How to Test a Welding Transformer in 10 Steps


Issues with welder performance can often be traced back to the welding transformer. You can find out whether the transformer is the source of the problem by performing a quick series of tests on the transformer. P you won’t have to pay someone else to diagnose problems with your welder.

How do I test my welding transformer? There are 10 steps for testing a welding transformer. Here they are:

  1. Perform a Visual Inspection
  2. Figure Out the Wiring Pattern
  3. Get a Multimeter
  4. Make Sure the Power Has Been Disconnected
  5. Double Check the Power
  6. Check the Input Voltage
  7. Check the Output Voltage
  8. Continuity Test in Primary Windings
  9. Continuity Test in Secondary Windings
  10. Troubleshoot Welder Performance Issues

You will be able to breeze through these 10 steps for testing a welder transformer with ease once you understand how to complete each step. Below, you will find detailed guidance on how to troubleshoot issues with welding transformers.

1.  Perform a Visual Inspection of the Transformer

Start out by obtaining the owner’s manual for your welder. The information in this document can be quite thorough in details, as you can see if you look at the owner’s manual for one specific model of a MIG welder manufactured by Hobart.

Deep within the owner’s manual is a cut-away parts diagram showing the numerous parts included in the assembly of the machine. This will help you locate the transformer for a visual inspection. It will also show you where different parts of the welder should be located in the event that you have to remove other parts to access the transformer.

I would strongly recommend taking good pictures as you take apart the welder to access the transformer. This will help you put the welder back together. Photos are also a great way to show anything unusual to the manufacturer or to a repairman without having to physically show them the welder.

Once you are able to access the transformer, look for the following signs that there may be a problem with your transformer:

  • Signs of overheating: deformities or melting at the exterior of the transformer or the parts around it
    • Do not bother testing the transformer if there are obvious signs of overheating
  • Loose connections: loose connections could be causing your transformer to go haywire.
  • Bulging: the transformer will need to be replaced if it appears that any part of it is bulging out, this is another sign of damage from overheating

2.  Figure Out the Wiring Pattern

Testing the transformer in your welder requires you to understand how the transformer has been put together. The wiring scheme should be shown in the owner’s manual. Most welders owner’s manuals will include a complex wiring diagram.

In general, the transformers used in welding follow this design scheme:

  • Primary Taps and Secondary Taps are located in the secondary windings
    • The secondary winding is connected to a receptacle or current switch
    • One side of the secondary winding is connected to the welding rod while the other is joined to the welded pieces
  • The primary and secondary Taps serve the purpose of lowering the voltage in the system
  • Tap switches (not included in all welders)
    • These allow the welder the opportunity to adjust the voltage by turning the tap

Once you have a general idea of the wiring pattern in your welding transformer, you can proceed to perform tests to determine how the transformer is performing, all while using equipment that is inexpensive.

3.  Get a Multimeter

The first step to testing your welding transformer is getting yourself a multimeter such as this Etekcity Digital Multimeter. An affordable multimeter such as the one made by Etekcity will provide you with the capabilities to:

  • Measure AC/DC voltage from a DC current supply
  • Resistance
  • Diode
  • Continuity

The multimeter listed above can only be used to measure DC current. If you need to measure these parameters in a system with AC current, you will need a multimeter such as this Etekcity Digital Multimeter for AC Current.

The Meterek Digital Clamp Multimeter is the more versatile option. It can accurately measure both AC and DC voltage and current. It also includes a dedicated mode for continuity testing, among other mode features.

The continuity test is an important step in the procedure for testing welding transformers, as will be discussed later in this article.

Related reading: What’s The Difference Between AC And DC Welding >> AC vs. DC

4.  Make Sure that the Power Has Been Disconnected

Before you connect the multimeter or do any testing, make sure that the system is disconnected from all power sources. Machines such as welders require “step-down ” transformers because they require the higher voltages coming into the system to be transformed into a lower voltage.

It is for this reason that those who try to make their own rudimentary arc welding machines at home will pull transformers from microwaves for their welders. Transformers pose significant safety standards. Working on a system that has not been fully disconnected from the power source will carry a high risk of electrical shock.

For this reason, the transformer must be removed prior to testing and the capacitors must be bled out. The term “bleeding out the capacitors” simply refers to the act of allowing the power to drain from the capacitors.

As long as the transformer has drain resistors, this process will not need to involve any further intervention before proceeding to the next step.

Related Article: Personal Protective Equipment for Welders – PPE | List, and Requirements

However, if the transformer does not have drain resistors, then you may need to short-circuit the capacitors. In all likelihood the transformer in your welder probably does have drain resistors, meaning that you can allow the resistors to drain power from the capacitors on their own.

5.  Double Check to Make Sure that the Transformer Has No Power

You are encouraged to double check that the transformer has no power through the use of a multimeter.  Start out by making sure that the multimeter or ohmmeter is set at its lowest setting on the voltage scale. You will do this by moving the knob located in the center of the multimeter.

If you are unsure of what to do, you are encouraged to read the instruction manual for your multimeter or watch this helpful video.

