A transformer is an electrical device that regulates the change in voltage of an alternating current (AC).

This electrical component transmits electrical energy between two circuits or even moreAs a result, it may sometimes breakdown or goes bad and causes the entire circuit to stop functioning.

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How can You Tell if a Transformer is Bad?

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A bad transformer will usually have a weak or no power supply at all. So, if a poor or no power enters the transformer unit, it has to be replaced or repaired. There are certain steps to check if a transformer is faulty or bad.

  • Primarily, perform a visual inspection. Check the voltage and inspect the meter. Secondly, you need to evaluate power.
  • Lastly, check the voltage. If there is a weak or no voltage, then the transformer is bad.

How to Test a Transformer with a Multimeter?

A DMM or digital multimeter takes the reading of voltage in a transformer. Moreover, it is readily available in any hardware store, electrical stores, or hobby shops.

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  • Firstly, you have to switch off the power supply of the transformer. Uncover all the tape wrappings or panels and have access to the wires connecting the transformers.
  • Attach the multimeter leads to the input lines and check if the primary of the transformer is not shorted.
  • Similarly, you have to carry out the same process and inspect the transformer secondary.
  • Confirm the proper input to the transformer. After that, apply the power and set the multimeter in AC mode to check the voltage in the transformer primary.
  • Lastly, measure the secondary output of the transformer. In case, if the shaping or filtering is functioning in the secondary circuit, then set the multimeter in DC mode. Else, set it in AC and take the voltage readings.

How to Test a Transformer with an Oscilloscope?

We use an oscilloscope to measure the performances of a transformer under operating conditions. But, you have to always ground your transformer while testing with an oscilloscope.

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  • Firstly, ground the circuit of the transformer. However, you can also use a ground clip and connect the common of your circuit.
  • If you need to consider differential readings, then use both the channels.
  • Using the different settings, you have to subtract one channel from the other and take the differential reading.
  • If you need a differential reading with a single channel, try taking an isolation transformer to float your circuit.

How to Test a Transformer with a Megger?

A megger is an insulation tester which provides a direct reading of insulation resistance in Ohms. It is a small, portable instrument that is available in any hardware store or hobby shop. To test a transformer with a megger you need to,

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  • Neutral all the ends of the transformers and disconnect all the lines. Connect the megger to the HV and LV bushing studs.
  • Calculate the IR resistance between the HV and LV turnings. Earth the transformer tanks and calculate the IR resistance between earth and the HV turnings.
  • Lastly, earth the transformer tanks and calculate the IR resistance between the ground and LV turnings.

How do You Test a Power Transformer?

A power transformer is one of the most important and commonly used transformers. One can use it for stepping up the voltage in an electrical power generating station. However, you can also use it for stepping down the voltage in a distribution station or a substation as well. There are some basic steps carried out almost everywhere to test a power transformer.

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  • The most common method of testing a power transformer is the Turns Ratio Testing. Here, one takes the calculated value of the ratio of the number of turns in the primary to that of the number of turns in the secondary coil.
  • This calculated value gives the corresponding voltage and expected output of the transformer.
  • Secondly, many power stations also use Insulation Resistance Testing and Power Factor Testing to test a power transformer.
  • However, you can also carry out some other methods like Phase Relation Testing, Resistance Testing, Oil Testing, and Polarity Testing to test a power Transformer.

How to Test a Plug-In Transformer?

You can use a plug-in transformer on your laptop or your cell phone. When such transformers break down, you can use an ohmmeter to test it.

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  • Firstly, you need to disconnect it from the circuit, before doing anything else.
  • Secondly, you have to touch the pins of your ohmmeter to the opposite side of the wirings of your transformer. Takedown the reading of the ohmmeter and compare it with the stated voltage of your transformer.
  • You must take at least three readings before coming to any conclusions. There are chances that your ohmmeter might not show accurate readings every time.
  • Lastly, if there is a massive difference between the reading of ohmmeter and that of the stated value of the transformer, it is most obvious that the transformer is faulty.

How to Test a Low Voltage Transformer?

You can find a low-voltage transformer generally in household appliances. We can find a low-voltage transformer from doorbells to washing machines to air conditioning systems. So here are some easy steps you can apply to test it.

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  • Using the label as a guide, you have to identify the transformer’s terminals.
  • Secondly, you need to turn a DMM to its VAC function. Then, you have to take the reading of the input voltage using the transformer’s label as a terminal guide.
  • Similarly, take the reading of the output voltage and compare it with the stated output voltage.
  • Switch off the power of the transformer and check if the voltage of the input terminals reads zero. Lastly, disconnect all the wires from both the terminals.

How to Test a 3 Phase Transformer?

A three-phase electrical power transformer has three high voltage terminals and three low voltage terminals. You have to consider the following steps to test a three-phase transformer:

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  • Provide the first plurality of voltage reduction circuits. Then, provide a second plurality of voltage reduction circuits.
  • Provide volunteering between the first voltage reduction circuit of the first plurality and the first voltage reduction circuit of the second plurality.
  • Similarly, provide the volunteering between all other respective polarities in symmetric order.
  • Check the readings with that of the stated reading and evaluate the data.

How to Check the Transformer Primary and Secondary?

A transformer consists of a primary winding and a secondary winding. The primary is a coil that draws the power from the source. On the other hand, the secondary is a coil that delivers the changed or transformed voltage to the load. You can check the primary and the secondary of a transformer by the Turns Ratio Testing method.

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  • The Turns Ratio is the ratio of the number of turns of the coil on the primary to that of the secondary. The resultant value gives you the voltage of the transformer.
  • Compare this value to the stated voltage of the transformer in the datasheet.
  • If you get massive fluctuations in the value, then the transformer is most likely considered to be bad.

How to Check Transformer Ampere?

An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. Now, if you want to check the amperage in a transformer, you can use a DMM. While there are several other methods but checking the ampere with a multimeter is the most common and widely used technique.

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  • Set the multimeter range to the level of amperage you expect to be testing. You have to run the circuit in the meter itself to test the current in a circuit.
  • Hold the black lead to the downstream and red lead to the upstream of the circuit. Take the reading in the digital multimeter and evaluate it.


  • You must read all the safety guidelines written on the transformer guide book before operating any operations.
  • Without proper knowledge of power and electricity, you must not try any kind of testing on a transformer.
  • You must get proper guidance before opening any circuit, coils, or any kind of electric products.

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