This chapter covers troubleshooting power supply issues, a critical skill for the CompTIA A+ 220-1101 exam, specifically under Objective 5.1 (Hardware Troubleshooting). Power supply problems are among the most common hardware failures and can manifest as intermittent crashes, failure to power on, or component damage. Expect roughly 5-10% of the exam to touch on power supply troubleshooting, often in scenario-based questions asking you to identify symptoms and select the correct replacement or diagnostic step.
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Think of a computer's power supply unit (PSU) as a city's water tower and distribution system. The utility company provides high-voltage AC power, just as a reservoir holds water at high pressure. The PSU's job is to step down and convert that AC to stable DC voltages (like 3.3V, 5V, 12V) — analogous to a water tower reducing pressure and distributing water to different zones. Each voltage rail is like a separate pipe: the 12V rail supplies high-current components (CPU, GPU) like a fire hose, while the 3.3V and 5V rails feed low-power parts (RAM, SSDs) like garden hoses. The PSU's regulation circuitry is like pressure regulators that maintain constant flow despite varying demand. If the water tower runs dry (PSU overload), pressure drops and appliances malfunction — in a PC, this causes crashes, reboots, or failure to boot. A failing PSU might deliver 'dirty' power (ripple or noise), like water with sediment, which can damage sensitive electronics. Just as a city needs a properly sized water system for its population, a PC needs a PSU with sufficient wattage and stable rails for its components. Overloading a PSU is like connecting too many sprinklers to a single hose — flow drops and nothing works right.
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 the computer's internal components. It also provides voltage regulation and protection against surges, brownouts, and short circuits. The PSU is typically rated by its total wattage output, but the true measure of its capability lies in the amperage available on each voltage rail (3.3V, 5V, 12V, and sometimes -12V and 5VSB).
How a PSU Works Internally
AC Input and Rectification: The PSU receives AC power (typically 110-120V in North America, 220-240V in Europe) and passes it through a bridge rectifier to convert it to pulsating DC.
Filtering and Smoothing: Capacitors smooth the pulsating DC into a rough DC voltage, but it still contains ripple.
Switching and Regulation: The switching regulator (transformer and MOSFETs) chops the DC into high-frequency pulses, allowing a smaller transformer to step down the voltage efficiently. Pulse-width modulation (PWM) controls the duty cycle to maintain stable output voltages despite load changes.
Secondary Rectification and Filtering: The stepped-down AC is rectified again and filtered by capacitors and inductors to produce clean DC rails.
Voltage Rails: The PSU outputs multiple rails: +3.3V, +5V, +12V, -12V, and +5VSB (standby). The +12V rail is the most critical for modern systems, powering the CPU and GPU. The +3.3V and +5V rails power older peripherals, RAM, and storage devices.
Key Specifications and Ratings
Total Wattage: The maximum continuous power the PSU can deliver. For a typical desktop, 500W-750W is common; high-end gaming or workstation systems may require 850W-1200W.
Amperage per Rail: Each rail has a maximum current rating. For example, a 650W PSU might provide 54A on the +12V rail (12V × 54A = 648W). The +3.3V and +5V rails combined are often limited to 100-150W.
Efficiency Rating: 80 PLUS certification (Bronze, Silver, Gold, Platinum, Titanium) indicates efficiency at various loads. Higher efficiency means less heat and lower electricity costs.
Form Factor: ATX (standard), SFX (small form factor), and others. The 220-1101 exam focuses on ATX and SFX.
Modular vs. Non-Modular: Modular PSUs allow detaching unused cables, improving airflow.
Common Power Supply Failure Symptoms
System fails to power on: No fan spin, no LEDs. This can indicate a dead PSU, faulty power button, or motherboard issue.
Intermittent crashes or reboots: Often under load (gaming, rendering). The PSU may be unable to supply sufficient current, causing voltage droop.
Burning smell or visible smoke: Indicates capacitor failure or short circuit.
Overheating: PSU fan runs at full speed constantly or stops.
