Power Supply and Cases

ObjectivesIn this chapter, you will: -Learn about the various power supply form factors used in PCs today

-Study the connectors used with power supplies, including a connector used by one major manufacturer that could destroy your motherboard if you’re not careful

-Learn how to understand power supply ratings and specifications

-Troubleshoot power supply problems – which are oft-overlooked culprit when tracking pesky system problems

-Learn how to test, remove and install a power supply

-Study power protection systems, surge suppressors and uninterruptible power supplies

Power Supply

Physically attached to case

Connects to Motherboard

Converts AC to DC at required Voltages

AT Connection ATX Connection

Primary Function and Operation

The basic function of the power supply is to convert the type of electrical power available at the wall socket to the type the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 115-volt (nominal) 60Hz AC (alternating current) or 230V (nominal) 50Hz AC power into +3.3V, +5V, and +12V DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch.

Power Supply Form Factors

The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. In the PC market, IBM originally defined the standards, and everybody else copied them. Intel gave the power supply a new definition in 1995 with the introduction of the ATX form factor.

Power Supply Form Factors

Technically, the power supply in your PC is described as a constant voltage half-bridge forward converting switching power supply.Seven main power supply physical form factors have existed that can be called industry standard. Five of these are based on IBM designs, whereas two are based on Intel designs. Of these, only three are used in most modern systems; the others are pretty much obsolete.

LPX Style LPX systems were designed to have a smaller footprint and a lower height than AT-sized systems. These computers used a different motherboard configuration that mounts the expansion bus slots on a "riser" card that plugs into the motherboard.

NLX Style

The NLX specification, also developed by Intel, defines a low-profile case and motherboard design with many of the same attributes as the ATX. In fact, for interchangeability, NLX systems were designed to use ATX power supplies, even though the case and motherboard dimensions are different.

SFX StyleIntel released the smaller Micro-ATX motherboard form factor in December of 1997, and at the same time also released a new smaller SFX (Small form factor) power supply design to go with it.

Motherboard Power Connectors

Every PC power supply has special connectors that attach to the motherboard, giving power to the system processor, memory, and all slotted add-on boards (ISA, PCI, AGP).Attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard.

AT Power Supply Connectors

These supplies feature two main power connectors (P8 and P9), each with 6 pins that attach the power supply to the motherboard.

ATX Main Power Connector The industry standard ATX power-supply to motherboard main connector is the Molex 39-29-9202 (or equivalent) 20-pin ATX style connector

ATX style 20-pin motherboard main power connector

Power Switch ConnectorsThree main types of power switches are used on PCs. They can be describes as follows: Integral power supply AC switch Front panel power supply AC switch Front panel motherboard controlled switch

Power supply remote push button switch connections

Peripheral Power Connectors

In addition to the motherboard power connectors, power supplies include a variety of peripheral power connectors for everything from floppy and hard drives to internal case fans.

Peripheral and Floppy Drive Power Connectors

The disk drive connectors on power supplies are fairly universal with regard to pin configuration and even wire color.

Power connector

Floppy power connector

Peripheral Connectors

A peripheral female power connector

To determine the location of pin 1, look at the connector carefully.

It is usually embossed in the plastic connector body; however, it is often tiny and difficult to read.

Fortunately, these connectors are keyed and therefore difficult to insert incorrectly.

Power Supply Ratings A system manufacturer should be able to provide you the technical specifications of the power supplies it uses in its systems. The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. IBM reports output wattage level as "specified output wattage."

Power Supply RatingsIf your manufacturer does not list the total wattage, you can convert amperage to wattage by using the following simple formula: Watts = volts X amps

For example, if a motherboard is listed as drawing 6 amps of +5V current, that would be 30 watts of power, according to the formula.

By multiplying the voltage by the amperage available at each output and then adding the results, you can calculate the total capable output wattage of the supply.