Your desktop won't turn on, or it starts then cuts out under load. This page gives you the exact steps to test the power supply safely and decide if you need a repair or a replacement.
In short: Start with a visual check for physical damage, then test the standby voltage using a multimeter to confirm if the unit is completely dead. If the standby rail is present but the main rails fail, swap in a known-good unit to isolate the fault before spending money on internal repairs.
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Lets you verify if the 5V and 12V rails are actually outputting the correct voltage.
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Tests if the wall outlet and cable are delivering power before you open the case.
Why the power supply fails
Most desktop power supply failures come from three main sources. Understanding which one applies saves time and prevents you from replacing a good part. The power supply converts AC wall power into stable DC rails for your components. If any of these internal circuits degrade, the system stops working. The other half of this decision is lenovo laptop charging port repair.
- Capacitor swelling: Electrolytic capacitors degrade over time due to heat, causing voltage instability and eventual failure.
- Failed fan motor: If the cooling fan seizes, the unit overheats and trips its internal thermal protection circuit.
- Short circuit: A shorted component inside the PC can overload the power supply, causing it to shut down to prevent damage.
Check the basics before opening the case
Never assume the power supply is dead until you rule out the simplest external causes. These steps take five minutes and avoid unnecessary internal work. Start by looking at the back of the PC. Ensure the physical switch on the power supply is set to ‘I’ (on). Many owners forget this, especially after moving the computer.
Next, verify the power source. Plug the PC into a known-working wall outlet. You can test this by plugging in a lamp. If the lamp works, the outlet is live. If not, check your home breaker box. A tripped breaker is a common reason for ‘dead’ computers.
Finally, inspect the power cable itself. Look for frayed wires, loose plugs, or bent pins on the connector. A bad cable can mimic a dead power supply. If the cable looks worn, replace it before opening the case.
Safety first: Always unplug the PC from the wall before touching internal components. Static discharge can damage sensitive parts.
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Test the standby voltage with a multimeter
This is the critical diagnostic step. If the standby rail is dead, the PSU has no power at all. If it works, the main switching circuitry may be faulty, or the motherboard might not be waking the unit. You need a digital multimeter for this test. Set it to DC voltage, specifically the 20V range if available.
Unplug the PC from the wall. Open the case. Locate the 24-pin ATX connector on the motherboard.
It is usually near the top left corner. Do not plug the PC in yet. Identify the pins you need.
Pin 9 is the purple wire (5V standby). Any black wire is ground. Pin 8 is the gray wire (Power Good).
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Insert the black multimeter probe into any black ground pin. Insert the red probe into pin 9 (purple wire). Now, plug the PSU back into the wall (do not press the power button yet).
Read the voltage. It should be between 4.75V and 5.25V. If you read 0V, the standby circuit is dead, or the main fuse inside the PSU has blown. If you read a correct 5V, the PSU has basic power.
Next, test pin 8 (gray wire) while the PC is off. It should be near 0V. Press the power button. The voltage on pin 8 should jump to above 2.4V. This signal tells the motherboard that power is good. If it stays low, the PSU cannot regulate properly.
A basic multimeter allows you to verify these specific voltage readings safely.
- Accurately Test Full Features: Accurately measures AC/DC voltage, DC current
- High Quality & Safety :Our multimeter is designed to safely and accurately troubleshoot
Use the paperclip test to wake the main rails
To test the 3.3V and 12V rails accurately, you often need to wake the PSU without a motherboard. This is called the paperclip test or jump test. It allows the PSU to engage its main switching transistors. This is crucial because some PSUs only output voltage when the power button is held, which happens automatically in a full system.
Unplug the PSU from the wall. Disconnect the 24-pin connector from the motherboard. Take a metal paperclip or a wire. Straighten one end. Insert it into pin 16 (the green wire, PS_ON) and pin 15, 17, or 18 (black ground wires). The clip connects these two pins.
Plug the PSU back into the wall. The fan should spin up. This confirms the main circuit is active. Now, use your multimeter to test the output voltages. Measure pin 10 (orange wire, +3.3V), pins 4, 6, 21, 22, 23 (red wires, +5V), and pins 11, 12, 13, 14 (yellow wires, +12V). Record the readings. We cover how to fix desktop no power problem in a guide of its own.
