This chapter covers power supply units (PSUs) and their connectors, a fundamental topic for the CompTIA A+ 220-1101 exam under objective 3.5 (Given a scenario, install and configure power supplies and components). Understanding PSU types, form factors, wattage ratings, efficiency certifications, and connector pinouts is critical because power issues cause roughly 15-20% of hardware failures. Expect 3-5 exam questions on this topic, often testing your ability to select the correct PSU for a build, identify connector types, and troubleshoot power-related symptoms.
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Think of a computer's power supply unit (PSU) as a city's water system. The utility company (wall outlet) provides high-voltage AC power, like a high-pressure water main. The PSU's transformer is the water tower that steps down the pressure to safe levels, converting AC to DC. The rectifier and filter capacitors are like settling tanks and filters that smooth out pressure spikes and remove debris, ensuring clean, steady DC. The PSU's voltage rails (3.3V, 5V, 12V) are like different pipe diameters feeding different neighborhoods: the 12V rail is a large pipe for high-demand components (CPU, GPU), while the 3.3V and 5V rails are smaller pipes for sensitive electronics (RAM, SSDs). The PSU's regulation circuit is the pressure regulator that maintains constant pressure even when a fire hydrant (GPU) opens suddenly. Connectors are like faucets: the 24-pin main connector is the master shutoff valve, the 4+4 pin EPS is a dedicated line for the CPU, and SATA power connectors are garden hoses for drives. A poor-quality PSU is like a water system with rusty pipes and weak pressure—it may cause brownouts (voltage drops) or surges (spikes) that damage your appliances (components).
What is a Power Supply Unit (PSU)?
The Power Supply Unit (PSU) is the component that converts alternating current (AC) from the wall outlet into direct current (DC) at the voltages required by computer components. Without a PSU, no component can operate. The PSU also provides overcurrent, overvoltage, and short-circuit protection. For the 220-1101 exam, you must know the different PSU form factors, wattage ratings, efficiency (80 PLUS certification), and the purpose of each connector.
How a PSU Works Internally
AC Input: The PSU receives 110-120V (US) or 220-240V (Europe) AC from the wall outlet. Many PSUs have a manual voltage selector switch on the back; modern units are often auto-switching (active PFC) and accept 100-240V automatically.
Rectification: The AC is passed through a bridge rectifier, which converts it to pulsed DC.
Filtering: Capacitors smooth the pulsed DC into a more stable DC voltage, but it still has ripple.
Switching/Regulation: A switching circuit (using MOSFETs) chops the DC into high-frequency pulses, which are then passed through a transformer to step down the voltage to the desired levels (3.3V, 5V, 12V).
Secondary Rectification and Filtering: The stepped-down AC is rectified again and filtered by capacitors and chokes to produce clean DC rails.
Regulation Feedback: The PSU monitors output voltages and adjusts the switching duty cycle to maintain stable outputs under varying loads.
Key PSU Specifications
- Wattage: The total power the PSU can deliver continuously. Common ratings: 300W, 500W, 650W, 750W, 1000W+. For a typical office PC, 300-400W is sufficient; for a gaming workstation with a high-end GPU, 650-850W is recommended. The exam may ask you to calculate total system power draw and select an appropriate PSU. - Voltage Rails: Modern PSUs provide +3.3V, +5V, and +12V DC. The +12V rail is the most important, supplying power to the CPU, GPU, and fans. Older PSUs had multiple +12V rails (e.g., +12V1, +12V2) for safety, but modern single-rail designs are common. - Efficiency (80 PLUS): PSUs convert AC to DC with some loss. 80 PLUS certification guarantees at least 80% efficiency at 20%, 50%, and 100% load. Higher tiers: Bronze (82-85%), Silver (85-88%), Gold (87-90%), Platinum (89-92%), Titanium (90-94% at 10% load). The exam may test which certification is appropriate for a given scenario. - Form Factor: The physical size and shape of the PSU. Common form factors: - ATX: Standard for desktop PCs, 150mm x 86mm x 140mm (depth varies). - SFX: Smaller form factor for compact builds (Small Form Factor), 125mm x 63.5mm x 100mm. - TFX: Thin form factor for slim desktops, 85mm x 64mm x 175mm. - EPS: Server form factor, often larger and with different connector requirements.
