Chemistry First Paper

Lesson 9 : Power Supply Unit 

9.1. Learning Objectives
On completion of this lesson you will be able to describe:
 Different types of power supply
 Power supply connections.

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9.2. Introduction 

Personal computers (at least, desktop PCs) don’t come with batteries included. You plug them into the wall socket and they work. The PC itself does not directly use wall current, as this is 220-240 volt alternating current. The PC doesn't need AC; it needs DC, usually 3.3 or 5 volts for its chips, 12 volts for the motors on older drives-newer drive motors run off 5 volts. Anyway, I just said that the PC uses DC, but the wall sockets provide AC power. How does the PC convert the power? The power supply actually doesn't supply power-it converts it from AC to DC. 

9.3. Types of Power Supply 

There are two kinds of power supplies, linear power supplies and switching power supplies. The PC's power supply is, in every PC I've ever seen, a switching power supply. Both linear and switching power supplies have their positive and negative aspects. 

9.3.1. Linear Power Supply 

Linear power supplies are based on transformers. That makes them hot, heavy and impervious to changes in current levels, while rendering them vulnerable to voltage swings. Linear power supplies are an older design than switching power supplies and you still find them on monitors and some external drive cases. Even small linear power supplies generate a relatively large amount of heat, which is why you should never cover the holes atop a monitor; you can fry a monitor quickly that way. 

9.3.2. Switch Mode Power Supply 

Switching power supplies are digital in nature. They step down voltage by essentially "switching" it on and off, hence their name. Think of how they work in this way: suppose you had a 1000-watt bulb in a light, but you only wanted the lighting value of a 100-watt bulb. You could get 100 watts' worth out of the 1000-watt bulb by switching it on and off, but leaving it off 90 percent of the time. I know it sounds goofy, but if you could switch the light on and off quickly enough, then you'd never see it flicker. (In fact, that's how fluorescent lights work. They're actually very bright, but they flash off and on 60 times per second, too quickly for most eyes to register and they're off over 90 percent of the time.) Switching power supplies are less sensitive to fluctuations in input voltage, although they are still a problem. These power supplies generate heat, but a lot less of it than linear power supplies.