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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0001:  Power Supplies - System Board

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II  The Relationship Between Power Supplies and System Boards
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IV  Power Supply Form Factors
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III  Power Supplies

The term power supply in the PC is actually a bit of a misnomer. Today’s power supply is a sealed box, with a cooling fan and power leads. Inside the power supply box is a step down transformer that converts 120 or 240 volts AC to approximately 12 and 5 volts AC. From this point, the AC voltage is run through four diodes filtering off the negative voltage. This is known as a bridge rectifier. Newer power supplies use switching transformers in place of the diodes to accomplish this. They are known as switching power supplies.

 

From there, the power is passed to a capacitor, which outputs clean DC power. Inside the case of the power supply are only two more parts. They are a fuse and a fan. The fan is used to pull air through the computer case as well as to cool components of the power supply itself.

Figure 18: How power is converted from household current to 5 and 12 Volts DC. Voltage steps down from transformer on the left to 12 Volts AC, then the four diodes in the center (the diamond shape) convert to positive only voltage. The --| (-- is the symbol for the capacitor that filters the AC ripple to straight DC voltage. The same process occurs for the 5 Volts.

(Actually, the modern PC power supply is actually a switched power supply. See
http://www.smpstech.com/tutorial/t01int.htm#SMPSDEF for details. The process described above is still popular with “wall warts” used for telephones, small CD players, etc.)

 


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II  The Relationship Between Power Supplies and System Boards
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IV  Power Supply Form Factors
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