Booting issues: The PC powers on for a second then shuts down (often due to short circuit protection or insufficient power).
BSODs with power-related codes: e.g., WHEA_UNCORRECTABLE_ERROR, KERNEL_POWER.
Troubleshooting Steps (Objective 5.1)
Verify AC power: Check that the wall outlet is live (use a known working device). Test with a multimeter: expect 110-120V AC (US) or 220-240V AC (EU).
Check the PSU switch: Ensure the rear switch is in the '1' (on) position. Also check the voltage selector switch if present (rare on modern PSUs).
Inspect cables and connections: Ensure the 24-pin motherboard connector, 4/8-pin CPU power, and PCIe power cables are fully seated. Loose connections cause intermittent issues.
Test with a PSU tester: A simple PSU tester checks voltages on each rail. Expected ranges: +3.3V (3.135-3.465V), +5V (4.75-5.25V), +12V (11.4-12.6V). The 'Power Good' signal should be present (typically 3-5V).
Perform the paperclip test: To test if a PSU powers on 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 does not test voltage regulation but confirms the PSU can turn on.
Use a multimeter: For more precise testing, measure voltages at the 24-pin connector or at peripheral connectors. Pinout:
- Pin 9: +5VSB (purple wire) — should be 5V even when PSU is off (standby). - Pin 14: PS_ON (green wire) — when shorted to ground, PSU turns on. - Pin 8: Power Good (gray wire) — goes high (3-5V) after PSU stabilizes. 7. Check for bulging capacitors: Visually inspect the PSU (if safe) or the motherboard for swollen or leaking capacitors. Bulging caps indicate failure. 8. Swap with a known good PSU: The definitive test is to replace the PSU with a unit of sufficient wattage and verify if the system operates normally.
How Power Supply Interacts with Other Components
Motherboard: The motherboard's voltage regulator modules (VRMs) further regulate the +12V rail to supply the CPU core voltage (Vcore). If the PSU's +12V rail is unstable, the VRMs cannot compensate fully, leading to CPU instability.
GPU: Modern GPUs draw heavily from the +12V rail via PCIe power cables. Insufficient amperage causes GPU throttling or crashes under load.
Storage: HDDs and SSDs use +5V (logic) and +12V (motor for HDDs). A failing +5V rail can cause drive detection issues.
RAM: RAM uses +3.3V and +5V. Drooping voltages cause memory errors.
Cooling: The PSU fan itself is powered internally; if the fan fails, the PSU overheats and may shut down.
Power Protection Features
Overcurrent Protection (OCP): Shuts down the PSU if current exceeds a threshold on any rail.
Overvoltage Protection (OVP): Shuts down if output voltage exceeds a safe level.
Undervoltage Protection (UVP): Shuts down if voltage drops too low.
Short Circuit Protection (SCP): Shuts down if a short is detected.
Overpower Protection (OPP): Shuts down if total power exceeds rating.
Brownout Protection: Some PSUs can tolerate brief dips in AC input.
Power Supply Testing Tools and Commands
Multimeter: Essential for measuring AC input, DC output, and ripple. Set to DC voltage range (20V) for rails.
PSU Tester: Plug-and-play device that displays voltages and Power Good signal.
Paperclip: A simple wire to short PS_ON to ground.
Software monitoring: Tools like HWMonitor or SpeedFan can report voltages from motherboard sensors, but these are less accurate than a multimeter.
Exam-Specific Details for 220-1101
The exam expects you to know the standard voltages: +3.3V, +5V, +12V, -12V, +5VSB.
The Power Good signal (gray wire) is a key diagnostic point: if it is not present, the motherboard will not start.
The PS_ON signal (green wire) is active low: shorting to ground turns on the PSU.
The exam may ask about the paperclip test as a quick way to determine if a PSU is dead.
Be aware that a PSU can partially fail: it may power on but provide incorrect voltages, causing intermittent issues.
Always consider the PSU when troubleshooting no power, random reboots, or burning smell.
The exam does not require deep electrical theory but expects you to understand symptoms and basic testing procedures.
