POWER SUPPLY SYSTEMS Lesson Note
Learning Objectives
This SS2 Computer Hardware & GSM Repair lesson note covers POWER SUPPLY SYSTEMS for First Term, Week 4.
Lesson Note
Learning Objectives
At the end of the lesson, students should be able to:
1. Define a computer power supply system.
2. Explain the importance of power supply in a computer.
3. Define Power Supply Unit (PSU).
4. Identify the major components of a computer power supply system.
5. Explain the functions of a PSU.
6. Differentiate between AC and DC power.
7. Explain the difference between linear and switching power supplies.
8. Identify common types of computer power supply units.
9. Explain common power supply problems and their symptoms.
10. State basic safety precautions when working with power supply systems.
A. MEANING OF POWER SUPPLY SYSTEM
A power supply system is the part of an electrical or electronic system that provides the appropriate electrical power required for the system to operate.
In a computer, the power supply system provides electrical energy to components such as: Motherboard, CPU, RAM, Hard disk drive, Solid-state drive, Cooling fans, Graphics cards, Optical drives and Other internal components
A computer cannot operate properly without a suitable and stable power supply.
POWER SUPPLY UNIT (PSU)
A Power Supply Unit (PSU) is a hardware component in a computer that converts electrical power from the mains supply into the voltages an’ currents required by the computer’s internal components.
Desktop computers normally use a PSU located inside the system case.
The PSU receives Alternating Current (AC) from an electrical outlet and provides suitable Direct Current (DC) outputs for the computer’s electronic components.
IMPORTANCE OF A POWER SUPPLY SYSTEM
The power supply system is important because it:
1. Provides electrical power to computer components.
2. Converts electrical power into suitable forms for the computer.
3. Helps maintain stable voltage levels.
4. Distributes power to different components.
5. Provides protection against certain electrical problems.
6. Allows the computer to start and operate.
7. Supports the proper functioning of storage devices, fans and other components.
AC AND DC POWER
1. Alternating Current (AC)
Alternating Current (AC) is electrical current that periodically changes direction.
The electricity supplied through ordinary household electrical outlets is AC.
A computer’s PSU receives AC power from the mains and converts it into suitable DC power.
2. Direct Current (DC)
Direct Current (DC) flows in one direction.
Most electronic components inside a computer operate using DC power.
Examples of DC-powered computer components include:
- CPU
- RAM
- Motherboard circuits
- SSD
- HDD electronics
- Cooling fans
DIFFERENCE BETWEEN AC AND DC
AC| DC
Current changes direction periodically| Current flows in one direction
Commonly supplied by electrical mains| Commonly used by electronic circuits
Supplied to a computer PSU| Supplied by the PSU to internal components
Suitable for power distribution over electrical grids| Commonly used by electronic devices
MAIN FUNCTIONS OF aA COMPUTER PSU
The major functions of a PSU include:
Conversion of Electrical Power
The PSU converts AC electricity from the wall outlet into appropriate DC voltages for computer components.
Voltage Regulation
It helps provide stable output voltages to the computer’s components.
Power Distribution
The PSU distributes electrical power through different cables and connectors to components such as the motherboard, drives and graphics card.
Protection
Modern PSUs may provide protection against certain conditions such as:
- Over-voltage
- Under-voltage
- Over-current
- Short circuits
- Excessive power or temperature conditions
The exact protection features depend on the PSU design.
Power Control
The PSU works with the motherboard to support computer power-on, shutdown and standby operations.
MAJOR COMPONENTS OF A POWER SUPPLY SYSTEM
A computer power supply contains several important parts.
These may include:
1. Input connector
2. Fuse and protection circuitry
3. EMI filter
4. Rectifier
5. Capacitors
6. Switching circuitry
7. Transformer
8. Output rectification and filtering circuits
9. Cooling fan
10. Output cables and connectors
1. Input Connector
The input connector is where the mains power cable connects to the PSU.
It allows electrical power from an outlet to enter the power supply.
2. Fuse
A fuse is a protective component designed to interrupt the circuit when excessive current flows through it.
It helps protect the power supply from serious electrical faults.
3. EMI Filter
EMI means Electromagnetic Interference.
