Power Supplies
The computer needs electrical power to operate. It is plugged into a 110-volt socket, which uses Alternating Current (AC). Computers use 5- and 12-volt direct current (DC) electricity. Therefore, a device is required to convert the power from the wall socket to the power required by the computer. This is the function of a power supply. Power supply connectors are either the Molex type (square shape) or the Bundy type (rectangular shape).
Although power supplies are rated in different ways, the most common rating is in watts. A watt is a unit of measurement of an electrically powered device and is affected by the electricity that is passed through it. Because computer components are becoming increasingly more efficient with large-scale integration and low-power designs, a 200 or 250 watt power supply is typically large enough for most systems. Do not be surprised, however, to see a 300-or-more-watt power supply in some of the larger network servers with large hard drives and other peripherals.
Power supplies are most often found in a rear corner of the inside of the computer, and have several colored wires running out of them. These are called power connectors and are used to connect peripherals such as floppies, tape backups, or hard drives. The power supply will most likely have a fan attached. Although some newer designs do not, most still use the fan in order to maintain an internal temperature acceptable to the design of the circuitry. The fan moves warm air outside the computer and brings cool air in.
The power supply will have an AC line that will connect into the wall. It will also have connectors for external peripheral devices. Usually there are three or more disk drive connectors and two system board connectors. Hard drives and 5-1/4" floppy drives use the same connector; 31/2" drives use a smaller one. Older models of computers typically connected the monitor to the power supply. Newer models of monitors plug directly into the wall, because they require more power than the power supply connector can provide.
Getting a bit more technical, the AC current is converted to DC in the power supply. There are four voltages present in a power supply: +12 volts, -12 volts, +5 volts, and -5 volts. The +12 voltage is for the motor, and the +5 voltage is for the processor. These are the four levels of electricity the computer needs to operate. Different devices use different voltage levels. The power supply not only converts these, but also ensures that the flow of electricity into the computer is received evenly, without bursts or sags.
Because of the voltages used and because not all technicians are certified electricians, repairing power supplies is not recommended. Replacement is the safest option. However, the wiring for the connectors can be tested for proper voltage levels.
The following table shows specifications for the output wiring of the power supply.
Wire Voltage Proper Range Current Range (Amps)
Yellow +12 +8.5 - +12.6 0.0 - 2.00
Blue -12 -8.5 - -12.6 0.0 - 0.25
Red +5 +2.4 - +5.2 2.3 - 7.00
White -5 -4.5 - -5.4 0.0 - 0.30
The specifications may be tested with a multimeter, which has a dial so you can specify what you want tested. You can select DC Volts, AC Volts, or Amps. Later in the course, we will demonstrate the use of multimeters.
Selecting Power Supplies
When selecting power supplies to be used as on-hand spares, when building PCs from component subsystems, or when given the option on vendor-built systems, you need to give some thought to what you are selecting. While all power supplies use the same input voltages and provide the same DC output voltages, there are some differences, which may influence your decision.
Peak Power
This determines the amount of power available for short term, high current situations, as it occurs when you first turn on the system. Large hard disks can increase your peak power requirements.
Number and type of connectors
You will need a connector for each hard disk and floppy disk you install in your system. Both hard disks and 5-1/4" floppy disk drives use the same power connector.
Many 3-1/2" disk drives use a different type of connector. There are jumper plugs available that convert one type of connector to the other.
For today's PCs, the minimum rating for a power supply is 200 to 250 Watts.
Power Requirements
Listed below are the average power requirements that you can expect from many common devices. Several factors can affect these requirements. Always check vendor documentation for information on specific devices.
Device AC Current Requirements at 120 Volts Power Requirements
System Board &expansion cards 0.80 Amps 96.0 VA
Diskette drive and adapter 0.16 Amps 19.2 VA
Additional diskette drive 0.05 Amps 6.0 VA
Hard Disk* 0.13 Amps 15.6 VA
* Average value for 3 1/2" hard disk. Increase to 40 VA for a 5 1/4" hard disk. VA (Volt Amps) is a measure of power and is equal to WATTS.