Verify AC Power Source
Start by ensuring the wall outlet is supplying power. Plug in a known working device (like a lamp) to confirm the outlet is live. If using a power strip or UPS, verify it is switched on and not tripped. Use a multimeter set to AC voltage: expect 110-120V (US) or 220-240V (EU). If no power, check the circuit breaker. This step eliminates external power issues before blaming the PSU.
Check PSU Switch and Cables
Ensure the PSU's rear rocker switch is set to the '1' (on) position. Verify that the 24-pin main power cable, CPU power (4/8-pin), and any PCIe power cables are fully seated. Loose connections can cause intermittent power loss. Reseat each connector firmly. Also check for bent pins in the connectors. If the system has a dedicated GPU, ensure its power cables are connected.
Perform the Paperclip Test
Disconnect the PSU from all components. On the 24-pin motherboard connector, locate the green wire (PS_ON, pin 16 on ATX v2.2) and any black wire (ground, e.g., pin 15 or 17). Use a paperclip to short the green wire to a black wire. Plug the PSU into AC and turn on the switch. If the PSU fan spins, the PSU can power on. If not, the PSU is likely dead. Note: This test does not verify voltage regulation.
Test Voltages with Multimeter
With the PSU connected to AC and the paperclip in place (or with the system on if safe), set your multimeter to DC voltage (20V range). Touch the black probe to a black wire (ground) and the red probe to the desired pin. Check +3.3V (orange wire, pin 1, 2, or 3): expect 3.3V ± 5% (3.135-3.465V). Check +5V (red wire, pin 4, 6, or 22): expect 5V ± 5% (4.75-5.25V). Check +12V (yellow wire, pin 10 or 11): expect 12V ± 5% (11.4-12.6V). Also check +5VSB (purple wire, pin 9): should be 5V even with PSU off. If any voltage is out of spec, the PSU is faulty.
Check Power Good Signal
The Power Good (PG) signal is on the gray wire (pin 8 on the 24-pin connector). With the PSU turned on (paperclip test or system running), measure voltage between gray and black. It should be between 3V and 5V (typically 5V). If it is 0V or below 2.5V, the PSU is not signaling that it is ready, and the motherboard will not start. A faulty PG signal can cause intermittent boot failures.
Test with a PSU Tester
A dedicated PSU tester simplifies testing: connect the 24-pin and any other connectors, then power on. The tester will display voltages for each rail and indicate if the PG signal is present. Some testers also simulate load. If any rail shows a red light or out-of-range value, the PSU is defective. This is quicker than a multimeter and is a common exam scenario.
Swap with Known Good PSU
The most definitive test is to replace the suspect PSU with a known good unit of sufficient wattage. If the system operates normally with the replacement, the original PSU is faulty. Ensure the replacement has enough wattage and the correct connectors for the system. This step confirms the diagnosis and is the final step in the troubleshooting process.
Enterprise Scenario 1: Server Room PSU Failure
In a data center, a rack server running a critical database begins randomly rebooting during peak load. The server has redundant power supplies (2x 750W hot-swappable). The first step is to check the PSU status LEDs: one shows amber (failure), the other green. The technician swaps the failed PSU with a spare. The system stabilizes. The failed PSU is later bench-tested: the +12V rail drops to 10.8V under load. This scenario highlights the importance of redundant PSUs and monitoring tools in enterprise environments. Common misconfiguration: using mismatched PSU wattages can cause load balancing issues.
Enterprise Scenario 2: Workstation Intermittent Crashes
A graphic design workstation with a 600W PSU crashes when rendering 3D scenes. The user replaced the GPU with a more powerful model without upgrading the PSU. The technician uses a PSU tester and finds the +12V rail at 11.2V under load (out of spec). The solution is to replace the PSU with a 850W unit that provides adequate amperage on the +12V rail. This is a classic example of undersizing a PSU. The exam frequently tests this: always ensure the PSU wattage matches the total system draw, especially after upgrades.