Interpret the voltage readings
Compare your multimeter readings against the ATX specification. The acceptable range for each rail is within ±5% of the nominal value.
For the +12V rail, this means 11.4V to 12.6V. For the +5V rail, it is 4.75V to 5.25V. For the +3.3V rail, it is 3.14V to 3.47V. If any of these readings fall outside the range, the power supply is out of specification.
A borderline reading, such as 11.3V on the 12V rail, is often a sign of impending failure. While it might boot now, it will drift further under load. If the 12V rail sags below 11.4V when you run a heavy program, the PSU cannot handle the peak current. This instability often causes crashes or reboots during gaming or rendering.
If all voltages are within spec, the power supply is likely fine. The problem may be a short on the motherboard or a faulty component that is dragging the voltage down. In this case, swap in a known-good PSU to test the rest of the system.
When to stop and replace the unit
Internal repair of a power supply is rarely worth the risk for home users. Most failures are caused by components that are difficult to replace safely. The primary side of the PSU holds high voltage even when unplugged.
Opening the metal shell exposes you to dangerous capacitors. Unless you are a qualified technician with proper discharge tools, do not open the case.
Replace the PSU immediately if you see any of these signs. Look through the fan grille for swollen or leaking capacitors. Smell for burnt dust or acrid smoke inside the case. Check for scorch marks on the circuit board if you can see it. If your multimeter tests show dead rails or out-of-spec voltages, the unit is faulty.
A replacement 500W to 600W 80 PLUS certified unit is a standard and safe fix for most modern systems. It protects your CPU, GPU, and storage drives from voltage spikes. Do not keep using a failing power supply, as it can damage other expensive components.
Safety first: Do not attempt to repair the internal circuitry of a desktop power supply. The main filter capacitors can hold 300-400V DC and cause severe shock.
How to stop it happening again
Power supplies fail primarily from heat and dust accumulation. Regular maintenance significantly extends their lifespan and improves system stability. Dust acts as an insulator, trapping heat inside the unit. This forces the fan to work harder and degrades capacitors faster.
Clean the vents every six months. Use compressed air to blow dust out of the PSU fan and the surrounding case area. Hold the fan blades with a finger to prevent them from spinning freely while you clean. Do not tilt the can while spraying, as liquid discharge can damage electronics.
Manage airflow within the case. Ensure you have a bottom intake fan and adequate front-to-back airflow. Use cable management to keep wires out of the PSU’s fan intake. Finally, monitor your system’s power draw. If you are consistently running at 80-90% of your PSU’s rated capacity, consider upgrading to a unit with more headroom for longevity.
The bottom line
If the standby voltage is missing or the main rails are out of spec, your power supply has failed. Do not attempt to repair it internally; instead, buy a new 80 PLUS certified unit. Start with a multimeter test to confirm the fault before spending money on replacement.
- Accurately Test Full Features: Accurately measures AC/DC voltage, DC current
- High Quality & Safety :Our multimeter is designed to safely and accurately troubleshoot
Frequently asked questions
Can a bad power supply damage my GPU?
Yes. If the power supply outputs unstable voltage or spikes, it can damage the Graphics Processing Unit. Always replace a failing power supply immediately to protect your expensive components. The GPU is very sensitive to dirty power rails.
What is the paperclip test?
It is a way to turn on the power supply without a motherboard. You short the green wire (PS_ON) to a black ground wire on the 24-pin connector. This wakes the main voltage rails so you can test them with a multimeter without booting the full system.
Is it safe to open the power supply casing?
No. The primary side capacitors hold high voltage even when unplugged. You can receive a severe electric shock. Never open the metal shell of a power supply unless you are a qualified technician with proper safety equipment.
What voltage should the 12V rail read?
The 12V rail should read between 11.4V and 12.6V. If it reads lower or higher, the power supply is out of specification and should be replaced. Consistent drift indicates failing capacitors or regulators.