Exercise 5.2: Testing a Computer Power Supply Unit (PSU)

A Comprehensive Hands-On Guide

A detailed, hands-on exercise on how to identify, inspect, and perform functional tests on an ATX-style computer PSU using both a multimeter and a dedicated PSU tester.
Author

Chuck Nelson

Published

October 20, 2025

1 Purpose

This exercise provides a comprehensive, hands-on guide to testing a computer Power Supply Unit (PSU), one of the most critical and failure-prone components in a PC. You will learn how to visually identify a PSU’s specifications and then use two different tools—a digital multimeter and a dedicated PSU tester—to verify its functionality.

2 What You’ll Accomplish

By the end of this exercise, you will be able to:

  • Identify a PSU’s form factor, type, and primary connectors.
  • Safely jump-start a disconnected ATX PSU for testing.
  • Measure the +12V, +5V, and +3.3V rails using a multimeter.
  • Operate a dedicated PSU tester and interpret its results.

This exercise maps to the following program and course learning outcomes:

  • Program Learning Outcomes (PLOs):
    • 3. Apply terminology and numeric or system concepts: You will identify PSU components by name and measure their output against the ATX voltage specification.
    • 6. Maintain environment: This is a core skill for PC repair and maintenance, ensuring system stability by validating the power source.
  • Course Learning Outcomes (CLOs):
    • 3. Troubleshoot hardware and basic network components: This exercise provides a direct, practical application of troubleshooting skills to power systems.

This exercise develops the following skills, which align with the O*NET SOC Code 15-1232.00 for Computer User Support Specialists.

Learning Objective O*NET KSAs Technologies Used
Identify PSU form factor, type, and connectors. Knowledge: Computers & Electronics
Abilities: Information Ordering, Perceptual Speed		 PC Hardware (PSU)
Test a PSU with a multimeter and PSU tester. Knowledge: Physics
Skills: Equipment Selection, Troubleshooting, Quality Control Analysis		 Digital Multimeter, PSU Tester

3 ULTIMATE SAFETY WARNING

This exercise is the most electrically dangerous one you will perform. You must follow these rules perfectly.

  1. NEVER, EVER OPEN THE PSU CASING. The capacitors inside a PSU can hold a lethal electrical charge for a long time, even after it is unplugged. There are no user-serviceable parts inside.
  2. The “Paperclip Test” is for a DISCONNECTED PSU ONLY. You will be forcing the PSU to turn on while it is not connected to a motherboard. All connectors must be unplugged from computer components.
  3. Handle the paperclip and probes with care. When the PSU is on, the paperclip and probe tips are live. Do not touch them with your bare hands.
  4. If you are unsure about any step, STOP and ask your instructor for help.

4 Materials Needed

  • A standard ATX, SFX, or other compatible computer PSU.
  • Digital Multimeter with probes.
  • A dedicated PSU tester.
  • A bent paperclip or a purpose-made PSU jumper.

5 Part 1: Visual Inspection

Before testing, you must know what you’re working with. Examine your PSU.

  1. Identify Form Factor and Type: Look at the PSU’s size and cables.
    • Form Factor: Is it a standard square shape (likely ATX) or a smaller, more compact shape (likely SFX)?
    • Type: Are all the cables permanently attached (Non-Modular), are some detachable (Semi-Modular), or are all cables detachable (Fully-Modular)?
  2. Identify Connectors: Lay out the PSU’s cables. Identify the following key connectors.
    • Main Motherboard Power (20+4 pin)
    • CPU Power (4+4 pin)
    • PCIe Power for graphics cards (6+2 pin)
    • SATA Power (for modern drives)
    • Molex Power (for older accessories and fans)

In your student notebook, record the Form Factor, Type, and a count of each connector type your PSU has.

6 Part 2: Testing with a Digital Multimeter

This test manually checks the voltage of the main power rails.

  1. PREPARE FOR JUMP-START: Ensure the PSU is unplugged from the wall. Take your paperclip or jumper and locate the 24-pin motherboard connector. Insert one end of the paperclip into the pin for the GREEN wire (PS_ON) and the other end into any pin for a BLACK wire (Ground).

  2. POWER ON: With the jumper securely in place, plug the PSU into the wall and flip its power switch to ON. The PSU fan should spin up. The PSU is now live.

  3. SET MULTIMETER: Turn your multimeter dial to the DC Voltage setting (V⎓), in the 20V range.

  4. TEST VOLTAGES: Take a 4-pin Molex connector. Connect the multimeter’s black probe to one of the two center black (Ground) wires. Then, use the red probe to test the outer wires:

    • Touch the red probe to the YELLOW wire. The reading should be close to +12V.
    • Touch the red probe to the RED wire. The reading should be close to +5V.

  5. RECORD AND POWER OFF: In your notebook, record the exact +12V and +5V readings. Once done, unplug the PSU from the wall and remove the paperclip jumper.

7 Part 3: Testing with a Dedicated PSU Tester

This tool provides a faster, safer, and more comprehensive test.

  1. CONNECT THE PSU: With the PSU unplugged, connect the 24-pin motherboard connector and the 4/8-pin CPU connector to the corresponding sockets on your PSU tester.
  2. POWER ON: Plug the PSU into the wall and flip its power switch to ON. The PSU tester will immediately power on, and its screen or indicator lights will turn on.
  3. INTERPRET RESULTS:
    • If your tester has indicator lights, they should all be lit (usually green), indicating that each voltage rail (+12V, -12V, +5V, +3.3V, +5VSB) is active.
    • If your tester has an LCD screen, it will display the measured voltage for each rail. The values should be within a +/- 5% tolerance of their target.

In your notebook, record the results from the PSU tester. Write down the voltage shown for each rail.

8 Part 4: Analyze and Conclude

Compare your results.

  • How did your multimeter readings for the +12V and +5V rails compare to the readings from the dedicated tester?
  • Were all voltages reported by the tester within the acceptable +/- 5% range? (e.g., +12V should be between 11.4V and 12.6V).

In your notebook, write a final conclusion about the health of your PSU based on all your tests.

9 Reflect and Review

ImportantReflection: 3-2-1

Reflect on this complex exercise in your Microsoft Teams Student Notebook:

  • 3 new skills you learned in this exercise.
  • 2 differences between testing with a multimeter vs. a dedicated tester.
  • 1 question you still have about how PSUs work.
TipCheck on Learning

Answer these questions in your notebook:

  1. What two pins on the 24-pin ATX connector do you connect to jump-start a PSU, and what are their signal names?
  2. What is the acceptable voltage range for the +12V rail according to the +/- 5% ATX specification?
  3. You are testing a PSU with a multimeter. You test a yellow wire and get a reading of 10.5V. Is this PSU good or bad? Why?
  4. What is one major advantage of using a dedicated PSU tester over a multimeter?
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