Exercise 5.1: Testing an AC-to-DC Power Adapter

Hands-On with a Wall Wart

A hands-on exercise guiding you through using a multimeter to test a common AC-to-DC power adapter and verify its output voltage.
Author

Chuck Nelson

Published

October 20, 2025

1 Purpose

This exercise provides essential hands-on practice with a digital multimeter, one of the most fundamental tools for an IT hardware technician. You will apply your knowledge of electrical concepts to safely test a common AC-to-DC power adapter (also known as a “wall wart”) to determine if it is functioning correctly.

2 What You’ll Accomplish

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

  • Safely configure and use a digital multimeter to measure both AC and DC voltage.
  • Read and interpret the input/output specifications printed on a power adapter.
  • Perform a functional test of an adapter to verify its output voltage is within an acceptable range.

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 apply the concepts of AC/DC voltage and use a multimeter to get precise numerical readings.
    • 6. Maintain environment: This exercise is a practical application of the troubleshooting and validation skills needed to maintain a stable hardware environment.
  • Course Learning Outcomes (CLOs):
    • 1. Identify hardware and basic network components: You will identify and test a critical power component.
    • 3. Troubleshoot hardware and basic network components: This is a foundational skill for diagnosing any “no power” issue.

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
Safely use a multimeter to measure voltage. Knowledge: Physics
Skills: Equipment Selection, Troubleshooting
Abilities: Arm-Hand Steadiness		 Digital Multimeter
Compare measured voltage to component specs. Knowledge: Computers & Electronics
Skills: Critical Thinking
Abilities: Information Ordering		 Power Adapter

3 Introduction

A common task in IT support is determining why a peripheral device isn’t turning on. The problem could be the device itself or the power adapter that supplies it. This exercise will teach you how to test a standard AC-to-DC power adapter to see if it’s delivering the correct voltage.

4 Safety is Your #1 Priority

Working with electricity is dangerous. The AC power from a wall outlet can cause serious injury or death. You must follow these safety rules without exception:

  • Never touch the metal tips of the multimeter probes with your hands when they are connected to a live circuit.
  • When testing a wall outlet, hold the probes firmly and insert them directly into the slots. Do not wiggle them or touch the metal prongs.
  • Always set the multimeter to the correct function (AC or DC) and a voltage range higher than what you expect to measure.
  • If you are unsure about any step, STOP and ask your instructor for help.

5 Materials Needed

  • Digital Multimeter with probes
  • A standard AC-to-DC power adapter (“wall wart”)
  • Access to a standard wall outlet

6 Step-by-Step Guide

6.1 Step 1: Inspect the Power Adapter

Before you measure anything, you need to know what to expect. Each student may have a different adapter. Look for the label on your specific power adapter. You must find the text that specifies the INPUT and OUTPUT.

  • The INPUT will be listed in AC Volts (e.g., 100-240V~).
  • The OUTPUT will be listed in DC Volts (e.g., 9V ⎓ 500mA). The symbol ⎓ indicates DC power.

In your student notebook, write down the specified INPUT AC Voltage and OUTPUT DC Voltage for your adapter.

6.2 Step 2: Test the Wall Outlet (AC Voltage)

First, let’s confirm the power source is working.

  1. Set your multimeter: Turn the dial to the AC Voltage setting (V~). Choose a range higher than the expected 120V. The 200V~ setting is usually perfect.
  2. Insert the probes: Carefully insert the red and black probes into the two vertical slots of the wall outlet.
  3. Read the voltage: The multimeter should display a value between 110V and 125V.

In your notebook, record the AC voltage you measured. Once done, remove the probes from the outlet.

6.3 Step 3: Test the Adapter Output (DC Voltage)

Now, let’s see if the adapter is converting the power correctly.

  1. Plug in the adapter: Plug your AC-to-DC adapter into the wall outlet.
  2. Set your multimeter: Turn the dial to the DC Voltage setting (V⎓). Choose a range higher than the adapter’s specified output. If the output is 9V, the 20V⎓ setting is appropriate.
  3. Measure the output: The output of the adapter is usually a barrel-style plug. To measure the voltage:
    • Carefully insert the red probe into the center hole of the barrel plug.
    • Touch the black probe to the outer metal sleeve of the barrel plug.
  4. Read the voltage: The multimeter will display the DC output voltage. It is normal for this reading to be slightly higher than the specified rating (e.g., a 9V adapter might read 9.5V), but it should be close.

In your notebook, record the DC voltage you measured.

6.4 Step 4: Analyze Your Results

Compare your measured DC output to the specified DC output you wrote down in Step 1.

  • Is the measured voltage within about 10-15% of the specified rating? If so, the adapter is likely working correctly.
  • Is the voltage zero, very low, or fluctuating wildly? If so, the adapter is faulty and should be replaced.

In your notebook, write a concluding sentence stating whether your adapter is within spec or appears to be faulty.

7 Reflect and Review

ImportantReflection: 3-2-1

Now that you have completed this hands-on exercise, reflect on your experience in your Microsoft Teams Student Notebook:

  • 3 things you learned about using a multimeter from this exercise.
  • 2 safety precautions you will always remember when handling live circuits.
  • 1 question you still have about testing power adapters.
TipCheck on Learning

Answer these questions in your notebook to solidify your understanding:

  1. Why is it important to set the multimeter to a voltage range higher than the voltage you expect to measure?
  2. What could happen if you forgot to switch the multimeter from AC to DC mode before testing the adapter’s output?
  3. You test an adapter rated for 12V DC output. Your multimeter reads 12.5V DC. Is the adapter good or bad? Why?
  4. You test a different 12V DC adapter and the multimeter reads 0V. Before declaring the adapter dead, what is one other simple thing you should check?
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