3.6 Given a scenario, install the appropriate power supply.
📘CompTIA A+ Core 1 (220-1201)
Understanding power supply output voltages is critical for the A+ exam. A computer power supply (PSU) does not send one single voltage to the system. Instead, it converts AC power from the wall into multiple DC voltage levels, each used by different computer components.
The three main output voltages you must know for the exam are:
- +3.3 volts (3.3V)
- +5 volts (5V)
- +12 volts (12V)
Each voltage serves a specific purpose inside a computer system.
Why Multiple Output Voltages Are Needed
Different computer components have different power requirements:
- Some components need low voltage for logic and communication
- Some need medium voltage for controller circuits
- Others need higher voltage to run motors, fans, and high-performance hardware
The PSU safely distributes the correct voltage to each component using dedicated power rails.
+3.3V Output
What 3.3V Is Used For
The 3.3V rail is mainly used by low-power electronic components on the motherboard.
Common Components Using 3.3V
- Motherboard chipsets
- RAM (memory modules)
- PCIe slots (part of the power delivery)
- CPU logic circuits (indirectly through voltage regulators)
Key Points for the Exam
- 3.3V is used for modern low-voltage components
- It supports data processing and signaling
- It is not used for motors or high-power devices
Why 3.3V Matters
Modern hardware is designed to use less voltage to reduce heat and improve efficiency. This makes the 3.3V rail very important in today’s systems.
+5V Output
What 5V Is Used For
The 5V rail supplies power to internal electronics and control circuits.
Common Components Using 5V
- USB ports and connected USB devices
- Storage drive logic boards (HDDs and SSDs)
- Some motherboard controllers
- Older expansion cards
Key Points for the Exam
- USB ports rely heavily on 5V
- Many internal components still depend on 5V for control logic
- Older systems used 5V more heavily than modern systems
Standby Power (5VSB)
- The PSU provides 5V Standby (5VSB) even when the PC is turned off
- This allows:
- Power button functionality
- Wake-on-LAN
- Sleep and hibernate features
⚠️ Exam tip: If a system can wake from sleep, 5VSB must be working.
+12V Output
What 12V Is Used For
The 12V rail is the most important output in modern computers. It supplies high-power components.
Common Components Using 12V
- CPU (through the motherboard’s voltage regulators)
- Graphics card (GPU)
- Hard drive motors and cooling fans
- Liquid cooling pumps
- PCIe power connectors
Key Points for the Exam
- 12V handles most of the system’s power load
- Modern CPUs and GPUs depend heavily on 12V
- A weak 12V rail can cause:
- System crashes
- Random shutdowns
- Failure to boot
Single-Rail vs Multi-Rail (Basic Awareness)
- Some PSUs have one 12V rail
- Others split 12V into multiple rails for safety
- A+ exam expects awareness, not deep electrical theory
Voltage Rails and Power Connectors (Exam-Relevant)
Different PSU connectors carry different voltages:
| Connector | Voltages Supplied |
|---|---|
| 24-pin ATX motherboard connector | 3.3V, 5V, 12V |
| CPU power connector (4-pin / 8-pin) | 12V |
| SATA power connector | 3.3V, 5V, 12V |
| Molex connector | 5V, 12V |
| PCIe power connector | 12V |
⚠️ Exam tip: If a GPU is not getting enough power, check the 12V PCIe connectors.
Power Distribution Summary (Easy to Remember)
| Voltage | Purpose | Typical Usage |
|---|---|---|
| 3.3V | Low-power logic | RAM, chipsets, motherboard circuits |
| 5V | Control and peripherals | USB, drive electronics, standby power |
| 12V | High-power components | CPU, GPU, fans, motors |
Common Exam Scenarios
Scenario 1: PC powers on but no display
- Possible cause: Insufficient 12V power to GPU
Scenario 2: USB devices not working
- Possible cause: 5V rail failure
Scenario 3: System cannot wake from sleep
- Possible cause: 5V Standby (5VSB) issue
Key Exam Takeaways
- A PSU outputs multiple DC voltages, not just one
- 3.3V = low-power electronics
- 5V = logic, USB, and standby power
- 12V = high-performance and mechanical components
- Modern systems rely most heavily on 12V
- Understanding voltage usage helps with troubleshooting scenarios
