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newSchoolWorks.com › VoTechWorks › Technology & Design › IT Support / Networking › PC Hardware: Installation, Maintenance, and Troubleshooting › Lesson 6
TEC-ITS43-02 · Lesson 6 · 10.5 clock hours · demo

Power Supplies: Selection, Testing, and Replacement

Objective. Students will use a digital multimeter to measure PSU output voltages on the 24-pin ATX connector and verify they fall within ATX specification tolerances.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Power Supplies Matter

The power supply unit (PSU) converts AC power from the wall outlet into the stable DC voltages that every component inside a PC depends on. A PSU that drifts out of specification can corrupt data, cause random reboots, shorten component life, or damage hardware entirely. Learning to test and evaluate a PSU with a digital multimeter (DMM) is a foundational IT support skill and appears on both the CompTIA A+ and Cisco CCST exams.

ATX Voltage Rails and Tolerances

Modern desktop PSUs follow the ATX specification maintained by PESA (Power Electronics and Standards Association, continuing Intel's original ATX work). The spec defines several DC output rails:

The ATX specification allows a ±5% tolerance on the +3.3 V, +5 V, and +12 V rails. That means:

| Rail | Nominal | Minimum | Maximum |

|------|---------|---------|--------|

| +3.3 V | 3.30 V | 3.135 V | 3.465 V |

| +5 V | 5.00 V | 4.750 V | 5.250 V |

| +12 V | 12.00 V | 11.400 V | 12.600 V |

| −12 V | −12.00 V | −11.400 V | −12.600 V |

A reading outside these ranges is a fail. The −12 V rail also carries a ±5% tolerance per the ATX spec, though its allowable current is very small.

Safety First

Before touching any PSU:

1. Lockout/Tagout (LOTO): If the system is connected to a live outlet for any reason other than intentional testing, follow your lab's LOTO procedure—unplug the power cord and tag the outlet before opening the case.

2. ESD precautions: Wear an anti-static wrist strap or regularly touch the bare metal of the grounded chassis before handling components.

3. Never probe inside the PSU enclosure. Lethal voltages (over 300 V DC on the internal filter capacitors) can persist for several minutes after the unit is unplugged. All measurements in this lesson are made on the low-voltage output side only—the connectors going to the motherboard and drives.

4. Meter lead placement: Insert the red probe lead into the VΩ input jack and the black lead into the COM jack of the Fluke 101 (or equivalent). Set the dial to DC Volts (V⎓). Never use the AC voltage setting for PSU output measurements.

5. Keep the paperclip jumper or PSU tester in place while probing. Without a load or a jumper bridging the PS_ON pin to ground on the 24-pin connector, the PSU will not power on at all.

The Paperclip Test and the 24-Pin ATX Connector

A PSU will not start without a signal from the motherboard. To test a PSU outside a fully assembled system, you can short the PS_ON pin (pin 16, green wire) to any adjacent ground pin (black wire) on the 24-pin ATX connector using a straightened paperclip or a dedicated PSU tester. This tricks the PSU into turning on standalone. Once the PSU fan spins, the output rails are live and ready to measure.

24-pin ATX breakout boards expose each pin individually, making probing much safer and easier than reaching into a live connector with bare probes.

Measuring Voltage Rails

1. Confirm the PSU is off and unplugged, then insert the paperclip jumper or attach the PSU tester.

2. Set your multimeter to DC Volts.

3. Plug in the PSU and switch it on. Confirm the fan spins.

4. Place the black (COM) probe on any ground pin (black wire) of the 24-pin connector.

5. Touch the red probe to the pin you want to measure (+3.3 V = orange wire, +5 V = red wire, +12 V = yellow wire, −12 V = blue wire, 5VSB = purple wire).

6. Read the voltage displayed. Record it immediately in your lab log.

7. Compare the reading to the ATX tolerance table above.

Always keep your non-probing hand away from the connector while power is applied. Move one probe at a time.

Selecting the Right PSU Wattage

Once you confirm a PSU passes voltage testing, you need to verify it has enough wattage capacity for the build. Add up the thermal design power (TDP) of the CPU, the rated power draw of the GPU, and estimates for storage drives, RAM, and peripherals. A commonly accepted rule of thumb is to add 20% headroom above the calculated total so the PSU operates efficiently and handles transient spikes without straining.

For example: CPU TDP 65 W + GPU 150 W + two SSDs 10 W + RAM 15 W + peripherals 20 W = 260 W total. Adding 20% headroom: 260 W × 1.20 = 312 W minimum PSU. You would select the next standard size up—typically a 350 W or 400 W unit.

Interpreting Results

If any rail reads out of the ±5% tolerance window, document the exact reading, label the PSU as marginal or failed, and flag it for replacement. Never return a suspect PSU to service—an underperforming +12 V rail under load degrades faster than it does at idle, meaning a marginal reading at rest can become a full failure during a CPU-intensive task.

