Power Supply Unit

Power Supplies

As a certified computer technician we tend to learn the basic information about power supplies for computer systems, we will often be call on to troubleshoot when needed. And we had a reason why we take a look at the different variations of power supplies? Of course, we try to identify the proper type, and how to connect it to motherboard and related devices. We also examine the safety considerations related to working with power supplies and if we tend to replace it when needed. A standard power supply draws power from a local, alternating current (AC) source usually in a wall outlet and converts it to either 3.3 or 5 volts direct current (DC) for on-board electronics, and 12 volts DC for motors and hard drives. In all cases, it delivers both positive and negative DC to the computer. Power supplies must , smoothing out the fluctuations power or any radical changes in its quality. Most PC power supplies also provide electricity to the system's cooling and processor fans that keep the machine from overheating. Most supplies have a universal input that will accept either 110 volts alternating current (VAC), 60 hertz (Hz) (U.S. standard power), or 220 VAC, 50 Hz (European and Asian standard). When replacing a power supply, there are three things to consider: physical size, wattage, and connectors.

NOTE:

A hertz is a measure of unit frequency: 1 cycle per second equals 1 hertz. A kilohertz (kHz) is 1,000 cycles per second; a megahertz (MHz) is 1 million cycles per second. Power Supply Sizes

Power supplies are available in a few standard sizes and shapes. They are based on the type of case in which they are installed and the types of motherboard connections they will support. This is because different styles of cases place items such as plug fittings, mounting screws, and fans in different places, and motherboard styles offer different connections and placement of plugs. These cables and fittings must be compatible to work together. The very first PCs all used the same type of power supply with a large, red switch on the side. Variations were manufactured for early portables and tower cases, with longer or shorter cables and different types of switches. They all shared a common pair of motherboard connectors. Collectively they are known as AT-style. When replacing an AT-style power supply, you generally only need to be concerned about the type of case that it will go in. The exceptions are some high-end network servers, which sometimes allow for an extra power lead to the motherboard. All newer desktop PCs (Pentium II and later) and servers use ATX-style power supplies. The ATX design simplifies motherboard connection by combining the two power leads in the AT-style power supply into one. The main issues to be aware of are how much wattage the PC needs to power its parts and how many peripheral connectors are required. Generally speaking, older Pentium-based computers and all 486-based and earlier PCs used AT-style supplies; almost all Pentium II and later-based systems use ATX-style supplies. The ATX design is preferable for two reasons:

1. The on–off power control circuit (not the button) on ATX boards is built into the motherboard. On AT-style PCs, it comes from the power supply.
2. AT-style power supplies connect to the motherboard through a pair of six-wire connectors. ATX-style power supplies connect through a single 20-pin connector.

When replacing a power supply, it's a good idea to compare the existing power supply to the new one. Make sure that they are physically the same size, have the same connectors, and that the new one has at least the same power rating. Some high-quality power supplies offer "silencer" fans that are much quieter than most models.

Power Supply Wattage

Power supplies are rated according to the maximum sustained power (measured in watts) that they can produce. A watt is a unit of electrical power equivalent to one volt-ampere. It is important to keep in mind that the power supply must produce at least enough energy to operate all the components of the system at the same time. You can determine a computer's power consumption by adding the power requirements, measured in watts, for all the devices in the unit. When evaluating a power supply, however, don't rely on the computer's operating consumption alone. Remember that a much larger drain occurs as the machine powers up, when hard drives and other heavy feeders simultaneously compete for the available startup power. Most general-use computers require 130 watts while running and about 200–205 watts when booting (at startup). Sound cards, modems, and (worst of all) monitors attached with an accessory plug in the case can push a weak power supply to its limit and beyond. Servers and high-performance workstations often have an abundance of random access memory (RAM), multiple drives, SCSI (Small Computer System Interface) adapters, and power-hungry video cards, along with one or more network cards. They often demand power supplies of 350–500 watts.

CAUTION

The label on a power supply that says "Don't Open" means just that! Opening a power supply is dangerous. It is better to completely remove and replace a defective power supply as needed.

Power Supply Connectors

Power supplies employ several types of connectors, all of which are easy to identify and use. On the outside of the computer enclosure, a standard male AC plug and three-conductor wire (two power wires and a ground) draws current from a wall outlet, with a female connection entering the receptacle in the back of the power supply. There are three types of connectors on the inside: the power main to the motherboard (which differs, as mentioned, in AT and ATX models) and two types of four-pin fittings to supply 5 volts and 3.3 volts of power to peripherals such as the floppy disk and hard disk drives. Let's take a close look at each in turn.

Two types of main power connectors: 1. AT-Style Connections to the Motherboard

A pair of almost identical connectors, designated P8 and P9, links the power supply to the motherboard. These connectors are seated into a row of six pins and matching plastic guides, or teeth, on the motherboard. The P8 and P9 connectors must be placed in the proper orientation. The motherboard manual will show which fittings are for P8 and P9. If the connectors are not marked, AT (advance technology) Switching Power Supply

NOTE :

Some computer makers employ proprietary power connections that require a special power supply. To install a new part in these types of computers, you will need to follow the instructions that come with the computer. Remember to install the P8 and P9 plugs so that the black wires are side by side. Installing them on the wrong receptacle can damage both the motherboard and the power supply. Figure 5.2 shows the P8 and P9 connectors and a motherboard.

