Exercise 5.0: Testing a Simple Transformer

Hands-On with AC Voltage

A hands-on exercise guiding you through the process of using a multimeter to test a simple iron core AC transformer and verify its voltage step-down ratio.
Author

Chuck Nelson

Published

October 20, 2025

1 Purpose

This exercise provides essential hands-on practice with a digital multimeter to test a fundamental electrical component: a simple iron core transformer. You will apply your knowledge of AC electricity to safely measure the input and output voltages, calculating the transformer’s step-down ratio to see if it is within specification.

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 AC voltage on both the primary and secondary coils of a transformer.
  • Read and interpret the input/output voltage specifications of a simple transformer.
  • Perform a functional test to verify a transformer’s output voltage is correct relative to its input.

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 concept of AC voltage and transformer ratios, using a multimeter to get precise numerical readings.
  • Course Learning Outcomes (CLOs):
    • 1. Identify hardware and basic network components: You will identify and test a fundamental power component.
    • 3. Troubleshoot hardware and basic network components: This is a foundational skill for diagnosing power issues at the component level.

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 AC 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		 Iron Core Transformer

3 Introduction

The first stage in any power supply is transformation. In this exercise, you will test a simple iron core transformer, the type used to step AC wall voltage down to a lower, more manageable AC voltage. Unlike a complete power adapter, this component does not convert the power to DC; it only changes the voltage.

4 Safety is Your #1 Priority

Working with electricity is dangerous. Both the input and output sides of the transformer will have live AC voltage.

  • Never touch the metal tips of the multimeter probes or the transformer’s terminals with your hands when it is plugged in.
  • Always set the multimeter to the correct AC voltage range higher than what you expect to measure.
  • Ensure the transformer is on a stable, non-conductive surface. Do not allow the input or output terminals to touch each other or any metal objects.
  • If you are unsure about any step, STOP and ask your instructor for help.

5 Materials Needed

  • Digital Multimeter with probes
  • A simple iron core transformer with clearly marked primary (input) and secondary (output) windings (e.g., 120V to 12V AC).
  • A power cord to safely connect the transformer’s primary windings to a wall outlet.
  • Access to a standard wall outlet.

6 Step-by-Step Guide

6.1 Step 1: Inspect the Transformer

Before you connect power, you must know what to expect. Look for markings or a label on the transformer. You should see specifications for the Primary (Input) and Secondary (Output) windings.

  • The Primary side will be rated for the wall voltage (e.g., 120V AC).
  • The Secondary side will be rated for a lower AC voltage (e.g., 12V AC).

In your student notebook, write down the specified Primary and Secondary AC voltages for your transformer.

6.2 Step 2: Test the Input Source (AC Voltage)

First, confirm the power source is working.

  1. Set your multimeter: Turn the dial to the AC Voltage setting (V~). Choose a range higher than 120V, such as 200V~.
  2. Measure the outlet: Carefully insert the probes into the wall outlet. You should read between 110V and 125V.

In your notebook, record the exact AC voltage you measured from the outlet. Unplug the probes.

6.3 Step 3: Test the Transformer Output (AC Voltage)

Now, let’s see if the transformer is stepping down the voltage correctly.

  1. Connect and power on: With the transformer unplugged, safely connect the power cord to the primary (input) terminals. Plug the cord into the wall outlet. The transformer is now live.
  2. Set your multimeter: Turn the dial to the AC Voltage (V~) setting. Choose a range higher than the transformer’s specified output. If the output is rated for 12V AC, the 20V~ or 200V~ setting is appropriate.
  3. Measure the output: Carefully touch the multimeter probes to the two secondary (output) terminals of the transformer.
  4. Read the voltage: The multimeter will display the stepped-down AC output voltage. It is normal for this reading to be slightly different from the rating, especially when there is no load connected.

In your notebook, record the AC voltage you measured on the secondary coil. Once done, unplug the transformer from the wall outlet.

6.4 Step 4: Analyze Your Results

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

  • A transformer’s job is to change voltage by a specific ratio. For example, a 120V to 12V transformer has a 10:1 ratio.
  • Calculate the actual ratio: (Measured Input Voltage) / (Measured Output Voltage).
  • Is this ratio close to the expected ratio? If so, the transformer is working correctly.
  • If the output voltage is zero or extremely low, the transformer is faulty.

In your notebook, write a concluding sentence stating whether your transformer is operating within its specified ratio.

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 testing a simple transformer.
  • 2 safety precautions that are most critical when working with this type of component.
  • 1 question you still have about AC voltage or transformers.
TipCheck on Learning

Answer these questions in your notebook to solidify your understanding:

  1. Why did you use the AC Voltage setting on your multimeter for both the input and output readings?
  2. If a transformer is rated for 120V input and 24V output, what is its step-down ratio?
  3. You measure 118V at the wall outlet and connect it to a 10:1 transformer. What voltage would you expect to read on the output terminals?
  4. You test a transformer and the output voltage is 0V. Before declaring the transformer dead, what is one other simple thing you should check first?
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