Key terms9

ATX specificationAn industry standard document defining the physical form factor, connector pinout, and DC output voltage tolerances for PC power supplies.
Voltage railA distinct DC output voltage (such as +12 V or +5 V) provided by the PSU to power different categories of components.
±5% toleranceThe allowable deviation from a nominal voltage value permitted by the ATX specification; for example, the +12 V rail may read between 11.4 V and 12.6 V.
PS_ON pinPin 16 (green wire) on the 24-pin ATX connector; when pulled to ground by the motherboard—or by a paperclip during standalone testing—it signals the PSU to turn on.
Paperclip testA bench technique that shorts the PS_ON pin to a ground pin on the 24-pin ATX connector, enabling a PSU to power on without a motherboard for testing purposes.
Digital multimeter (DMM)A handheld electronic instrument used to measure DC and AC voltage, current, and resistance; the primary tool for PSU voltage rail testing.
Thermal design power (TDP)The maximum heat (in watts) that a CPU or GPU is designed to generate under sustained load; used as an approximation of power consumption when sizing a PSU.
Wattage headroomExtra PSU capacity beyond the calculated system load—commonly 20%—to ensure stable operation and accommodate power spikes.
Lockout/Tagout (LOTO)An OSHA-recognized safety procedure requiring that energy sources be isolated and physically locked or tagged before maintenance begins, preventing accidental energization.

Worked examples4

1. Your multimeter reads 11.35 V on the +12 V rail of a PSU under test. Does this reading pass or fail the ATX specification?
  1. Identify the nominal voltage: +12.00 V.
  2. Calculate the lower tolerance limit: 12.00 × (1 − 0.05) = 12.00 × 0.95 = 11.40 V.
  3. Calculate the upper tolerance limit: 12.00 × (1 + 0.05) = 12.00 × 1.05 = 12.60 V.
  4. Compare the reading to the range: 11.35 V is below 11.40 V.
Answer: FAIL — 11.35 V is outside the acceptable range of 11.40 V to 12.60 V.
2. Your multimeter reads 3.28 V on the +3.3 V rail. Does this reading pass or fail?
  1. Identify the nominal voltage: +3.30 V.
  2. Calculate the lower tolerance limit: 3.30 × 0.95 = 3.135 V.
  3. Calculate the upper tolerance limit: 3.30 × 1.05 = 3.465 V.
  4. Compare the reading: 3.28 V is between 3.135 V and 3.465 V.
Answer: PASS — 3.28 V is within the acceptable range of 3.135 V to 3.465 V.
3. A build has a CPU with a 95 W TDP, a GPU rated at 200 W, two HDDs at 8 W each, 16 GB of RAM estimated at 10 W, and a USB hub at 5 W. What is the minimum PSU wattage needed with 20% headroom?
  1. Sum all component loads: 95 + 200 + 8 + 8 + 10 + 5 = 326 W.
  2. Apply 20% headroom: 326 × 1.20 = 391.2 W.
  3. Round up to the next practical PSU size: 400 W.
Answer: Minimum PSU: 400 W (the next standard size above 391.2 W).
4. A PSU reads +5.28 V on the +5 V rail. Does it pass or fail?
  1. Identify the nominal voltage: +5.00 V.
  2. Calculate the upper tolerance limit: 5.00 × 1.05 = 5.25 V.
  3. Calculate the lower tolerance limit: 5.00 × 0.95 = 4.75 V.
  4. Compare the reading: 5.28 V exceeds 5.25 V.
Answer: FAIL — 5.28 V is above the maximum allowable value of 5.25 V.

The math in this lesson2

Learners compute a plus or minus 5% band on +12 V (11.40-12.60 V) and fail an 11.35 V multimeter reading.
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Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates safe PSU testing procedure: verify system is powered off, use a PSU paperclip test or PSU tester to enable the PSU standalone, then probe pins with the multimeter—students observe and note expected voltage rails (+3.3 V, +5 V, +12 V, −12 V) and acceptable tolerance (±5% for most rails per ATX spec). · Students take turns measuring voltage rails on a powered test bench using a Fluke 101 or equivalent multimeter; each student records five readings and flags any out-of-tolerance value. · Students calculate the total system wattage for a hypothetical build (CPU TDP + GPU TDP + storage + peripherals) using a wattage estimation worksheet and select an appropriate PSU wattage with 20% headroom.
Materials: Two ATX PSUs (one known-good, one known-marginal), 24-pin ATX breakout board or paperclip jumper, Fluke 101 or equivalent digital multimeter, PSU wattage estimation worksheet, ATX specification reference (current version from PESA/Intel ATX spec document), lab log notebooks
Check: Each student's five recorded voltage readings are compared against ATX tolerances by the instructor; student must correctly identify whether each rail passes or fails—pass mark is 4 of 5 rails correctly evaluated, and no unsafe probing technique observed.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

CompTIA A+ — Hardware
1. Which wire color on a standard 24-pin ATX connector carries the +12 V rail?
CompTIA A+ — Hardware
2. What is the minimum acceptable voltage on the +5 V rail according to the ATX specification ±5% tolerance?
CompTIA A+ — Safety and Professionalism
3. A technician is about to probe PSU output pins while the unit is energized. Which safety action must be completed FIRST?
CompTIA A+ — Safety and Professionalism
4. Why should a technician never probe inside the PSU enclosure even after the unit is unplugged from the wall?
CompTIA A+ — Hardware
5. The PS_ON pin on a 24-pin ATX connector must be connected to which signal to turn on the PSU during a standalone paperclip test?
CompTIA A+ — Hardware
6. A student measures 11.90 V on the +12 V rail of a PSU under no-load conditions. What is the correct evaluation?
CompTIA A+ — Hardware
7. A technician estimates a system's total power draw at 340 W. Applying 20% headroom, what is the minimum PSU wattage the technician should select?
CompTIA A+ — Hardware
8. Which of the following BEST describes the role of the +12 V rail in a modern PC?
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