Hardware Systems
© Copyright Brian Brown, 1992-2000. All rights reserved.

Part 5: Power Supplies

Introduction
Power supplies are devices that provide the energy necessary to operate the unit that they are associated with. For example, batteries are the power supply for a torch or walkman radio. Computers may be powered by both batteries or mains power. For example, a portable notebook can operate on either a battery or mains power.

This module looks briefly at power supplies in relationship to computers.

1: Power Supplies
A power supply supplies power. In computers, the power supply converts the electrical power from the wall socket (250V AC, or 120V AC) into that which can be used by the computer.

Computers use the following power voltages. In addition, modern Pentium based computers also require 3.3V

The power supply unit fitted in the PC has a rating, measured in watts. This wattage rating is used as a measure of defining how many drives and adapter cards can be powered by the power supply.

For instance, a 200 watt power supply can supply more power than a 150 watt power supply. Each adapter card and disk drive in the computer draws power, and if the power supply cannot supply it, it becomes overloaded and turns itself off (or in some cases, burns out).

Large 250 watt power supplies are required for computers with multiple hard disk drives.

1.1 The Power Good Signal
The PSU (power supply unit) should, if properly designed, not allow the computer to function unless all the correct voltage levels are present. After the unit is turned on, the PSU performs an internal check and if all the levels are correct, sends a signal to the computer motherboard, known as Power_Good.

If the motherboard does not get this signal, it will not run. Some cheaper power supply units do not provide a genuine Power_Good signal.

1.2 Power Supply Connectors
First lets look at the connection points for power to the system unit and to the monitor. Most PCs today have 2 sockets at the back for power and these usually sit one above the other.

The picture on the right shows the 2 power sockets on the PC on the left hand side and the connecting leads or plugs that go into these sockets on the right. The bottom plug and socket is where the power is sent to the PC from an outside source (a power socket in the wall). The top plug and socket is where power is supplied to the monitor from the PC.

These sorts of configuration means that the Monitor doesn't have to be supplied with power from a second external source (another power socket in the wall).

Note that some of the plugs and sockets have pins and others have holes that accommodate pins. We refer to any plug or socket that has pins as being type Male, and any plug or socket that has holes to accommodate pins as being type Female. This is regardless of the number of holes or pins.

1.3 Orientation of Connectors
To prevent incorrect insertion of connectors, you will note on closer inspection that sockets are generally keyed, which means plugs can only be inserted into the socket one way. Incorrectly inserted plugs could cause significant damage, not only to the connector, but also the computer or device being attached.

Sometimes, orientation is achieved by having different numbers of pins on the top or bottom of the socket, little cut-away parts, or a larger width at the top compared to the bottom of the socket. You must ensure the plug you are inserting into the socket is the correct type and is the correct orientation.

1.4 Disk Drive Power Connector
The following table lists the pins and signals associated with the disk drive power connector.

1.5 Switched Mode Power Supplies
Virtually all power supplies used in modern computers are called "switched mode". The use a high speed oscillator to generate the required output voltages, which is directly derived from the mains power without the use of a transformer.

One characteristic of switched mode power supplies is they do not supply power unless being connected to something. If they are disconnected and turned on, they can quickly become damaged, or will shut off.

In order to test these power supplies, they must be connected to a load of some sort.

1.6 Voltage Regulation
This refers to how the output of the power supply is controlled, within allowable limits. If the load placed on the power supply exceeds its ability to supply that power, the voltage levels begin to drop and this can cause damage to the computer hardware.

Installing too many adapter cards in a computer with a low rated power supply is an example of exceeding the power rating of the power supply, and in this case, the power supply cannot maintain the correct power levels to ensure the computer can function correctly.

1.7 CMOS Battery Backup
Most computers have a small battery (sometimes replaceable) which is used to keep vital system information held in CMOS RAM active. If the battery wears out, this vital system configuration is lost. Some batteries are rechargeable, and whilst the computer is turned on, these batteries are continually recharged. When the computer is turned off for extended periods, the battery might lose all its power. A typical battery of this type could last several months before losing its power.

1.8 Portable Battery Packs for Laptops
Laptop computers have rechargeable batteries for powering the computer when mains supply is not available. Most rechargeable batteries have a limit to the number of recharge cycles (often about 1000 recharges).

The batteries should be recharged once the power level reaches a low stage. They should not be recharged all the time, for instance, recharging the batteries when the power level is already high or even half way will shorten the life of the battery.

You should let the battery reach minimum charge before recharging it, and then recharge it to the full level before using it again.

2: Power Supply Problems
The following summarizes some of the problems associated with the mains derived power supply. All can cause the computer to either shutdown or reboot, causing loss of data and potential corruption of open files.

2.1 Blackout
A blackout is a total loss of power.

2.2 BrownOut
A brownout is a momentary lowering of the power level below its correct level, lasting less than one second.

2.3 Surge
A surge is a momentary raising of the power level above its correct level.

2.4 Spike
A spike is an instaneous raising of the power level above its correct levl, and is very short in duration.

3: Power Supply Protection Devices
The following summarizes some of the common devices used in protecting computers from power supply problems.

3.1 Circuit Breaker
A circuit breaker is a device which activates when the current drawn from the power exceeds a set level. This prevents damage to the equipment and power supply.

3.2 Line Conditioner
This is a device which protects the computer from variations in the power supply, such as spikes and brownouts. It is connected to the wall outlet, and the computer is then plugged into it.

3.3 Surge Suppressor
This device protects the computer against surges.

3.4 UPS: Uninterruptible Power Supplies
This device, which incorporates batteries, provides protection against total loss of power. If this was to happen to servers, which require proper shut down procedures in order to save data and close open files, severe damage could result.

A UPS, when it detects that power is lost, continues to provide power to the computer from internal batteries. It also provides a signal to the computer that power has been lost (either via a dedicated hardware card or rs232 port). The computer then can start its orderly shutdown procedure.

Most UPS systems can supply a computer with power for about 10-15 minutes before the batteries run down. Thus, its important that the computer recognize that power has been lost and shut itself down. Normally, the UPS system is provided with software which performs this task.

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© Copyright Brian Brown, 1992-2000. All rights reserved.