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Home and Garden
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Electrical Maintenance
Motors Generators and Transformers
How to Test a Transformer Download Article Co-authored by Jesse Kuhlman
Last Updated: July 14, 2025 Fact Checked
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Identifying Key Transformer Information
|
Testing a Transformer with a DMM
|
Troubleshooting Your Transformer
|
Video
|
Expert Q&A
|
Tips
|
Warnings
|
Things You'll Need
|Show more|Show less X
This article was co-authored by Jesse Kuhlman. Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 786,173 times.
Transformers are an electrical component that transmit electrical energy between at least two circuits. Transformers regulate the voltage in circuits, but in some cases these can go bad and cause a circuit to not work. You'll need to identify key information about your transformer, like whether it's suffered visible damage and what its inputs and outputs are. After that, it should be relatively simple to test the transformer with a digital multimeter (DMM). Should you continue having problems with the transformer, you'll need to troubleshoot it.
Steps
Part 1 Part 1 of 3:
Identifying Key Transformer Information
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1 Inspect the transformer visually. Overheating, which causes the internal wiring of the transformer to run at elevated temperatures, is a common cause of transformer failure. This often causes a physical deforming of the transformer or the area surrounding it.
If the transformer exterior is bulged or shows what appear to be burn marks, don't test the transformer. Instead, replace it.
2 Determine the wiring of the transformer. The wiring should be clearly labeled on the transformer. However, it is always best to obtain a schematic of the circuit containing the transformer to determine how it is connected.
The schematic for the circuit will be available in the product information or on the website of the circuit manufacturer.[1]XResearch source
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3 Identify the transformer inputs and outputs. The first electrical circuit will be connected to the primary of the transformer. This is its electrical input. The second circuit receiving power from the transformer is connected to the transformer secondary, or the output.[2]XResearch source
The voltage being supplied to the primary should be labeled both on the transformer and the schematic.
The voltage being generated by the secondary should be labeled in the same fashion as the primary.
4 Determine the output filtering. It is common to attach capacitors and diodes to the transformer secondary to convert the AC power from the output to DC power. This information will not be available on the transformer label.
Generally, you can find the transformer conversion and output filtering information on the schematic.
Look for whether the transformer is AC or DC wherever the voltage is listed on the label.[3]X Expert Source Jesse KuhlmanMaster Electrician, Kuhlman ElectricExpert Interview
Advertisement Part 2 Part 2 of 3:
Testing a Transformer with a DMM
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1 Prepare to measure the circuit voltages. Turn off the power to the circuit. Remove covers and panels as necessary to gain access to the circuits that contain the transformer. Acquire a digital multimeter (DMM) to take the voltage readings. DMMs are available at electrical supply stores, hardware stores, and hobby shops.
Generally, you'll need to attach the leads of your DMM to the input lines to verify that the primary of the transformer is not shorted. The same process will be used to check the transformer secondary.
2 Confirm the proper input to the transformer. Apply power to the circuitry. Use the DMM in AC mode to measure the transformer primary. If the measurement is less than 80 percent of the expected voltage, the fault could lie in either the transformer or the circuitry providing the primary with power. In that case:
Separate the transformer from the input circuit. Test the input with your DMM. If the input power climbs to the expected value, the primary of the transformer is bad.
If the input power does not climb to the expected value, then the problem lies not with the transformer, but with the input circuitry.
The input and output on the transformer may be labeled with "input" and "output," or the input might be a black and white pigtail.[4]X Expert Source Jesse KuhlmanMaster Electrician, Kuhlman ElectricExpert Interview
If the transformer has terminals, the input will usually be L, which stands for "line," or hot power, and N, which stands for neutral, or the neutral power going into that wire. The output will be the low voltage side.[5]X Expert Source Jesse KuhlmanMaster Electrician, Kuhlman ElectricExpert Interview
3 Measure the secondary output of the transformer. If there is no filtering or shaping being performed by the secondary circuitry, use the AC mode of the DMM to read its output. If there is, use the DC scale of the DMM.
If the expected voltage is not present on the secondary, either the transformer or a filtering or shaping component is bad. Test the filtering and shaping components separately.
If the testing of the filtering and shaping components show no problems, then the transformer is bad.