How to Use a Multimeter for Beginners – How to Measure Voltage, Resistance, Continuity and Amps >> Check out the video below

Then touch the 2 leads from the multimeter together to make sure that you get a reading of 0. If the display screen on the multimeter shows any value other than 0, adjust the center knob in the voltage section until there is a reading of 0 on the display screen.

6.  Check the Input Voltage in the Transformer

The first test that you will want to perform is to check the input voltage of the transformer. Welding transformers feature a primary and secondary coil, as is described in the book Welding Principles & Applications.

The step-down transformer used in welding has more turns of wire in the primary coil then it will in the secondary coil.

This enables the welder to receive high-voltage, low-amperage current and convert it into low-voltage, high-amperage current for welding purposes.

The input and output sides of the transformer should be marked on the exterior of the transformer. If this is not the case, then you will need to check the wiring diagram included in the user manual for the welder.

Then take the multimeter and make sure that it is set up to test voltage. Place one multimeter lead on each side of the input-voltage terminal and record the voltage as shown on the display screen.

Make sure that you are repeating this test multiple times to ensure accurate results. Strange readings may be a result of you not using the multimeter correctly.

Once you have determined that you have an accurate reading and consistent results, you can check the voltage reading against the stated input voltage shown in the specifications section of the user manual. 

If the input voltage does not match the specified input voltage, proceed to checking the source of the voltage before troubleshooting the transformer.

7.  Check the Output Voltage

You will also want to check the output voltage from the transformer. The terminals carrying the output voltage should be marked on the transformer.

In the event that you cannot tell which terminal carries the output voltage, look at the wiring diagram in the welder user manual.

At the very least the output voltage should be less than the input voltage in a step-Down transformer, the type of transformer commonly used in welding. If the output voltage is greater than or equal to the input voltage in a step-down transformer, then there may be an issue in the secondary coil.

To measure the output voltage of a welding transformer, make sure that the center knob of the multimeter is set to read voltage. Place one lead at each end of the output terminal. Test the voltage multiple times to ensure that readings are accurate and consistent.

The output voltage readings should at least be within a reasonable range of the value listed in the specifications section of the user manual for your welder.

The cheaper multimeters are not necessarily the most precise, but they do work quite well for these purposes and will certainly let you know whether your tested value falls within the expected range.

If you come away with an unexpected value you will need to troubleshoot problems with the welding transformer. If the input voltage is fine, but the output voltage is too low or high, then there is likely an issue with the secondary windings, as was alluded to earlier.

 This may or may not be a repair you can make yourself. You may consider looking for an electrician or welding machine repair company who can fix the secondary coil for a price that is lower than the cost of replacing the transformer entirely.

Before you continue reading, here is an article we wrote: If Your Welder Keeps Tripping the Breaker, Read This Guide

8.  Perform a Continuity Test in the Primary Windings

Start out by moving the knob on the multimeter to read resistance. The knob will need to be moved to the ohmmeter section of your multimeter for you to start measuring resistance.

Start out by touching the separate leads of the multimeter together. The multimeter should indicate continuity.

Continuity is generally shown on a multimeter by a beep in continuity. Many multimeters do not have a dedicated continuity mode like this versatile multimeter does.

Fortunately, you can still measure continuity on multimeters without a dedicated continuity mode. The resistance reading should be near zero in such systems.

Remove the wiring on the input side of the transformer. Then touch the positive and negative leads from the multimeter to opposite input terminals.

The resistance reading should be close to 0, indicating continuity. If this is not the case, make sure that you test the wiring multiple times to make sure that the issue is a false reading from multimeter.

If you are still getting a resistance reading that falls out of the expected range of values, then you likely have a faulty transformer.

This issue would likely indicate that the transformer needs to be replaced entirely. In all likelihood, there is not a repair that can be done that can remedy a welding transformer that is simply not functioning at all.

How Welding Transformers Work. Teardown and Explanation >> Check out the video below

9.  Perform a Continuity Test on the Secondary Windings

You will also need to perform a continuity test on the secondary windings of the transformer. Disconnect the output wires from the transformer. Make sure that the multimeter is set to read resistance.

Just to be accurate, first touch the 2 leads of the multimeter together while reading resistance to ensure that the multimeter is capable of testing continuity. The multimeter will either beep and/or display a resistance value close to 0.

Then place each lead on each output terminal. The multimeter should indicate continuity.

If the multimeter does not indicate continuity, then you should inspect the secondary circuit for a short to ground, which is often caused by a bare wire. In this case the transformer will need to be replaced entirely.

10. Troubleshooting Issues Caused by the Transformer

Problems with welder performance can often be traced back to the transformer. Operators often do not instinctively think that this could be the reason that their welder is not performing adequately.

The owner’s manual for this arc welding transformer identifies an open transformer circuit as one of the potential reasons that a welder will not be able to weld at all. You may also notice that the welder works fine when you first start it, only to stop working soon thereafter.

If your welder is running off of DC current, then sporadic welder performance like this can be a result of poor internal connections.

Part of your troubleshooting routine should include performing a series of tests on the transformer to ensure that a faulty transformer is not the cause of performance issues.

Why Still Use Transformer-Based Welders?