The EMI filter helps reduce unwanted electrical noise entering or leaving the power supply.
4. Rectifier
A rectifier converts AC electricity into pulsating DC.
The resulting electrical signal is then processed and filtered by other parts of the power supply.
5. Capacitors
Capacitors are electronic components that store electrical energy temporarily.
In a PSU, capacitors are used for purposes such as:
- Filtering
- Smoothing voltage
- Energy storage
6. Switching Circuit
Modern computer PSUs commonly use high-frequency switching circuits to control electrical energy efficiently.
The switching circuit helps regulate the power supplied to the output stages.
7. Transformer
A transformer is used to transfer electrical energy between circuits and provide appropriate voltage levels while maintaining electrical isolation where required.
In modern switching power supplies, a high-frequency transformer is commonly used.
8. Cooling Fan
The cooling fan removes heat generated by components inside the PSU.
Proper cooling helps prevent excessive temperature and supports reliable operation.
9. Output Cables and Connectors
These carry electrical power from the PSU to the computer’s internal components.
Common connectors include:
- 24-pin ATX motherboard connector
- 4-pin or 8-pin CPU power connector
- SATA power connector
- PCI Express power connector
- Peripheral/Molex connector on some older systems
COMMON PSU CONNECTORS
1. 24-Pin ATX Connector
The 24-pin ATX connector is commonly used to supply power to the motherboard.
It provides several voltage rails and control signals needed by the motherboard.
2. CPU Power Connector
CPU power connectors are commonly available as 4-pin or 8-pin connectors, including configurations based on the 4+4-pin design.
They provide dedicated power to the CPU power circuitry on the motherboard.
3. SATA Power Connector
A SATA power connector supplies power to SATA storage devices such as:
- SATA HDDs
- SATA SSDs
- Optical drives
4. PCI Express Power Connector
PCI Express power connectors provide additional power to certain graphics cards and other expansion hardware.
5. Peripheral/Molex Connector
Molex connectors were widely used in older computers to power devices such as certain drives and accessories.
Some modern systems may no longer use them.
TYPES OF COMPUTER POWER SUPPLIES
Computer power supplies can be discussed according to their design and application.
1. AT Power Supply
AT power supplies were used in older generations of personal computers.
They are largely obsolete in modern desktop computers.
2. ATX Power Supply
ATX stands for Advanced Technology Extended.
ATX power supplies became widely used in desktop computers and support modern motherboard power requirements.
They are available in different power ratings and physical sizes.
3. SFX Power Supply
SFX is a smaller power-supply form factor commonly used in compact desktop computer systems.
It Is designed for smaller computer cases where a standard ATX PSU may not fit.
Power Rating of a PSU
A PSU’s capacity is commonly expressed in watts (W).
For example:
- 300 W
- 400 W
- 500 W
- 650 W
- 750 W
- 850 W
The required PSU capacity depends on the power requirements of the computer’s components.
A system with a powerful CPU and dedicated graphics card may require a higher-capacity PSU than a basic office computer.
EFFICIENCY OF A POWER SUPPLY
Power supply efficiency describes how effectively the PSU converts electrical input power into usable output power.
For example, if a PSU receives 500 W from the mains but supplies 450 W to the computer, some energy is lost, mainly as heat.
Higher efficiency generally means:
- Less energy wasted as heat.
- Lower heat production.
- Potentially lower electricity consumption.
- Better overall power efficiency.
MODULAR AND NON-MODULAR POWER SUPPLIES
1. Non-Modular PSU
In a non-modular PSU, the cables are permanently attached to the power supply.
Advantages
i) Usually simpler to install.
ii) No detachable cable compatibility issues.
Disadvantage
i) Unused cables can create cable-management difficulties.
2. Modular PSU
In a modular PSU, some or all cables can be connected or disconnected as required.
Advantages
1. Better cable management.
2. Easier to avoid unnecessary cables.
3. Can improve airflow inside a computer case when cables are properly arranged.
Important Safety Note:
Only cables designed and supplied for the particular PSU should be used. Modular PSU cables are not necessarily interchangeable between different PSU models.