The computer needs electrical power to operate. It is plugged into a 110-volt socket, which uses Alternating Current (AC). Computers use 5- and 12-volt direct current (DC) electricity. Therefore, a device is required to convert the power from the wall socket to the power required by the computer. This is the function of a power supply. Power supply connectors are either the Molex type (square shape) or the Bundy type (rectangular shape).
Although power supplies are rated in different ways, the most common rating is in watts. A watt is a unit of measurement of an electrically powered device and is affected by the electricity that is passed through it. Because computer components are becoming increasingly more efficient with large-scale integration and low-power designs, a 200 or 250 watt power supply is typically large enough for most systems. Do not be surprised, however, to see a 300-or-more-watt power supply in some of the larger network servers with large hard drives and other peripherals.
Power supplies are most often found in a rear corner of the inside of the computer, and have several colored wires running out of them. These are called power connectors and are used to connect peripherals such as floppies, tape backups, or hard drives. The power supply will most likely have a fan attached. Although some newer designs do not, most still use the fan in order to maintain an internal temperature acceptable to the design of the circuitry. The fan moves warm air outside the computer and brings cool air in.
The power supply will have an AC line that will connect into the wall. It will also have connectors for external peripheral devices. Usually there are three or more disk drive connectors and two system board connectors. Hard drives and 5-1/4" floppy drives use the same connector; 31/2" drives use a smaller one. Older models of computers typically connected the monitor to the power supply. Newer models of monitors plug directly into the wall, because they require more power than the power supply connector can provide.
Getting a bit more technical, the AC current is converted to DC in the power supply. There are four voltages present in a power supply: +12 volts, -12 volts, +5 volts, and -5 volts. The +12 voltage is for the motor, and the +5 voltage is for the processor. These are the four levels of electricity the computer needs to operate. Different devices use different voltage levels. The power supply not only converts these, but also ensures that the flow of electricity into the computer is received evenly, without bursts or sags.
Because of the voltages used and because not all technicians are certified electricians, repairing power supplies is not recommended. Replacement is the safest option. However, the wiring for the connectors can be tested for proper voltage levels.
The following table shows specifications for the output wiring of the power supply.
Wire Voltage Proper Range Current Range (Amps)
Yellow +12 +8.5 - +12.6 0.0 - 2.00
Blue -12 -8.5 - -12.6 0.0 - 0.25
Red +5 +2.4 - +5.2 2.3 - 7.00
White -5 -4.5 - -5.4 0.0 - 0.30
The specifications may be tested with a multimeter, which has a dial so you can specify what you want tested. You can select DC Volts, AC Volts, or Amps. Later in the course, we will demonstrate the use of multimeters.
Selecting Power Supplies
When selecting power supplies to be used as on-hand spares, when building PCs from component subsystems, or when given the option on vendor-built systems, you need to give some thought to what you are selecting. While all power supplies use the same input voltages and provide the same DC output voltages, there are some differences, which may influence your decision.
Peak Power
This determines the amount of power available for short term, high current situations, as it occurs when you first turn on the system. Large hard disks can increase your peak power requirements.
Number and type of connectors
You will need a connector for each hard disk and floppy disk you install in your system. Both hard disks and 5-1/4" floppy disk drives use the same power connector.
Many 3-1/2" disk drives use a different type of connector. There are jumper plugs available that convert one type of connector to the other.
For today's PCs, the minimum rating for a power supply is 200 to 250 Watts.
Power Requirements
Listed below are the average power requirements that you can expect from many common devices. Several factors can affect these requirements. Always check vendor documentation for information on specific devices.
Device AC Current Requirements at 120 Volts Power Requirements
System Board &expansion cards 0.80 Amps 96.0 VA
Diskette drive and adapter 0.16 Amps 19.2 VA
Additional diskette drive 0.05 Amps 6.0 VA
Hard Disk* 0.13 Amps 15.6 VA
* Average value for 3 1/2" hard disk. Increase to 40 VA for a 5 1/4" hard disk. VA (Volt Amps) is a measure of power and is equal to WATTS.
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