3.6 Given a scenario, install the appropriate power supply.
📘CompTIA A+ Core 1 (220-1201)
1. What Is Input Voltage in a Power Supply?
A power supply unit (PSU) converts electricity from a wall outlet into safe, usable power for computer components like:
- Motherboard
- CPU
- RAM
- Storage devices
- Expansion cards
The input voltage is the amount of electrical voltage the PSU receives from the wall outlet before it converts it to DC power.
For the CompTIA A+ exam, you must understand the two main input voltage ranges:
- 110–120 VAC
- 220–240 VAC
VAC means Volts Alternating Current
2. Why Input Voltage Matters (Exam Focus)
Choosing the correct input voltage is critical because:
- Using the wrong voltage can damage the PSU
- It can cause system instability
- It can destroy internal components
- The system may not power on at all
In an IT environment, technicians must ensure the PSU is compatible with the available electrical supply.
3. 110–120 VAC (Low Voltage Input)
Definition
- This voltage range supplies lower AC voltage
- Common in North America and some other regions
PSU Behavior at 110–120 VAC
- The PSU must draw more current to provide the same power
- Generates more heat
- Less energy-efficient compared to higher voltage
IT Environment Use
- Desktop PCs in offices
- Home and small business workstations
- Entry-level servers (with compatible PSU)
Exam Points to Remember
- Requires proper PSU compatibility
- Older PSUs may have a manual voltage selector switch
- Incorrect selection can destroy the PSU
4. 220–240 VAC (High Voltage Input)
Definition
- This voltage range supplies higher AC voltage
- Used in many countries outside North America
PSU Behavior at 220–240 VAC
- Draws less current
- Produces less heat
- More power-efficient
- Better for high-load systems
IT Environment Use
- Data centers
- Enterprise servers
- Network equipment rooms
- High-performance workstations
Exam Points to Remember
- Preferred for professional IT environments
- Improves PSU efficiency and reliability
- Reduces electrical stress on components
5. Manual Voltage Selector Switch (Very Important for Exam)
What Is It?
Some older power supplies have a red voltage selector switch on the back.
- 110V setting → for 110–120 VAC
- 220V setting → for 220–240 VAC
Exam Warning ⚠️
- Setting the switch incorrectly can cause:
- Immediate PSU failure
- Smoke or burning smell
- Permanent hardware damage
Modern PSUs usually do NOT have this switch
6. Auto-Switching Power Supplies (Modern Standard)
What Is Auto-Switching?
Most modern PSUs are auto-sensing, meaning they automatically detect:
- 110–120 VAC
- 220–240 VAC
No manual selection is needed.
Advantages (Exam-Relevant)
- Safer
- Reduces human error
- Used in modern desktops and servers
- Required for global IT deployments
Exam Tip
If the question mentions:
- Modern PSU
- No voltage switch
- Universal input
→ It is an auto-switching PSU
7. Power Supply Label and Specifications
Technicians should always check the PSU label, which usually shows:
Input: 100–240 VAC, 50–60 Hz
What This Means
- Supports both voltage ranges
- Works worldwide
- Automatically adjusts input voltage
Exam Focus
- Always verify PSU input rating
- Never assume voltage compatibility
8. Power Efficiency and Voltage (Exam Concept)
| Voltage Range | Efficiency | Heat | Current Draw |
|---|---|---|---|
| 110–120 VAC | Lower | Higher | More |
| 220–240 VAC | Higher | Lower | Less |
Why This Matters
- High-voltage input is better for:
- Servers
- Continuous operation
- High-power systems
9. Safety Considerations (Tested on Exam)
Technicians must:
- Match PSU input voltage to available power
- Never force incompatible connections
- Use proper power cables rated for voltage
- Avoid modifying PSU internals
⚠️ PSUs should never be opened by technicians
10. Common Exam Scenarios
You may see questions like:
- A system fails after installing a PSU → voltage mismatch
- A PSU works in one country but not another → input voltage difference
- Burnt PSU after installation → manual switch set incorrectly
- New PSU works worldwide → auto-switching PSU
11. Key Exam Takeaways (Memorize This)
- 110–120 VAC → lower voltage, higher current, more heat
- 220–240 VAC → higher voltage, lower current, more efficient
- Older PSUs may have a manual voltage switch
- Modern PSUs are auto-switching
- Always check PSU input rating
- Incorrect voltage can destroy the PSU