PSU Connectors (Exam Critical)
Connectors are a major exam topic. You must be able to identify each connector by appearance and know its purpose.
24-pin Motherboard Connector (ATX Main Power): The largest connector, supplies power to the motherboard. It provides 3.3V, 5V, and 12V. Older motherboards used a 20-pin connector; the extra 4 pins provide additional 12V and ground. The connector is keyed so it can only be inserted one way. Often, the 24-pin is split into a 20+4 pin configuration for backward compatibility.
4+4 pin CPU Power Connector (EPS12V): Provides dedicated 12V power to the CPU. It is two 4-pin connectors that can be combined into an 8-pin connector. Some high-end motherboards require two 8-pin EPS connectors. This connector is square and has a notch. Note: Do not confuse with PCIe power connectors (see below).
6+2 pin PCIe Power Connector: Provides 12V power to graphics cards. It can be used as a 6-pin or 8-pin connector. The 6-pin delivers 75W, the 8-pin delivers 150W. High-end GPUs may require two 8-pin connectors. The connector is rectangular with a clip.
SATA Power Connector: A flat, 15-pin connector that provides 3.3V, 5V, and 12V to SATA drives (HDDs, SSDs, optical drives). It is L-shaped to prevent incorrect insertion.
Molex (4-pin Peripheral) Connector: An older, larger connector used for PATA drives, case fans, and some legacy devices. It provides 5V and 12V. It is round with four pins arranged in a trapezoid shape.
Floppy Drive (Berg) Connector: A small 4-pin connector used for floppy drives. Rarely used today but still appears on some PSUs.
Power Supply Troubleshooting
Common symptoms of PSU failure: - No power: System does not turn on. Check PSU switch, power cord, and motherboard LED. - Intermittent crashes: Random shutdowns or restarts, especially under load (e.g., gaming). - Burning smell: Indicates capacitor failure or short circuit. - Overheating: PSU fan not spinning or running at full speed all the time. - Voltage fluctuations: Measured with a multimeter or software (e.g., HWMonitor). Rails should be within ±5% of nominal: 12V rail should be 11.4V–12.6V; 5V rail 4.75V–5.25V; 3.3V rail 3.135V–3.465V.
How to Test a PSU
Paperclip test: To turn on a PSU without a motherboard, short the green wire (PS_ON) to any black wire (ground) on the 24-pin connector. The PSU fan should spin. This is a basic test but does not check voltage stability.
Multimeter: Measure voltages on the 24-pin connector (e.g., pin 9 is +5V standby, pin 8 is power good).
PSU Tester: A dedicated device that plugs into the connectors and displays voltages.
Efficiency and Power Factor Correction
Active PFC: Modern PSUs use active power factor correction to improve power factor to near 1.0, reducing reactive power and complying with regulations. PSUs with active PFC are typically auto-switching (100-240V).
Passive PFC: Older, less efficient method using a large inductor.
Interaction with Other Components
The PSU affects system stability. An underpowered PSU can cause GPU throttling, random reboots, or failure to post. A high-quality PSU with good ripple suppression (less than 50mV on 12V) ensures stable operation. Modular PSUs allow detachable cables for better cable management. Semi-modular PSUs have fixed essential cables (24-pin, CPU) and modular others. Fully modular PSUs have all cables detachable.