2. ATX-Style Motherboard Connections

The newer ATX main power connection, found on Pentium II computers and later, is much easier to install. A single 20-pin plug is set into a fitted receptacle and secured with a catch on the side of the plug that snaps over the fitting. Figure 5.3 shows how to properly seat the connection. A small, flat-tip screwdriver is a handy tool for easing the pressure on the catch to remove the plug. In some cases, you can use a screwdriver to ease installation as well. Note; show pictures

ATX (Advance Technology Extended) Switching power supply

Connections to Peripheral Hardware

Two standard types of connectors are used to connect the power supply to peripheral hardware:

1. Molex connector. This is the most commonly used power connector. It provides both 12-volt and 5-volt power. Hard disk drives, internal tape drives, CD-ROM drives, DVD (digital video disc) drives, and older 5.25-inch floppy disk drives all use this fitting. The Molex connector has two rounded corners and two sharp corners to ensure that it installs properly.

2. Mini connector. Most power supplies provide one or more mini. The mini is used primarily for 3.5-inch floppy disk drives. It has four pin-outs and, usually, four wires. Most are fitted with keys that make it difficult, but not impossible, to install upside down. Be sure to orient the connector correctly; applying power with the connector reversed can damage or destroy the drive.

Two- and Three-Pin Mini Plugs A less common type of power connector is used to connect the fan of a Pentium II or III processor to the motherboard for power, to connect a CD-ROM drive to a sound card, and to provide power for 3.5-inch floppy disk drives. These connectors have two or three wires that are usually red and black or red, yellow, and black.

CAUTION Do not connect power-carrying mini plugs to audio or data devices such as a CD drive or a sound card, because you could damage or destroy those devices.

Extenders and Splitters PCs can run out of power connections, and large cases can have drives beyond the reach of any plug on the supply. A good technician has a quick solution on hand to both of these common problems: extenders and splitters. Extenders are wire sets that have a Molex connector on each end; they are used to extend a power connection to a device beyond the reach of the power supply's own wiring. Splitters are similar to extenders, with the exception that they provide two power connections from a single power supply connector.

Power Supply Problems

Power supply problems can come from both internal and external sources. Component failure within a computer can cause a power supply to fail, but the most common failures come externally from the power source itself. In this lesson, we look at common problems associated with power supplies and what you, as a technician, can do about them. Power Failures

Power supplies are affected by the quality of the local power source. Common power delivery problems such as spikes, surges, sags, brownouts, and blackouts affect the stability and operation of the main power supply and are passed on to the computer. Although most users don't notice sudden changes in the quality of electrical power, computers and other sensitive electronics are affected. Although we can't fully control these problems, there are a few things we can do, noted in the following table, to protect our equipment and data and ensure a reasonably clean electrical supply.

Power Protection Devices

Surge suppressors are devices used to filter out the effects of voltage spikes and surges that are present in commercial power sources and smooth out power variations. They are available from local computer dealers and superstores. A good surge suppressor will protect your system from most problems, but if you purchase an economy model, it might not work when you need it most. Keep in mind that almost nothing will shield your hardware from a very close lightning strike.

Two devices protect against external power problems:

1. Surge suppressors

Most power strips with surge protection have a red indicator light. If the light goes out, this means that the unit is not providing protection. These types of surge suppressors need to be replaced every year or so. If the indicator light starts flashing before then, it means the power strip is failing and should be replaced immediately. These units protect up to a point; however, for complete protection from power fluctuations and outages, an uninterruptible power supply (UPS) is recommended.

2. Uninterruptible power supply (UPS)

A UPS is an inline battery backup. When properly installed between a computer and the wall outlet, a UPS protects the computer from surges and acts as a battery when the power dips or fails. It also provides a warning that the power is out of specification (above or below acceptable levels). Many models can also interact with the computer and initiate a safe shutdown in the event of a complete power failure using software that runs in the background and sends a signal through one of the computer's COM ports when the power goes down. The amount of time that a UPS device can keep a system running is determined by battery capacity and the power demands of the equipment connected to it. A more powerful UPS device will need its own line and circuit breaker. One of the principal power drains is the monitor. To keep a system online as long as possible during a power failure, turn off the monitor immediately after the failure commences. When considering a UPS, take into account how much protection is needed, as well as the importance of peace of mind to the user. The VA rating (voltage × amps = watts) must be sufficient to supply the computer and all its peripherals with power for enough time to safely shut down the system. The easiest way to calculate this number is to add the power rating (watts) for all pieces of equipment that are to be connected to the UPS, as shown in the following.

caution Never plug a laser printer into a UPS unless the UPS is specifically rated to support that type of device. Laser printers often require more power than a UPS is able to provide, potentially placing the printer, the UPS, and the computer at risk.

Power Supply Problems

The most easily recognized problem is a complete failure of the power supply. This is easy to detect because, in the event of a failure, the computer will not boot up (no lights, no sound). If there is apparently no power, be sure to check the power source and the plug at both ends: the outlet and the computer. If you are experiencing intermittent failures such as memory loss, memory corruption, or unexplained system crashes, don't rule out the power supply—it is often the culprit. Fortunately, it is easy to check and replace. Good power supplies have line-conditioning circuits, but these might not be sufficient in locations where the power source has substantial quality flaws. If you have problems with several systems, or if a second power supply still does not fix a related complaint, add a UPS with good line-conditioning features. Most power grids in the United States provide current that is far from ideal for sensitive electronic components. Line-conditioning hardware added in the chain just before the current reaches the machine adds a much needed level of protection from spikes (very fast jumps in power levels), surges (longer ones), and drops. All can cause transient problems with operations. These are often erroneously blamed on the operating system or software.

NOTE: For more information and clarification with regard on this topics, feel free to read “A+ Certification Training Kit / Microsoft Corporation.--3rd Ed.” PUBLISHED BYMicrosoft PressA Division of Microsoft CorporationOne Microsoft WayRedmond, Washington 98052-6399 Copyright © 2001 by Microsoft Corporation