SYMPTOMS OF A FAULTY POWER SUPPLY
A faulty or inadequate PSU may cause symptoms such as:
1. Computer fails to power on.
2. Computer suddenly shuts down.
3. Computer restarts unexpectedly.
4. System becomes unstable.
5. Storage devices fail to operate properly.
6. Fans may not spin.
7. Graphics card may fail to operate correctly.
8. Burning smell or unusual electrical noise may occur in serious faults.
9. The computer may repeatedly attempt to start.
10. System may fail under heavy workload.
These symptoms can also be caused by other faulty components, so proper troubleshooting is necessary.
COMMON CAUSES OF POWER SUPPLY PROBLEMS
Common causes include:
1. Electrical Surge
A sudden increase in electrical voltage can damage power-supply components.
2. Overheating
Poor ventilation or a failed PSU fan can cause excessive heat.
3. Dust Accumulation
Dust can restrict airflow and contribute to overheating.
4. Component Age
Electronic components can deteriorate with prolonged use.
5. Overloading
A PSU that cannot safely provide the power required by the system may cause instability.
6. Short Circuit
A short circuit in the computer or PSU can trigger protective shutdown or cause damage.
7. Poor-Quality Power Supply
A poorly designed or unsuitable PSU may provide unreliable power and inadequate protection.
BASIC POWER SUPPLY TROUBLESHOOTING
When troubleshooting a computer that does not power on, the following basic checks can be performed safely:
Step 1: Check the Wall Outlet
Confirm that the electrical outlet is working.
Step 2: Check the Power Cable
Ensure that the power cable is properly connected and not visibly damaged.
Step 3: Check the PSU Power Switch
Some desktop PSUs have a rear power switch. Ensure it is in the correct position.
Step 4: Check Internal Power Connections
With the computer completely disconnected from mains power, trained personnel can check that the motherboard and component power connectors are properly seated.
Step 5: Check for Signs of Damage
Look for obvious signs such as:
- Burn marks
- Damaged cables
- Unusual smell
- Swollen or leaking components
Step 6: Test With Appropriate Equipment
A qualified technician can use suitable diagnostic equipment, such as a PSU tester or multimeter, following proper safety procedures.
Step 7: Replace a Confirmed Faulty PSU
If the PSU is confirmed to be faulty, it should be replaced with a compatible, adequately rated unit.
POWER SUPPLY SAFETY PRECAUTIONS
Power supplies can contain dangerous electrical voltages, even after a computer has been switched off and unplugged.
Therefore:
1. Do not open a PSU unless properly trained and qualified to work on it.
2. Never touch internal PSU components unnecessarily.
3. Disconnect the computer from mains power before working inside the computer case.
4. Do not work on electrical equipment with wet hands.
5. Keep liquids away from the computer.
6. Do not use damaged power cables.
7. Do not bypass protective devices such as fuses.
8. Use properly grounded electrical equipment where required.
9. Use the correct PSU for the computer.
10. Allow qualified technicians to handle dangerous electrical faults.
Important: A PSU should generally be replaced rather than opened and repaired by an untrained student because hazardous voltages can remain inside it.
PREVENTIVE MAINTENANCE OF POWER SUPPLY SYSTEMS
Proper maintenance can help extend the life of a computer power system.
Preventive measures include:
1. Keep the computer in a clean environment.
2. Ensure adequate airflow.
3. Keep ventilation openings clear.
4. Remove accumulated dust safely.
5. Use suitable electrical protection.
6. Avoid overloading electrical outlets.
7. Use a PSU with adequate capacity.
8. Replace damaged cables.
9. Monitor unusual noises, smells or shutdowns.
10. Keep the computer away from moisture.
IMPORTANCE OF A GOOD QUALITY PSU
A good-quality PSU is important because it:
1. Provides stable power.
2. Helps protect computer components.
3. Supports system reliability.
4. Reduces the risk of power-related instability.
5. Provides appropriate connectors.
6. Can operate efficiently.
7. Provides suitable protection features.
Choosing a PSU should be based on the computer’s actual power requirements and compatibility, not simply on the highest wattage available.
POWER SUPPLY AND OTHER COMPUTER COMPONENTS
The PSU works with several other computer components.