Enterprise Scenario 3: Office PC No Power
An office PC fails to power on. The technician checks the outlet (working), the PSU switch (on), and finds the 24-pin connector slightly loose. After reseating, the PC boots. This simple fix is common. However, if the PC still doesn't power on, the technician uses a multimeter and finds 0V on the +5VSB rail, indicating a failed standby circuit. The PSU is replaced. This scenario underscores the importance of checking the +5VSB, as it powers the power button logic on the motherboard.
What the 220-1101 Exam Tests
Objective 5.1 (Hardware Troubleshooting) includes power supply issues. The exam expects you to:
Identify symptoms of a failing PSU: no power, intermittent shutdowns, burning smell, overheated PSU, system fails to boot.
Know the standard voltages and their acceptable ranges: +3.3V (3.135-3.465V), +5V (4.75-5.25V), +12V (11.4-12.6V).
Understand the paperclip test and when to use it.
Recognize that a PSU can partially fail (e.g., good +12V but bad +5V).
Know that the Power Good signal must be present for the motherboard to start.
Be able to differentiate between PSU failure and other hardware issues (e.g., motherboard, power button).
Common Wrong Answers and Why
'Replace the motherboard' — When a system has no power, many candidates jump to motherboard failure. However, the PSU is the most likely culprit and should be tested first. The exam tests your ability to follow a logical troubleshooting process.
'The PSU is dead' — If the paperclip test spins the fan, the PSU is not completely dead. It may still have bad voltages. Candidates often stop at the paperclip test and assume the PSU is good, but voltage testing is needed.
'Upgrade the BIOS' — For intermittent crashes, some candidates think of BIOS updates. But if the crashes occur under load, insufficient PSU wattage is more likely.
'Check the RAM' — RAM errors can cause crashes, but power supply issues often cause complete shutdowns or failure to power on, not just blue screens. The exam expects you to prioritize PSU testing when symptoms match.
Specific Numbers and Terms
+5VSB: Standby voltage, always present when PSU is plugged in (even if off). If missing, the power button won't work.
Power Good: Gray wire, must be 3-5V within 100-500ms of power-on.
PS_ON: Green wire, active low (short to ground to turn on).
ATX v2.2 pinout: Pin 16 is PS_ON, Pin 8 is Power Good, Pin 9 is +5VSB.
80 PLUS ratings: Bronze, Silver, Gold, Platinum, Titanium — not directly tested but may appear in context.
Edge Cases and Exceptions
Some PSUs have a voltage selector switch (110V/220V) — if set wrong, the system may not power on or may be damaged.
A failing PSU can cause data corruption on HDDs due to unstable +5V or +12V.
The paperclip test does not load the PSU; a PSU may pass the paperclip test but fail under load.
Power supply issues can mimic GPU or CPU failure (e.g., artifacts on screen due to insufficient +12V to GPU).
How to Eliminate Wrong Answers
If the symptom is 'no power at all', eliminate motherboard, CPU, and RAM first — they are less likely to cause complete power loss.
If the symptom is 'intermittent crashes under load', the PSU is a prime suspect because load increases current draw.
If the system powers on for a second then shuts down, suspect a short circuit or PSU protection tripping.
Always test the simplest things first: outlet, switch, cables.
Standard PSU voltages: +3.3V (±5%), +5V (±5%), +12V (±5%), +5VSB (5V always).
Power Good signal (gray wire) must be 3-5V for motherboard to start.
PS_ON (green wire) is active low; short to ground to turn on PSU.
The paperclip test only checks if the PSU can power on, not voltage quality.
Intermittent crashes under load often indicate insufficient +12V amperage.
A burning smell or bulging capacitors means immediate PSU replacement.
Always test the wall outlet first when troubleshooting no power.
Swapping with a known good PSU is the definitive diagnostic step.
These come up on the exam all the time. Here's how to tell them apart.