Exam-Specific Numbers and Values
24-pin main connector (20+4 for backward compatibility)
4+4 pin CPU (EPS12V) – provides up to 235W (8-pin) or 155W (4-pin)
6+2 pin PCIe – 6-pin provides 75W, 8-pin provides 150W
SATA power: 15-pin
Molex: 4-pin
Floppy: 4-pin (small)
ATX12V 2.0 specification: introduced 24-pin main connector
80 PLUS Bronze: 82% efficiency at 50% load
Voltage tolerance: ±5%
Determine System Power Requirements
Calculate the total power draw of all components. Use a PSU calculator or sum TDPs: CPU (e.g., 125W), GPU (e.g., 250W), motherboard (50W), RAM (10W), drives (15W each), fans (5W each). Add 20-30% headroom for future upgrades and peak loads. For example, a gaming PC with a 125W CPU and 250W GPU needs at least 500W PSU. The exam may ask you to select a PSU based on a given component list.
Select Appropriate Form Factor and Connectors
Choose a PSU that fits the case (ATX, SFX, etc.) and has the necessary connectors. Verify the motherboard requires a 24-pin (not 20-pin) and the CPU power connector (4-pin or 8-pin). Ensure the PSU has enough PCIe power connectors for the GPU (e.g., two 6+2 pin for a high-end card). Check for SATA power connectors for drives. The exam loves to test connector compatibility.
Check Efficiency Rating and Certification
For energy-efficient builds, choose an 80 PLUS certified PSU. The higher the tier (Bronze, Gold, Titanium), the less power wasted as heat. In data centers, Platinum or Titanium can save significant electricity costs. For a typical home PC, Bronze or Gold is sufficient. The exam might ask which certification is best for a given scenario, e.g., 'A client wants to reduce electricity bills; which 80 PLUS tier is most cost-effective?'
Install PSU and Connect Cables
Place the PSU in the case, usually at the bottom or top, with the fan facing downward (if bottom-mounted) or upward (if top-mounted) for proper airflow. Connect the 24-pin motherboard power, the 4+4 pin CPU power, and then PCIe power to the GPU. Connect SATA power to drives and Molex to any legacy peripherals. Ensure all connectors are fully seated and locked. Route cables neatly to avoid blocking airflow.
Test and Verify Operation
After installation, power on the system. Listen for the PSU fan spinning. Check motherboard LEDs for standby power. Use BIOS or software (e.g., HWMonitor) to verify voltage rails are within spec. If the system fails to boot, recheck all connections, especially the 24-pin and CPU power. A common exam scenario: 'After installing a new GPU, the system won't boot. What is the most likely cause?' Answer: The PSU is underpowered or the PCIe power connectors are not plugged in.
Enterprise Server Deployment: In a data center, servers often use redundant PSUs (2+2 configuration) for high availability. Each PSU is typically 800W-1600W with Platinum or Titanium efficiency. The PSUs are hot-swappable: if one fails, the server continues running on the remaining PSU(s). Engineers monitor PSU health via BMC (Baseboard Management Controller) and set alerts for voltage or temperature anomalies. Misconfiguration example: Plugging a 120V-only PSU into a 208V data center outlet without checking the voltage selector (if not auto-switching) can destroy the PSU instantly.
Gaming PC Build: A custom gaming PC might use a 750W Gold-rated PSU to power an overclocked i9-13900K (250W) and RTX 4090 (450W). The builder must ensure the PSU has at least three 8-pin PCIe connectors (or use a 12VHPWR connector for newer GPUs). Common mistake: Using a daisy-chained PCIe cable (one cable with two connectors) instead of two separate cables, which can cause overheating and shutdowns under load.
Office PC Upgrade: An IT technician upgrades an old office PC with a new SSD and more RAM. The existing 250W PSU may be sufficient if no GPU is added. However, if the technician installs a low-profile graphics card, they must verify the PSU has a spare PCIe power connector (some low-power GPUs draw all power from the PCIe slot, up to 75W). A real-world failure: The technician forgets to flip the PSU switch back on after cleaning, causing a 'dead' system that takes hours to troubleshoot.