*PSU → Motherboard
The PSU supplies power to the motherboard through appropriate connectors.
*PSU → CPU Power Circuit
The PSU provides power through the dedicated CPU power connector.
*PSU → Storage Devices
The PSU supplies power to SATA storage devices through SATA power connectors.
*PSU → Graphics Card
Some dedicated graphics cards require additional power directly from the PSU.
*PSU → Cooling Fans
The PSU supplies power to system components that require it, including some cooling systems through the appropriate connections.
SIMPLE POWER SUPPLY SYSTEM FLOW
The basic flow of electrical power can be represented as:
Mains AC Power → PSU → Conversion and Regulation → DC Power → Computer Components
This shows how electrical energy from the mains is converted into suitable power for the computer.
DIFFERENCE BETWEEN PSU AND UPS
A Power Supply Unit (PSU) is a component inside a computer that converts and supplies electrical power to the computer’s internal components.
A Uninterruptible Power Supply (UPS) is an external device that provides temporary backup power to a computer when the mains electricity fails.
PSU| UPS
Usually located inside the computer| Usually external to the computer
Converts AC to suitable DC outputs| Provides backup power during outages
Powers internal components| Keeps the computer running temporarily during power interruption
Part of the computer system| Separate power-protection/backup device
ROLE OF UPS IN COMPUTER POWER PROTECTION
A UPS can help provide temporary power during an electrical outage.
Depending on its design, it may also provide protection against certain power disturbances.
Benefits may include:
- Temporary backup power.
- Time to save work.
- Time to shut down the computer properly.
- Reduced interruption during short power failures.
SUMMARY
A power supply system provides the electrical power required by a computer. The Power Supply Unit (PSU) converts AC mains electricity into appropriate DC power for the computer’s internal components.
Important PSU functions include power conversion, voltage regulation, power distribution and electrical protection.
Computer PSUs contain components such as rectifiers, capacitors, switching circuits, transformers, cooling fans and output connectors.
Common desktop PSU types include ATX and SFX, while power supplies may also be classified as modular or non-modular.
Proper selection, maintenance and safe handling of power supplies are important because power-related faults can damage computer components and electrical equipment can present serious hazards.
WEEKLY EVALUATION OBJECTIVE QUESTIONS
Choose the correct answer from options A–D.
1. What does PSU stand for?
A. Power Storage Unit
B. Power Supply Unit
C. Processing Supply Unit
D. Personal System Unit
2. The main function of a computer PSU is to ______.
A. store files
B. display images
C. convert and supply electrical power to computer components
D. process instructions
3. The electrical power commonly supplied by household mains is ______.
A. DC
B. AC
C. USB
D. SATA
4. Which component converts AC electricity into pulsating DC in a power supply?
A. Rectifier
B. Keyboard
C. Monitor
D. RAM
5. Which connector commonly supplies power to a desktop computer motherboard?
A. HDMI connector
B. 24-pin ATX connector
C. Audio connector
D. VGA connector
6. The power rating of a PSU is commonly measured in ______.
A. metres
B. volts only
C. watts
D. kilograms
7. Which of the following is a common symptom of a faulty PSU?
A. Computer suddenly shuts down
B. Keyboard becomes a printer
C. Monitor becomes a scanner
D. Mouse stores files
8. Which device provides temporary backup power to a computer during a power outage?
A. CPU
B. UPS
C. RAM
D. GPU
9. Which of the following is an important safety precaution when working with a PSU?
A. Open the PSU while it is connected to mains power
B. Touch internal components immediately after unplugging it
C. Keep liquids away from electrical equipment
D. Bypass the fuse
10. A modular PSU is one in which ______.
A. all cables are permanently attached
B. some or all cables can be connected or disconnected as needed
C. no cables are required
D. only USB cables can be used
THEORY QUESTIONS
1a. What is a Power Supply Unit (PSU), and state five functions of a PSU.
1b. Differentiate between Alternating Current (AC) and Direct Current (DC).
2a. List and explain five major components of a computer power supply.
2b. State five common symptoms of a faulty power supply and explain two possible causes.
2c. State ten safety precautions that should be observed when working around computer power supply systems.