A great debate within the realm of welding pits inverter welders against transformer welders. For most of industrial history, the transformer-based welders have been the norm. However, in the late 1980s software engineers began designing welders based off of inverters.

Inverter welders make use of a silicon-based technology. They are computerized welders that can easily adjust the current without having to make use of the clunky transformers and rectifiers seen in the traditional welders.

Related reading: What Are the Advantages of An Inverter Welder?

Transformer welders still hold their merits though. For one thing, they are much easier to repair. Think about how much easier it is to repair an older vehicle then it is to repair modern vehicles with more complex computer systems.

Many operators are more comfortable with transformer-based welders for this reason.

Transformer welders have also been around a lot longer. This means that there has been more time spent perfecting the performance of transformer welders than there has with inverter welders. To be fair, inverters have caught up quite a bit in recent years.

Inverter welders are still more expensive than transformer welders, although the average cost of inverters has come down over the years. If you are mostly welding mold steel in your home shop, then you will find that a transformer will still work quite well with your demands.

The transformer welders are really the best option for the DIY welder when you take price into consideration.

Inverters also cost more to repair once they are beyond their warranty, says this article appearing in The Fabricator. Inverters are an expensive piece of equipment to fix, especially if you run into persistent issues with the computer system. 

Transformers are less costly to repair or replace because you may be able to draw from a scrap supplies of parts.

How Many Ohms Should a Transformer Read?

Your ohmmeter reading should not have significant differences between its test result and the resistance listed on your transformers datasheet.

The resistance of an alternating current is held in the wires wrapped around its core. You measure this by touching an ohmmeter to the red and black pins at opposite ends of the transformer’s wiring.

If there is a substantial difference between your transformer’s datasheet, you should consider replacing the transformer immediately.

Any reading of infinite resistance or OL can be measured as a fault in your transformer and should be replaced.   

Which Side of a Transformer Has Higher Resistance?

Which side of a transformer has higher resistance? The transformer’s input side (or primary side) typically has a higher value because the main electrical power is connected at this point. The output (or secondary) side is where the electrical current is sent to the load.

The voltage to the primary coil of s step-down transformer is always more significant than the secondary winding, so it has a higher resistance than the secondary wiring.

So the side with the greater resistance should be the primary side. Other ways to find your primary source is to:

  • Use a multimeter on continuous mode, and you may test the primary and then secondary wires to see the higher multimeter value.
  • If your transformer is a center tap transformer, you will find the primary typically has two wires, and the secondary has three wires.
  • If your transformer is labeled, the primary voltage is displayed on the top of the transformer, and the secondary lesser voltage is displayed on the bottom of the display.

How Do You Demagnetize a Transformer Core?

How do you demagnetize a transformer core? To demagnetize a transformer coil, a direct current should be applied, and its magnitude should be lowered as the polarity of the directed current is reversed several times.

Demagnetization is crucial for a transformer as the core might have residual magnetism after being disconnected from the power source, or residual magnetism from winding resistance measurements.

If your transformer is not properly demagnetized is can cause high inrush currents when the core is re-energized. This residual magnetism may cause damage to the coils or reduce clamping pressure.

These mechanical shocks caused by overcurrent may lead to loose windings and mechanical failure.

Are Inverter Welders Better Than Transformer Welders?

Both machines have their advantages and disadvantages depending on factors such as space, efficiency, and longevity.

Inverters typically use fewer amperes to reach the same voltage as a transformer, so they are more efficient and produce a more stable arc. Transformers have higher duty cycles and can handle more heavy-duty operations than inverters.

They also have a longer track record for durability as the technology has been around for substantially longer.

Inverters take less space than a transformer, so they would suit welders that work in a limited space area. Inverters have more working parts, so repairs are not as easy as the more simple structured transformer.

The initial cost for an inverter is higher than that of a transformer, but with their comparatively low electricity consumption ( around 10%), your inverter saves money over time. 

Inverters have more versatility in terms of materials than a transformer with GMAW and GTAW programmability.

However, if your needs are simple and your focus on mild steel, the transformer is all you need for a durable and dependable machine that will last you a long time. 

Which Transformer is Used in Arc Welding?

Which transformer is used in Arc Welding? Most commonly, arc welders opt for converters that are IGBT or MOSFET based on mains to DC or synthesized AC, such as the Dekopro Arc Welder.

Although there are five power sources available for arc welding, most modern welders will not opt for mains frequency transformers.

Although simple primary tapped systems may be robust enough for MIG welding, supply fluctuations may be problematic. Thyristor controls allow continuous variable adjustment of output and can be used for most welding purposes.

Related reading: What Is Arch Welding?

Inverter power sources offer the most advantages in terms of efficiency and performance.

They convert mains AC (50Hz) to high-frequency AC before rectifying to DC suitable for welding. 

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Thank you for reading this article. I hope it helps you find the most recent and accurate information for your welding project. Here are some tools that I use daily and hope you´ll also find helpful.

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David Harper

David is the Co-Founder and Senior Editor at David's an experienced fitter and tuner/welder who's passionate about helping others develop in life through new skills and opportunities.

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