Paperclip Test
Only confirms PSU can turn on
Requires shorting PS_ON to ground
No voltage or PG signal measurement
Free and quick
Does not load the PSU
PSU Tester
Measures voltages on all rails
Checks PG signal and +5VSB
Displays results with LEDs or LCD
Costs $10-30
Provides more comprehensive diagnosis
Mistake
A PSU that spins its fan is definitely working.
Correct
The fan spinning only indicates the PSU can turn on, not that it provides correct voltages. A PSU can have a working fan but fail to deliver proper +12V or +5V, causing system instability. Always test voltages with a multimeter or PSU tester.
Mistake
A higher wattage PSU will damage components.
Correct
A PSU only delivers the power demanded by the components, not its maximum rating. A higher wattage PSU is safer because it operates at lower load percentage, reducing stress and heat. Damage only occurs if the PSU is of poor quality and fails.
Mistake
If the system powers on, the PSU is fine.
Correct
A PSU can be partially failed. For example, the +12V rail might be low under load, causing crashes during gaming or rendering. The system may boot normally at idle but fail under stress. Always test under load if possible.
Mistake
The paperclip test is a definitive test of PSU health.
Correct
The paperclip test only checks if the PSU can turn on. It does not load the PSU or measure voltages. A PSU can pass the paperclip test but still have faulty regulation or ripple. Always follow up with voltage measurements.
Mistake
You can replace a PSU with any model that has the same wattage.
Correct
Wattage is not the only factor. The amperage on the +12V rail is critical, especially for modern systems with powerful GPUs. Also, the form factor (ATX vs. SFX) and connector types (e.g., 4+4 pin CPU, 6+2 pin PCIe) must match. Always check compatibility.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Set the multimeter to DC voltage (20V range). Connect the black probe to a black wire (ground) on the 24-pin connector. Use the paperclip trick to turn on the PSU. Touch the red probe to the colored wires: orange (+3.3V), red (+5V), yellow (+12V), purple (+5VSB), gray (Power Good). Acceptable ranges: +3.3V: 3.135-3.465V; +5V: 4.75-5.25V; +12V: 11.4-12.6V; +5VSB: 4.75-5.25V; PG: 3-5V. If any reading is out of spec, the PSU is faulty.
The paperclip test forces the PSU to turn on without a motherboard. By shorting the green wire (PS_ON) to any black wire (ground) on the 24-pin connector, you mimic the signal the motherboard sends to start the PSU. If the PSU fan spins, the PSU is capable of powering on. However, this test does not check voltage regulation or load handling. It's a quick way to rule out a completely dead PSU.
This is often caused by a short circuit or a PSU protection feature (OCP, OVP, SCP) tripping. Common causes: a loose motherboard standoff shorting the board, a faulty component, or the PSU failing to provide proper voltages. The PSU turns on, detects an issue, and shuts down to prevent damage. Check for shorts, reseat all components, and test with a known good PSU.
Yes. A failing PSU can deliver unstable voltages (ripple or spikes) that can damage the motherboard, GPU, RAM, or storage. Overvoltage can fry circuits, while undervoltage can cause data corruption on hard drives. A shorted PSU can also send a surge through the system. Always replace a suspect PSU promptly to avoid collateral damage.
The +5VSB (standby) rail provides power to the motherboard's power button logic, USB ports (for wake-on-USB), and other standby functions even when the PC is off. If this rail is missing, the power button will not work, and the system cannot be turned on. It should always measure close to 5V when the PSU is plugged in.
Calculate the total power draw of all components: CPU (e.g., 125W), GPU (e.g., 250W), drives, RAM, fans. Add 20-30% headroom. Use online calculators or component specs. For a typical gaming PC, 650W-750W is common. If your PSU wattage is close to the total draw, upgrade to a higher wattage unit. Also ensure the +12V rail amperage meets the GPU's requirements.
The Power Good (PG) signal is sent from the PSU to the motherboard to indicate that all output voltages are stable and within spec. The motherboard waits for this signal before initializing the CPU. If PG is missing or low, the motherboard will not start. It typically goes high (3-5V) within 100-500ms of power-on. A faulty PG signal can cause intermittent boot failures.
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