power supply unit

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.

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. Te 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.

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.

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

Power Supply Connections

On the side of the power supply is the on/off switch for the computer. Sprouting out of the other side are the power connectors. Most power supplies have two connectors that are sometimes labeled P8 and P9 that connect to the motherboard; other nonstandard power supplies use a single connector. Looking at P8 connected to P9, you might wonder just why IBM designed a power supply interface to the motherboard that can so easily cause troubles; after all, if you reverse P8 and P9, as some of my friends have done in the past, then you irreparably smoke the motherboard. Why didn’t IBM use a single larger connector? Oddly enough, IBM did; the first prototype PC-which never saw production had a single large connector from the power supply to the motherboard. But, in the final days of setting up the new microcomputer for production, the company supplying the connectors went out of business, and the only way to not get behind was to just use two connectors that were readily available from another vendor. Since then, some other companies have gone to a single-connector design, but that hasn’t become part of the de facto PC standard, sadly Power supplies also sport from two to four identical Molex connectors for attaching to drives (hard disks, floppy disks, and tape drives), and most modern power supplies also include the smaller Berg connectors used to power 3.5-inch floppy drives. The motherboard receives power through the power strip in the northeastern corner of the board. (This assmes that you are looking at the board so that the memory chips are closest to you and the expansion connectors and the keyboard connector are farthest from you.)

P8 has only five wires, and connects above P9, which has six wires. For a detail of an 8088 motherboard; it’s the same for even the newest Pentium Pro clone motherboard. The power supply lines – the yellow, blue, and red wires – can be tested against a ground – any of the black wires. If you are actually testing a power supply, all of the black wires should be tested.

power supply unit