In a personal computer the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan.
Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are:

• orange = +3.3 volts digital section [ram,processor] 15 A aprox.
  
• red = +5 volts digital section [drive circuits] 35 A aprox
  
• yellow = +12 volts servo section = motor [drives,fans] 15 A aprox
  
• white = -5 volts .5A aprox
  
• blue = -12 volts 1 A aprox
  
• violet [purple] = +5 vsb [voltage standby]
  
• green = power on [ you can also test the power supply manually by linking green + black [any black]
  
• grey = power good [PG]
  
• black = zero volt [ground / negative]

importent
There are more than one wire with the same colour contain the same volt [v] and the same current [i]
but share the total wattage[w]
for more details
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satheesanonakkunnu@yahoo.com

The 3.3- and 5-volts are typically used by digital circuits, while the 12-volt is used to run motors in disk drives and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps. If you have been around PCs for many years, you probably remember that the original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or off, you knew you were doing it. These switches actually controlled the flow of 120 or 220 volt power to the power supply. Today you turn on the power with a little push button, and you turn off the machine with a menu option. These capabilities were added to standard power supplies several years ago. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.
Switcher Technology Prior to 1980 or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 or 220 volts and 60 or 50 hertz into 5 volts and 12 volts DC. The switching power supplies used today are much smaller and lighter. They convert the 60or 50 Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

In this photo you can see three small transformers (yellow) in the center. To the left are two cylindrical capacitors. The large finned pieces of aluminum are heat sinks. The left heat sink has transistors attached to it. These are the transistors in charge of doing the switching -- they provide high-frequency power to the transformers. Attached to the right heat sink are diodes that rectify AC signals and turn them into DC signals.

A switcher power supply draws only the power it needs from the AC line. The typical voltages and current provided by a power supply are shown on the label on a power supply.

Personal computer power supply label. VSB is the standby voltage provided to the power switch.

Switcher technology is also used to make AC from DC, as found in many of the automobile power inverters used to run AC appliances in an automobile and in uninterruptible power supplies. Switcher technology in automotive power inverters changes the direct current from the auto battery into alternating current. The transformer uses alternating current to make the transformer in the inverter step the voltage up to that of household appliances (120 -220VAC).

Power Supply Standardization Over time, there have been at least six different standard power supplies for personal computers. Recently, the industry has settled on using ATX-based power supplies. ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard. PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply.
Advanced Power Management
Advanced Power Management (APM) offers a set of five different states that your system can be in. It was developed by Microsoft and Intel for PC users who wish to conserve power. Each system component, including the operating system, basic input/output system (BIOS), motherboard and attached devices all need to be APM-compliant to be able to use this feature. Should you wish to disable APM because you suspect it is using up system resources or causing a conflict, the best way to do this is in the BIOS. That way, the operating system won't try to reinstall it, which could happen if it were disabled only in the software.

Power Supply Wattage A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity. According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a personal computer are:
Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.
Power Supply Problems The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a PC receive their DC power via the power supply. A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan, which allows components in the power supply to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason. For any problems you suspect to be the fault of the power supply, use the documentation that came with your computer. If you have ever removed the case from your personal computer to add an adapter card or memory, you can change a power supply. Make sure you remove the power cord first, since voltages are present even though your computer is off

Power Supply Improvements Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New designs offer fan control so that the fan only runs the speed needed, depending on cooling needs. Recent designs in Web servers include power supplies that offer a spare supply that can be exchanged while the other power supply is in use. Some new computers, particularly those designed for use as servers, provide redundant power supplies. This means that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.



Power Supply Functions and Signals The power supply's main function is simple: take the power input to it from the power system of your home or office, and turn it into a form that the PC can use. However, the PC power supply's job isn't as simple as that of a standard power converter, such as the kind you might plug into your car's lighter socket to let you run a TV off your car battery. The PC power supply must provide several different voltages, at different strengths, and must also manage some additional signals that the motherboard uses. This section takes a look at what the power supply does and the voltages and secondary signals it generates. In addition to the topics covered here, power supply output and ratings are covered in this section. Power signal quality issues are covered in the section on power supply electrical specifications.

AC-DC Voltage Conversion The electricity you get from your utility company is in the form of alternating current (AC), while the electricity your PC requires is direct current (DC). (Read this page of the electrical basics section for more details on AC and DC.) Therefore, the primary function of your power supply is to convert your wall AC power into a DC form that your PC can use. In fact, the supply normally provides several different voltage levels, to meet the demands of different components in the machine. In fact, while almost everything in your home runs off standard AC power, many devices actually use DC internally. Just a few examples of this are telephone answering machines, many types of audio equipment, some kinds of battery chargers, and in the PC world, certain types of printers, external modems and other peripherals. Two indicators that a device actually uses DC inside are: the ability of the device to run on batteries, and the presence of a device outside the unit that powers it. These small "bricks" with one plug for the wall and another for the device are often called AC adapters (sometimes the "term of endearment" wall wart is used, in honor of the way they are usually designed to hang right out of a wall socket. :^) ). AC adaptors are really DC power supplies that converts the AC of the wall into DC for the device. The difference between this sort of DC power supply and the kind in your PC is the design. AC adapters are linear power supplies. These supplies are cheap and simple to make. The main problem with them is that they are tremendously wasteful; typically, 50% or more of the energy supplied to one of these adapters is wasted as heat. You can feel this readily--just touch the adapter when the unit is working; many become quite hot to the touch. A hot AC adapter means electricity is being wasted. This is an acceptable compromise for small appliances, but unacceptable for a PC power supply. Instead of the linear design, PCs use switching power supplies. (The full name for this sort of design is actually "constant-voltage, half-bridge forward-converting switching power supply", but don't worry, that won't be on the final exam. :^) ) Explaining in detail how the design works would take many paragraphs and make your eyes gloss over unnecessarily. In a nutshell, the switching power supply uses a transistor switch and a closed feedback loop to produce DC output that is properly regulated regardless of the load on it, with only the amount of AC power required to draw the DC load being taken from the utility. The main advantage of a switching power supply is that it is far more efficient than a linear design. When you are dealing with hundreds of watts of power, this is a more serious issue than when you are talking about an answering machine. The second advantage is that all the energy wasted in the power supply as heat has to be removed by the PC's cooling system. Therefore, more efficient power supplies produce less heat that the system has to exhaust. The main disadvantage of a switching supply is that it generates high-frequency signals within it as part of its conversion process, which can radiate out of the unit and cause interference to other electronic devices (inside or outside the PC). For this reason, you will always see PC power supplies encased in metal boxes for shielding. The power factor of a device refers to the ratio of the actual power used by the device to the product of the current and voltage supplied to it. Traditional power supplies have a power factor of about 0.6 to 0.7. The power factor is important especially for determining the sizing of UPSes as well as circuits that supply larger units. Some newer power supplies, especially larger ones for servers, have additional circuitry added to correct the power factor of the supply. These are, unsurprisingly, called power-factor-corrected supplies, and have a power factor of (or near) 1.0. They make UPS sizing either easier or more confusing, depending on how you look at it :^) Another reason why power-factor correction is being added to some supplies is that low power factor devices, if used in sufficient quantities, cause problems for electrical utilities. In some parts of the world the utility companies are starting to impose surcharges on companies with excessive loads at low power factors, though this is not really a concern for a home PC user.