A technician suspects that a power supply is faulty. How can it be checked?
- by checking the temperature of the power supply
- by using a multimeter
- by taking apart the power supply
- by powering up the PC after disconnecting each connector in turn
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✅ Correct Answer: By using a multimeter
🔧 Introduction: Diagnosing a Suspected Faulty Power Supply
The power supply unit (PSU) is a critical component of any computer system. It converts electrical power from an outlet into usable voltages for the motherboard, CPU, GPU, storage devices, and other peripherals. If the PSU fails or behaves erratically, it can lead to system instability, boot failures, or even hardware damage.
When a technician suspects that a power supply is faulty, one of the safest and most accurate ways to verify the issue is by using a multimeter. This tool allows for measuring the output voltages of the power supply and verifying whether they fall within acceptable ranges.
🧰 What is a Multimeter?
A multimeter is a diagnostic instrument used to measure electrical values such as:
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Voltage (V)
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Current (A)
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Resistance (Ω)
When testing a PSU, the voltage function is used to check whether the power supply outputs the correct voltages on each connector.
Common PSU voltages include:
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+12V (for CPU, GPU, fans)
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+5V (for motherboard components, USB)
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+3.3V (for RAM, chipset)
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-12V (for legacy components)
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5VSB (standby voltage)
🛠️ Step-by-Step Guide: Testing a Power Supply with a Multimeter
🖥️ What You’ll Need:
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A digital multimeter
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A paperclip or PSU jumper (to start the PSU if it’s not connected to a motherboard)
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Optional: Power supply tester (for convenience)
✅ Step 1: Safety First
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Turn off the power supply and unplug it from the wall.
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If you’ve just shut the PC down, wait a few minutes for residual charges to dissipate.
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Never open the PSU casing — it contains capacitors that may hold a charge even when unplugged.
✅ Step 2: Start the PSU Manually (If Not Connected to PC)
If the PSU is not connected to a motherboard:
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Use a paperclip to jumper the green wire (PS_ON) to any black wire (ground) on the 24-pin ATX connector.
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This simulates the power-on signal.
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Plug in the PSU and turn it on — the fan should spin.
✅ Step 3: Set the Multimeter
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Turn the multimeter dial to DC voltage (V⎓).
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Use a range that allows you to read up to at least 20V.
✅ Step 4: Test Voltages
Insert the black multimeter probe into a ground wire pin (black wire) on the 24-pin ATX connector.
Then, use the red probe to test the following pins:
| Pin Voltage | Wire Color | Acceptable Range |
|---|---|---|
| +12V | Yellow | 11.4V – 12.6V |
| +5V | Red | 4.75V – 5.25V |
| +3.3V | Orange | 3.14V – 3.47V |
| -12V | Blue | -10.8V – -13.2V |
| 5VSB | Purple | 4.75V – 5.25V |
If any reading is significantly outside these ranges, the power supply may be faulty.
🧪 Interpreting Results
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All voltages in range: The PSU is likely functioning normally.
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No voltage on any rail: PSU may be dead or not turned on properly.
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Low or fluctuating voltage: PSU may be degraded or unstable.
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Only standby voltage (5VSB) present: PSU may not be receiving the power-on signal (green wire).
❌ Why the Other Options Are Incorrect
❌ Checking the Temperature of the Power Supply
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The PSU might be warm or cool depending on usage, but temperature alone doesn’t determine health.
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A cold PSU could mean it’s not working at all.
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A hot PSU could mean it’s working too hard or failing, but this is not a diagnostic method.
Conclusion: Not a reliable or recommended method for diagnosis.
❌ Taking Apart the Power Supply
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Extremely dangerous and not recommended for non-specialists.
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Internal capacitors can retain lethal voltages even when unplugged.
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Opening the PSU voids warranty and may violate safety standards.
Conclusion: Never disassemble a PSU unless you are trained and certified in power electronics.
❌ Powering Up the PC After Disconnecting Each Connector
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Disconnecting components one by one does not isolate PSU faults.
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You risk damaging components, especially if the PSU is delivering erratic voltages.
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This is trial-and-error troubleshooting, not methodical diagnosis.
Conclusion: Not safe or effective as a first diagnostic step.
🧪 Why a Multimeter is the Preferred Tool
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Safe (when used properly)
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Accurate voltage readings
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Inexpensive and portable
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Immediate results
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Helps identify issues such as:
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Dead rails
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Inconsistent power output
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Over-voltage or under-voltage conditions
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🔁 When to Replace the Power Supply
If your multimeter readings are out of spec, or the PSU:
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Makes burning smells
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Emits popping or buzzing sounds
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Fails to power up the PC even with proper voltages
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Randomly reboots or shuts down
… then it’s time to replace the power supply.
📌 Best Practices for PSU Testing
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Never work on an open PSU
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Use proper tools (multimeter or PSU tester)
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Follow manufacturer’s wiring guide
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Document all readings
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Test under load if possible, as some issues only show up when the PSU is powering actual components
🧾 Conclusion
When a technician suspects that a power supply is faulty, the most effective and safe method to verify this is by using a multimeter. It allows the technician to directly measure voltage output from each rail and compare it to expected values. This diagnostic approach avoids dangerous or unreliable practices and provides accurate, actionable information.
✅ Final Answer: By using a multimeter
It is the safest, most reliable, and industry-recommended method for diagnosing potential power supply failures.