Power Supply Unit / How to Buy

 Basic items we will cover in this Presentation:

Power Supply Unit (PSU) intro

How to buy
Power Supplies in Common Use
Things to remember

Intro

If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just a box full of plastic and metal. The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer.
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:

• 3.3 volts
• 5 volts
• 12 volts 

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.

We turn on the power to our PC with a little push button, and you turn off the machine with a menu option. 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 on your PC will work.

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.

Items needed before determining PSU

1. Case

2. CPU

3.MOBO

4.RAM

5.Hard Drives

6.Video Card

7.NIC

8.CD/ DVD

9.Sound Card

Calculate the watts for 1-9 to give you and idea of what size PSU to shop for. 

Website to help Calculate your watts

 http://www.extreme.outervision.com/psucalculatorlite.jsp

Good Rule of thumb, is to never buy 50 over or 50 under the watts you need. 


Three factors to consider when buying a PSU

• 1. Determine the wattage you need.    

Don't purchase a PSU just above your requirements unless you plan not to upgrade the system. Also, PSUs age, losing power over time. Purchase a PSU that will take you through your next few upgrades, over a multiple-year period.

Power Supply Wattage

  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.

Here is an example of watts needed for some PC components 

PC Item	Watts
Accelerated Graphics Port (AGP) card	20 to 30W
Peripheral Component Interconnect (PCI) card	5W
small computer system interface (SCSI) PCI card	20 to 25W
floppy disk drive	5W
network interface card	4W
50X CD-ROM drive	10 to 25W
RAM	10W per 128M
5200 RPM Integrated Drive Electronics (IDE) hard disk drive	5 to 11W
7200 RPM IDE hard disk drive	5 to 15W
Motherboard (without CPU or RAM)	20 to 30W
550 MHz Pentium III	30W
733 MHz Pentium III	23.5W
300 MHz Celeron	18W
600 MHz Athlon	45W

• 2. Research which connectors you need.

Check your MOBO to determine which connector you need. Newer PSUs will often provide both a 24-pin ATX connector that doubles as a 20-pin connector. Higher-end models may only provide a 24-pin connector, and lower-end models may provide just a 20-pin connector.

• 3. Look for PSUs with high-efficiency ratings

And, ones rated under load temperatures, not room temperatures. Anything 80% and above is good. At 83%, approximately 17% of the wattage is lost as heat. Therefore, a PSU that may be advertised as a 500W PSU, will actually be drawing almost 600W at the wall. Efficiency drops over time and during the life of the PSU. A year-old PSU is most likely not capable of producing the same amount of energy it once did when it was new.

Examples of 3 types of power supply in common use:

• AT Power Supply - still in use in older PCs.
• ATX Power Supply - commonly in use today.
• ATX-2 Power Supply - recently new standard.

The voltages produced by AT/ATX/ATX-2 power supplies are:

• +3.3 Volts DC (ATX/ATX-2)
• +5 Volts DC (AT/ATX/ATX-2)
• -5 Volts DC (AT/ATX/ATX-2)
• +5 Volts DC Standby (ATX/ATX-2)
• +12 Volts DC (AT/ATX/ATX-2)
• -12 Volts DC (AT/ATX/ATX-2) 

note: A power supply can be easily changed and are generally not expensive, so if one fails (which is far from uncommon) then replacement is usually the most economic solution.

 Things to remember

Always replace a power supply with an equivalent or superior power output (Wattage).

Never open a power supply. A power supply contains capacitors that hold a charge even while a computer is turned off; this discharge can seriously injure the user.

Check the warranty. Compare each manufacturer's guarantee, return policy, and customer-service history. There are some PSU manufacturers that release great products with horrible support, and vice versa.