How to Test a Transformer

A transformer is a device that transmits electrical energy between two or more circuits. A transformer helps regulate the voltage in a circuit, but sometimes it can fail and cause the circuit to fail. You need to identify key information about the transformer, such as physical damage to the machine, and the input and output locations of the current. After that, the rest is simply using an electronic multimeter (DMM) to test the transformer. If the transformer problem is not solved then you need to fix it.

Table of Contents

Find key information about transformers

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Check transformer appearance. Overheating causes lines inside the transformer to operate at high temperatures, which is a common cause of transformer failure. This often causes physical distortion of the machine or the surrounding area.

If the transformer enclosure swells or shows signs of burning, do not conduct the test. Instead, you should buy a new machine.

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Determine the wiring of the transformer. Wiring directions will be clearly marked on the transformer. However, it is best to find the circuit diagram of the transformer to know how to connect the wires to the machine.

Circuit diagrams are provided in the product information or on the circuit manufacturer’s website. ^{[1] X Research Source}

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Determine the input and output of the transformer. The first circuit will be connected to the primary winding of the transformer. This is the input current. The second circuit receiving power from the transformer will be connected to the secondary or output winding. ^{[2] X Research Source}

The voltage applied to the primary winding will be noted on both the transformer and the circuit diagram.
The voltage generated from the secondary coil will be noted in the same way as in the primary coil.

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Locate the output filter. It is common to attach capacitors and diodes to the secondary coil to convert the alternating current from the output into direct current. This information will not be labeled on the transformer. ^{[3] X Research Sources}

In general, you can find information about the filter and output converter on the circuit diagram.
Determine if the transformer is a.c. or d.c. at locations marked with voltage.

Check transformer with electronic multimeter

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Prepare before measuring circuit voltage. Turn off the power supply to the circuit. Remove the covers and covers to gain access to the circuits inside the transformer. Buy a digital multimeter (DMM) to measure voltage. DMM is available at electrical and plumbing stores or hardware stores. ^{[4] X Research Sources}

In general, you need to connect the DMM’s test lead to the input of the transformer to verify if the primary winding of the machine is short-circuited. Use the same way to test the secondary circuit of the transformer.

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Determine the correct input of the transformer. First, you will supply power to the machine. Use the DMM in AC mode to measure the current in the primary coil. If the reading is below 80% of the expected voltage, the fault may be in the transformer or in the circuit that powers the primary winding. In that case:

Separate the transformer from the input circuit. Test the input circuit with the DMM. If the input current reaches the expected value, the primary winding of the transformer has failed.
If the input current does not reach the expected value then the problem is not with the transformer but with the input circuit. ^{[5] X Research Sources}
The input and output power of the transformer can be marked with a label, or the input is made of two black and white wires.
If the transformer is bridged, the input will be denoted by the letter L representing hot current, and the letter L representing neutral current. The output of the transformer is the side that produces the lower voltage.

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Measure the secondary voltage at the output of the transformer. If there is no filter or converter attached to the secondary circuit, you can use the AC mode of the DMM to measure the output voltage. Otherwise, you would use the DMM’s DC scale.

If there is no voltage on the secondary circuit then the transformer, filter or converter has failed. Check the filter and adapter separately.
If inspection of the filter and converter shows no problems, the transformer has failed. ^{[6] X Research Source}

Troubleshooting transformers

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Find out the root cause of the problem. A transformer failure is often a sign of another cause that originates somewhere in the circuit. Transformers usually have a long service life and rarely spontaneously burn out. ^{[7] X Research Sources}

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Monitor transformer to be replaced. If the cause of the transformer short-circuiting arises somewhere in the circuit, it is more likely that the transformer will continue to burn. After replacing the transformer with a new one, monitor to make sure this does not happen. If the damage continues, you need to perform other tests.

An overloaded transformer will often produce a crackling sound. If you hear this sound, disconnect the input power to avoid burning. ^{[8] X Research Sources}

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Verify condition of external fuses if necessary. If the transformer has an internal fuse then there probably won’t be a fuse on the line leading to the transformer. Otherwise, a fuse will be installed on the power supply line to the transformer. Check to make sure the fuses are working properly and replace the damaged ones.

Blacked, melted, and deformed fuses are obvious signs of failure. You just need to replace it with a new one.
In some cases, it is difficult to judge whether the fuse is in good working order or not. Attach the DMM to the fuse by connecting each probe to one side of the fuse. If there’s current flowing through the fuse, it’s fine. ^{[9] X Research Source}

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Check the secondary coil for overload. In some cases, the secondary winding of a transformer draws too much power and causes a short circuit. If you use a multi-output transformer and the DMM shows “OL” on the secondary, the secondary may be short-circuited.

Check this by attaching the secondary coil to the circuit and using a DMM to test the secondary circuit. If the measured value is higher than the transformer’s ampere rating, too much current is being drawn.
Common transformers have 3 fuses. The ampere rating of a transformer fuse may be labeled on the transformer, but also noted in the circuit diagram. ^{[10] X Research Source}

Advice

A crackling sound is often an early sign that a transformer is about to burn out.
Do not assume that the primary and secondary side of the transformer are both referenced to the same earth voltage level. The primary and secondary windings of a transformer are often referenced to different earth voltage levels. Pay attention to this difference when you measure the current.

Warning

While opening a circuit for testing, accidental contact with current in the circuit may result in electric shock or injury. Use only DMM probes to access the circuit.

Things you need

Map of electric
Electronic multimeter (DMM)

Steps