Advertisement Part 3 Part 3 of 3:
Troubleshooting Your Transformer
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1 Understand the root of the problem. Transformer failure is usually a symptom of a different kind of failure somewhere in the electrical circuit. Transformers generally have long lives, and rarely burn out by themselves.[6]XResearch source
2 Observe replaced transformers. If the problem causing your transformer to short out is coming from elsewhere in your circuit, it's likely the transformer will burn out again. After you've replaced the transformer, observe it to make sure this does not happen. If it does, you'll need to perform additional tests.
An overloaded transformer will often make snapping and crackling noises. If you hear sounds like these, cut power to the transformer to prevent burnout.[7]XResearch source
3 Verify the condition of external fuses, if necessary. If your transformer has an internal fuse, you may not have fuses in the line leading up to the transformer. Otherwise, there should be fuses in the power supply line to the transformer. Check to make sure these are in good condition and replace any that aren't working properly.
Blackness, melting, and deforming in fuses are good indications that the fuse has been damaged. Simple remove and replace these.
In some cases, it might be hard to tell if a fuse is in good condition. Attach your DMM to the fuse with one lead on each fuse end. If current runs through the fuse, it's good.[8]XResearch source
4 Check for overdraw on your secondary. In some cases, the secondary of your transformer may be drawing too much current, causing it to short out. If you have a multi-tap transformer and you receive the reading "OL" from the secondary, it's likely the secondary is shorted.
Test this by hooking up the secondary to its circuit and using your DMM to test the secondary lines. If the reading is above the amperage rating for the transformer, the circuit is drawing too much power.
Many common transformers have 3 amp fuses. The amperage rating for your transformer fuse may be labeled on the transformer, but will also be available in the circuit schematic.[9]XResearch source
5 Remove inputs and outputs to determine the source of the failure. For linear fuses, you'll only have one input and output. In this case, your trouble is either coming from the input circuit or the output circuit. For more complex fuses, remove inputs and outputs to the transformer one by one to determine what component of the entire circuit is causing the short.[10]XResearch source
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Expert Q&A
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Question Where should I look to find the input and output of a transformer? Jesse Kuhlman Master Electrician, Kuhlman Electric Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes. Jesse Kuhlman Master Electrician, Kuhlman Electric Expert Answer The input and output on a transformer is almost always going to be labeled—usually simply with "input" and "output." Thanks! We're glad this was helpful.Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 31 Helpful 21
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Video
Tips
A buzzing or cracking sound is frequently an early sign that a transformer is about to develop a burn. Thanks Helpful 12 Not Helpful 8
Do not assume that the primary side and secondary side of the transformer are referenced to the same electrical ground. The transformer primary and secondary are frequently referenced to different grounds. Be aware of this split grounding while taking measurements. Thanks Helpful 6 Not Helpful 10
Submit a Tip All tip submissions are carefully reviewed before being published Name Please provide your name and last initial Submit Thanks for submitting a tip for review! Advertisement
Warnings
While the circuits are exposed and energized for testing, accidental contact with circuit power could cause shock or injury. Touch the circuit only with the probes of the DMM. Thanks Helpful 5 Not Helpful 1
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Things You'll Need
Schematic of the circuit
Digital Multimeter (DMM)
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Co-authored by: Jesse Kuhlman Master Electrician, Kuhlman Electric This article was co-authored by Jesse Kuhlman. Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes. This article has been viewed 786,173 times. 20 votes - 75% Co-authors: 19Updated: July 14, 2025 Views: 786,173 Categories: Motors Generators and TransformersArticle SummaryX
To test a transformer with a digital multimeter (DMM), first turn off power to the circuit. Next, attach the leads of your DMM to the input lines. Use the DMM in AC mode to measure the transformer primary. If the measurement is less than 80% of the expected voltage, your problem could be with the transformer or the circuitry providing the primary power. To measure the secondary output, use the AC mode if there’s no filtering or shaping. Otherwise, use the DC scale to determine what the issue is. To learn how to troubleshoot your transformer, keep reading!Did this summary help you?YesNo
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Reader Success Stories
George Pfenzen
Mar 9, 2022
"I am na 80 year old man, could not sleep and was thinking of a light with a bulb in fixture lit and other not. You were helpful but as expected over my head somewhat, old fixture with starter and will buy starter and replace and go from there. Thanks."..." more
