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Power Supply

Power Supply Page

The power supply is usually a silvery box at one corner of your case. It takes power from a power cord and has a connection to apply monitor power for those monitors having the correct kind of power cord. It will have a power switch that turns both it and a connected monitor off.

Power is provided through wires and connectors to the mother board and disk drives. There are positive and negative 12 volts direct current, +/-5VCD and ground. All voltages are in reference to chassis voltage (chassis ground). In other words, if you have a 230Watt power supply and touch the +12Volt connection and ground(any metal part of the case), the power supply will do its level best to provide your body with twelve volts at 230Watts. This can kill you. Don't do it.

Motherboard Connection

The motherboard cable normally is in two parts. Each has a connector that fits onto the motherboard power header. They should only fit on facing one way. Because they are identical, the only way to KNOW that you are putting them onto the right parts of the motherboard power header is if you read and follow your motherboard manufacturer's instructions. However, as a general rule, the black wires of both connectors go toward the center.

Disk Drive Power

Power is also provided to the disk drives. These four-pin connections can only be plugged in one way. The square "bottom" of the connector will not fit into the round "top" of the drive power jack. If it doesn't fit, it doesn't go that way. Some 3.5" floppy drives use a smaller, pin-type connector. An adapter from a computer store has a large jack that takes the large connector and a small plug that fits into the small jack on the 3.5" disk drive. Again, the plug should fit in only one direction.

Processor Fan

Advanced processors require additional cooling. They get hot and if not cooled, they pop/fry/disintegrate/meltdown/slag/char/whatever. The cooling fan requires power or pop/fry etc. happens. Power normally is stolen from a disk drive power connector. Read and follow the instructions Fan without cable included with your fan and motherboard. By putting the power supply fan power adapter in line with a drive power cable, both the drive and fan get power. If not, then there is a problem with the connection or your cabling. Shut down. Then try again or get help.

Cableless fans attached to the processor get their power from the motherboard.

Expansion boards get their power from the motherboard.

A word about static discharge


Transistors are made up of two diodes with a part in common. They are designed to work in a range of zero volts direct current to something like 7 volts. This will get you in the ball park. A zener diode is a diode that runs backward (reverse biased). It regulates voltage by "breaking down" as voltages rise. That is, if voltage goes over 5 volts, its resistance goes down so voltage goes down too. As more voltage is applied its resistance goes down further until it can't any more. Then it gets hot, like the resistor on an electric stove, turns red and becomes a smoking mass of former zener diode. In the forward bias direction, a diode is happy to allow current to pass unrestricted. If enough current passes, however, the diode gets hot and the electric stove effect occurs. This, all in the range of, oh say, 30VDC, assuming a good robust diode.

Integrated circuit chips may contain thousands of transistors or more.

Now, we get to static discharge. Ever touch something metal and get a shock? Yes. The voltage in that contact is more than 1000 volts DC. Compare that with 30VDC. 33 times greater. Pop goes the transistor. You don't even feel most electrostatic discharges that you make. Perhaps one of 100 VDC. Get the picture?


So, now we get back to our power supply. When working inside the computer, always disconnect power from the computer. Since phone lines also carry 12 VDC (500VDC when ringing, ouch, now you know WHY it knows to ring) you always disconnect modem connectors too! Before touching anything else, you should touch the power supply or metal that is connected to the power supply. Remember, every voltage present on the motherboard is relative to ground (chassis voltage). Properly grounding yourself means that you will NOT shock your computer into inoperability.

One recommended method is to use a grounding strap. It electrically connects your wrist to the computer chassis.

a strap

By the same token, when handling expansion boards, touch first the metal spoon that screws to the back of the case. This is called grounding yourself to the card. Always ground yourself to (touch) the surface you are going to place the expansion board on before putting it down. The same goes for any semiconductor device you have to handle such as a memory SIMM or chip.

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Updated January 24, 1998, 10:52pm.