATX standard was created, a 3.3 volt rail was added to power the newer chips. So a PC of the time had a mix of 3.3 volt and 5 volt chips directly connected to their respective voltage rails. The table below shows the rail sizes from an old 300 watt ATX PSU. Most of the wattage is delivered on the 3.3 and 5 volt rails. It also has a fairly powerful 12 volt rail for computers with multiple disk drives.
300 Watt ATX PSU
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Voltage
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Maximum current
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Maximum wattage
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+3.3 volts
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20.0 amps
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66 watts
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+5 volts
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30.0 amps
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150 watts (180 watts maximum combined +5 and +3.3)
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+12 volts
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10.0 amps
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120 watts
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5 volts standby
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1.0 amps
|
5 watts
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-5 volts
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0.5 amps
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2.5 watts
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-12 volts
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0.8 amps
|
0.96 watts
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