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Yes No Advertisement Cookies make wikiHow better. By continuing to use our site, you agree to our cookie policy. Co-authored by: Jesse Kuhlman Master Electrician, Kuhlman Electric 20 votes - 75% Click a star to vote Co-authors: 19Updated: July 14, 2025 Views: 786,173
George Pfenzen
Mar 9, 2022
"I am na 80 year old man, could not sleep and was thinking of a light with a bulb in fixture lit and other not. You were helpful but as expected over my head somewhat, old fixture with starter and will buy starter and replace and go from there. Thanks."..." more
Wayne Szabo
May 5, 2018
"It is always good to read a step by step article about something that you do not work with all the time. I find it very helpful going over the process in case I missed something and in this case I did overlook a potential problem with the circuit."..." more
Jelord Guardiano
Mar 14, 2017
"This article helped me a lot because it's kind of short, but very informative and very easy to do. Applying this kind of info, we can get a lot without hassle because it is straight to the point and the lay out is very good!"..." more
Robbie Carrobie
May 27, 2020
"Explained accurately and with illustrations and a video presentation of how a transformer can be tested and how to trouble shoot it when it does not. Top notch education."..." more
Pillay Tom
Mar 8, 2017
"I want to test my fridge transformer, it clicks off in 3 seconds. " Share yours! More success stories Hide success stories
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Co-authored by: Jesse Kuhlman Master Electrician, Kuhlman Electric This article was co-authored by Jesse Kuhlman. Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes. This article has been viewed 786,173 times. 20 votes - 75% Co-authors: 19 Updated: July 14, 2025 Views: 786,173 Categories: Motors Generators and Transformers Article SummaryX
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1 Inspect the transformer visually. Overheating, which causes the internal wiring of the transformer to run at elevated temperatures, is a common cause of transformer failure. This often causes a physical deforming of the transformer or the area surrounding it. 
2 Determine the wiring of the transformer. The wiring should be clearly labeled on the transformer. However, it is always best to obtain a schematic of the circuit containing the transformer to determine how it is connected. 
3 Identify the transformer inputs and outputs. The first electrical circuit will be connected to the primary of the transformer. This is its electrical input. The second circuit receiving power from the transformer is connected to the transformer secondary, or the output.[2] 
4 Determine the output filtering. It is common to attach capacitors and diodes to the transformer secondary to convert the AC power from the output to DC power. This information will not be available on the transformer label. 
1 Prepare to measure the circuit voltages. Turn off the power to the circuit. Remove covers and panels as necessary to gain access to the circuits that contain the transformer. Acquire a digital multimeter (DMM) to take the voltage readings. DMMs are available at electrical supply stores, hardware stores, and hobby shops. 
2 Confirm the proper input to the transformer. Apply power to the circuitry. Use the DMM in AC mode to measure the transformer primary. If the measurement is less than 80 percent of the expected voltage, the fault could lie in either the transformer or the circuitry providing the primary with power. In that case: 
3 Measure the secondary output of the transformer. If there is no filtering or shaping being performed by the secondary circuitry, use the AC mode of the DMM to read its output. If there is, use the DC scale of the DMM. 
1 Understand the root of the problem. Transformer failure is usually a symptom of a different kind of failure somewhere in the electrical circuit. Transformers generally have long lives, and rarely burn out by themselves.[6] 
2 Observe replaced transformers. If the problem causing your transformer to short out is coming from elsewhere in your circuit, it's likely the transformer will burn out again. After you've replaced the transformer, observe it to make sure this does not happen. If it does, you'll need to perform additional tests. 
3 Verify the condition of external fuses, if necessary. If your transformer has an internal fuse, you may not have fuses in the line leading up to the transformer. Otherwise, there should be fuses in the power supply line to the transformer. Check to make sure these are in good condition and replace any that aren't working properly. 
4 Check for overdraw on your secondary. In some cases, the secondary of your transformer may be drawing too much current, causing it to short out. If you have a multi-tap transformer and you receive the reading "OL" from the secondary, it's likely the secondary is shorted. 
5 Remove inputs and outputs to determine the source of the failure. For linear fuses, you'll only have one input and output. In this case, your trouble is either coming from the input circuit or the output circuit. For more complex fuses, remove inputs and outputs to the transformer one by one to determine what component of the entire circuit is causing the short.[10] Advertisement 
Jesse Kuhlman Master Electrician, Kuhlman Electric Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes.