Whether you own a desktop or laptop, your computer needs to be powered somehow. But since a computer is full of different types of electronic circuits, several of which operate at different voltages, a simple connection to a wall outlet simply will not suffice. So how do we convert the electricity coming from the wall to something that the computer can use? A power supply was designed for this very purpose.
What Does A Power Supply Do?
A power supply is one of the most important pieces of your computer system. For a desktop PC, it converts the electricity coming from your wall outlet (in the U.S., that is 110-120 volts Alternating Current, 60 Hz) into four different flavors that your computer commonly uses, all Direct Current (DC) power: - 5 volts, +5 volts, +12 volts and -12 volts.
In addition to supplying the computer's power needs, a desktop computer's power supply also provides much-needed cooling with its power supply fan. Some computers (such as servers, gaming systems and high-end PCs) have multiple power supplies with multiple fans. This addresses two needs: the increased need for power due to high-speed or high-workload components, and increased cooling capacity, since high-speed and high-workload components generate a lot of heat.
A laptop power supply, however, is slightly different. The job of converting electrical power from a wall socket into something a laptop can use is divided between two components: the external power adapter and the internal power supply. A laptop power adapter converts the electricity from a wall outlet to something the internal power supply can use. Typically this is a higher DC voltage than a desktop power supply furnishes. The adapter converts the power you get at the wall outlet and converts it to another voltage, often between 10 and 18 volts DC. From there, the internal power supply further converts the voltage to suit the needs of the computer.
AT Power Supply
There are two major types of computer power supplies: AT and ATX.
An AT power supply is what was used for most older computers. This type of power supply powered the first personal computers made by IBM, and the standard was adopted for other manufacturers as well. Specifically, it powered all AT and AT-compatible motherboards.
The AT-compatible motherboard obtained its power from a special two-part power connector from the AT power supply. This power connector contained four +5 volt DC wires, four ground (0 volt) wires, one - 5 volt wire, one +12 volt wire and one -12 volt wire. The remaining wire was a signal wire that allowed the power supply to tell the motherboard that "Power is good." With an AT power supply, you were required to manually turn off your computer by pressing the power switch (which generally was a dedicated On/Off switch).
ATX Power Supply
With advances in software and operating systems, computers could do more, such as enter "Power-Save" or "Sleep" mode. Software could now be used to turn off the computer, rather than having to turn off a computer with a power switch. This has all been made possible by use of ATX power supplies and ATX-compatible motherboards. The ATX power supply, therefore, is more complex.
An ATX power supply has more outputs that connect to an ATX-compatible motherboard. Whereas the AT power supply only had 8 outputs, the ATX power supply uses either 20 or 24 outputs. Most ATX power supplies take into account that an ATX motherboard can contain either 20 or 24 outputs, so the extra 4 outputs are often split out as a separate plug that will only fit one way into a 24 output motherboard connector.
The ATX power supply also supports more voltage settings, and is capable of accepting signals from the ATX-compatible motherboard other than simply "Power Good." The 24-pin ATX power supply has the following number of outputs, all voltages DC: three +3.3 volt, eight ground (0 volt), five +5 volt, one -5 volt, two +12 volt, one -12 volt and four "signal" wires ("Power Good," "+5 volt standby," "+3.3 volt sense" and "Power on").
Warning
AT and ATX power supplies are incompatible with each other's motherboards. Trying to modify an ATX power supply to work with an AT-compatible motherboard will be unsuccessful. The extra inputs on an ATX power supply will not allow the power supply to function properly, since there are more "signal" inputs on an ATX power supply than there are on an AT motherboard.
Similarly, an ATX-compatible motherboard will not work with an AT power supply because there are too few varieties of voltage available. All computer processors that are Pentium MMX generation or newer use a primary (core) voltage of +3.3 volts DC. There are no +3.3 volt outputs on an AT power supply, so any attempt to power an ATX motherboard with an AT power supply will damage the computer processor.
Which Type Does My Computer Use?
If you have a computer that was manufactured after 1997, chances are that you have an ATX power supply. Most modern operating systems (post-Windows 98) support the advanced power functions that an ATX power supply/motherboard combination can offer, such as Power Save/Sleep mode. If your computer was manufactured previous to 1997, it's much more likely that you have an AT power supply.
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