Objective 3.5: 'Given a scenario, install and configure power supplies and components.' The exam tests your ability to select the right PSU for a build, identify connectors, and troubleshoot power issues.
Common Wrong Answers: 1. 'A 500W PSU is sufficient for any system' – WRONG because high-end GPUs and CPUs can draw 600W+ under load. Candidates often underestimate power requirements. 2. 'The 4+4 pin CPU connector is the same as PCIe power' – WRONG. They look similar but are keyed differently. The CPU connector has square pins; PCIe has rounded pins with a different clip shape. Forcing a PCIe cable into a CPU header can damage the motherboard. 3. '80 PLUS Bronze is the most efficient' – WRONG. Bronze is the entry-level certification. Gold is more efficient (87-90% vs 82-85%). Candidates often confuse the tiers. 4. 'A modular PSU provides better electrical performance' – WRONG. Modularity only affects cable management, not electrical quality.
Specific Numbers and Terms: - 24-pin, 4+4 pin EPS, 6+2 pin PCIe, SATA 15-pin, Molex 4-pin. - 80 PLUS Bronze, Gold, Platinum, Titanium. - Voltage tolerance ±5%. - 6-pin PCIe provides 75W; 8-pin provides 150W. - ATX12V 2.0 introduced 24-pin connector.
Edge Cases:
Some motherboards require both 8-pin and 4-pin CPU power (e.g., for extreme overclocking).
Older PSUs may have a 20-pin main connector; using it on a 24-pin motherboard will prevent boot.
PSUs with active PFC may not work with some older UPS units (simulated sine wave).
Eliminating Wrong Answers:
If a question asks about a system that 'turns on for a second then shuts down,' suspect a short circuit or PSU overload. Look for answers involving 'PSU wattage too low' or 'incorrect connector.'
If a question mentions 'burning smell,' the correct answer is usually 'replace the PSU immediately.'
If a question asks about 'no power at all,' check the PSU switch, power cord, and motherboard power button connection before blaming the PSU.
The 24-pin motherboard connector is the main power connector; it can be split into 20+4 for older boards.
The 4+4 pin EPS12V connector provides CPU power; never use a PCIe connector for the CPU.
The 6+2 pin PCIe connector supplies power to graphics cards; 6-pin = 75W, 8-pin = 150W.
SATA power connectors are 15-pin and L-shaped; they provide 3.3V, 5V, and 12V.
Molex connectors are 4-pin and used for legacy devices; they provide 5V and 12V.
80 PLUS certification guarantees at least 80% efficiency; tiers: Bronze, Silver, Gold, Platinum, Titanium.
Voltage tolerance for all rails is ±5%: 12V rail (11.4V-12.6V), 5V rail (4.75V-5.25V), 3.3V rail (3.135V-3.465V).
Always leave 20-30% headroom when selecting PSU wattage to account for peak loads and future upgrades.
Modular PSU cables are not interchangeable between different PSU models or brands.
A PSU that turns on then immediately off may indicate a short circuit or overload.
These come up on the exam all the time. Here's how to tell them apart.
ATX PSU
Standard size for most desktop cases (150mm x 86mm x 140mm).
Higher wattage options available (up to 1600W+).
More space for larger fans (120mm or 140mm) for quieter operation.
Typically lower cost per watt.
Commonly used in mid-tower and full-tower cases.
SFX PSU
Compact size for small form factor (SFF) cases (125mm x 63.5mm x 100mm).
Lower maximum wattage (usually up to 750W, some up to 1000W).
Smaller fan (80mm or 92mm) may be noisier under load.
Higher cost per watt due to compact design.
Requires an SFX-to-ATX bracket for installation in ATX cases.
Mistake
A higher wattage PSU always consumes more power from the wall.
Correct
A PSU draws only the power required by the components, plus efficiency losses. A 1000W PSU powering a 300W system draws about 350W from the wall (at 85% efficiency), while a 500W PSU under the same load draws the same 350W. Higher wattage does not mean higher electricity bill; it provides headroom.
Mistake
Modular PSUs provide better electrical performance than non-modular ones.
Correct
Modularity only affects cable management and convenience. The electrical performance (voltage regulation, ripple, efficiency) depends on the PSU's design and components, not whether the cables are detachable. A high-quality non-modular PSU can outperform a cheap modular one.
Mistake
You can mix and match modular cables from different PSUs.
Correct
Modular cables are not standardized between brands or even models. Using a cable from a different PSU can cause short circuits because the pinout may differ. Always use the cables that came with the PSU or manufacturer-specific replacements.
Mistake
The 4+4 pin CPU connector and 6+2 pin PCIe connector are interchangeable.
Correct
They are physically different. The CPU connector has square pins and a different clip shape; the PCIe connector has rounded pins. Forcing a PCIe cable into a CPU power header can damage the motherboard and PSU. They are keyed differently to prevent this, but force can overcome the keying.
Mistake
80 PLUS certification guarantees a PSU is high quality.
Correct
80 PLUS only certifies efficiency at specific loads. It does not test voltage regulation, ripple, transient response, or build quality. A PSU can meet 80 PLUS Gold but have poor ripple suppression or use low-quality capacitors. Look for independent reviews for overall quality.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
The 4+4 pin CPU connector (EPS12V) provides power to the CPU and has square pins with a specific clip shape. The 6+2 pin PCIe connector provides power to graphics cards and has rounded pins with a different clip shape. They are keyed differently to prevent accidental swapping. Using the wrong connector can damage components. Visually, the CPU connector is typically black or dark, while PCIe connectors are often black with a different latch.
Add the TDP of the CPU, GPU, and estimate 50W for motherboard, RAM, and drives. Add 10-20W per fan. Then add 20-30% headroom. For example, a 125W CPU + 250W GPU + 50W motherboard + 20W drives + 10W fans = 455W. Adding 20% headroom gives 546W, so a 550W or 600W PSU is appropriate. Online PSU calculators can help. The exam may give you component specifications and ask you to choose a PSU wattage.
80 PLUS Gold is a certification that guarantees the PSU is at least 87% efficient at 50% load, 90% at 100% load, and 87% at 20% load. Higher efficiency means less power wasted as heat, lower electricity bills, and typically better cooling. Gold is a common choice for mid-range to high-end builds. The exam may ask which certification is best for a given scenario, e.g., 'Data center' might require Platinum or Titanium.
No, a 20-pin PSU will not supply enough power to a 24-pin motherboard. The extra 4 pins provide additional 12V and ground for the PCIe slot and other components. While some motherboards may power on with only 20 pins, it can cause instability or damage. Always use a PSU with a 24-pin connector (or 20+4) for modern motherboards. The exam will test this compatibility issue.
The paperclip test is a basic way to check if a PSU powers on. Unplug the PSU from the motherboard, then insert a paperclip into the green wire (PS_ON, pin 16) and any black wire (ground, e.g., pin 15) on the 24-pin connector. Plug in the PSU and flip the switch. If the fan spins, the PSU is likely functional. This does not test voltage stability or load capacity, but it can rule out a completely dead PSU.
Random shutdowns under load often indicate an insufficient or failing PSU. When the CPU and GPU draw high current, the PSU may not be able to supply stable voltage, causing the overcurrent or undervoltage protection to trip. Other causes: overheating, driver issues, or motherboard problems. Check PSU wattage against system requirements and monitor voltages with software. If voltages drop below spec during load, replace the PSU.
A single-rail PSU has one +12V rail that can deliver all its current to any connector. A multi-rail PSU splits the +12V into two or more rails, each with its own overcurrent protection (OCP). Multi-rail designs are safer because a short on one rail won't affect others, but they can cause tripping if a component draws too much from one rail. Modern high-end PSUs often use single-rail or switchable modes. The exam does not heavily test this distinction.
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