How to Test a Transformer with a Multimeter (4-Step Guide)

Transformers are vital electrical components that transfer power between two or more circuits. However, sometimes they can fail and cause circuit failure. Therefore, it is very important to test the transformer so that your devices work without the risk of fire or any hazardous events.

There are various methods for testing transformers, and the most effective is a digital multimeter. So, read on and find out how to test a transformer with a multimeter! This guide will take you step by step!

Identifying Transformer Problems

There are several ways to determine if your transformer is bad, and a digital multimeter is one of them. A DMM is the most effective tool for detecting transformer faults, apart from its basic function of checking voltage, current, etc. If all goes well, you should be able to find any transformer faults and learn how to fix them. it can function normally again.

Therefore, before you start testing a transformer with a multimeter, it would be best to first identify critical information about transformers. Therefore, you must:

Visually inspect the transformer

A typical cause of transformer failure is overheating, which heats up the internal wire of the transformer to high temperatures. As a result, the transformer or the space around it is often physically deformed. Do not check the transformer if it is externally swollen or burnt, but replace it instead.

Find out the wiring of the transformer

The wiring must be clearly marked on the transformer. However, the easiest way to figure out how a transformer is connected is to get a circuit diagram. You can find the circuit diagram in the product information or on the circuit manufacturer's website. (1)

Know the sides of the transformer

A 24V transformer has a primary (high voltage) side and a secondary (low voltage) side.

• The primary (high voltage) side is the line voltage of the transformer and the electrical connection to the supply voltage, typically 120 VAC.
• The secondary (low voltage) side is the power converted to 24 volts.

In a transformer used for a 24V application, there is no direct electrical connection between the high and low side sections.

How to Test a Transformer with a Multimeter (Steps)

In this guide, we will be testing a 24V transformer and you will need the following:

• Screwdriver
• multimeter

So, how to check a power transformer with a multimeter? Do the following:

Step 1: Remove the electrical covers 

Turn off circuit power. Remove all electrical covers covering the transformer with a screwdriver. I recommend checking the manufacturer's instructions to confirm transformer access.

Step 2: Insert the wires into the multimeter

Change the multimeter setting to "Ohm", then insert the red and black test leads into the multimeter. The black probe goes into the standard hole, and the red probe goes into the Ohm socket. After that, connect the ends of the two wires together. It should show zero ohms or a closed circuit.

Step 3: Connect Leads to the Main Side 

Connect the multimeter leads to the high side or primary leads of the transformer. The meter must identify the resistance reading, and the type of transformer used in the circuit will affect this reading. If the meter shows an open circuit or infinite resistance, you need to replace the high side transformer.

Step 4: Do the same with the secondary side 

Follow the same procedure in step 3 for connections on the low voltage side or in the secondary circuit. The meter should report an accurate measurement of resistance in ohms for the bottom side. Then, if the multimeter shows an infinite or wide open reading, the low voltage side is damaged internally and the transformer needs to be replaced.

 Basic Tips

• A buzzing or crackling sound is a common warning that a transformer is about to burn out.
• When you touch the probes and only one side of the transformer is not working, you may hear a buzzing sound. In this case no current flows through the transformer and it tries to work against itself.
• Do not assume that the primary and secondary sides of the transformer are connected to the same electrical ground. They are usually referred to on different grounds. Therefore, be careful with separate grounding when making measurements.
• You can also check the integrity of the transformer. Checking the continuity of the transformer is critical to see if there is a path for electricity to pass between the two contact points. If there is no current path, something has gone wrong inside your transformer and needs to be fixed.

Safety measures

To safely test the transformer, the following must be taken into account:

• Disconnect all power from the appliance or device before performing any tests. Never test a device connected to an external power source.
• Always test in a safe, dry area away from children and pets.
• Accidental contact with circuit power while circuits are open and energized for testing may result in electrical shock or damage. Use only DMM leads to touch the circuit.
• Working with electricity is extremely dangerous. Therefore, be careful when doing so. Do not turn on a transformer with frayed wires or visible damage, as this can lead to electric shock.
• Only test a transformer if you are familiar with electrical equipment and have used a multimeter to test voltage, current, and resistance over a wide range of values.

Transformer: how does it work? (Bonus)

A transformer is an important electrical device that changes the voltage of an alternating current (AC) signal. This is achieved by converting AC electricity into high or low voltage signals. This is important because it ensures the safe transmission of electricity over long distances. Alternatively, you can use a transformer to step up or step down the voltage of an AC signal before it enters the building.

Transformers come in a variety of sizes and configurations, but they all work by creating a magnetic field around two coils of wire, known as windings. One winding is connected directly to the AC source, like a power line. On the other hand, the other winding is connected to an electrical load, such as a light bulb. When current passes through one coil, it creates a magnetic field that surrounds both coils. If there are no gaps between these two windings, they will always have opposite polarity, one pointing north and the other pointing south. So the transformer generates alternating current.

Primary and Secondary

The primary and secondary coils of a transformer are wire coils that generate alternating current. The primary coil is connected to a power line and the secondary coil is connected to an electrical load. You can change the output voltage of a transformer by changing the amount of current through each winding. (2)

Other multimeter learning guides below which you can also check out.

• How to check the voltage of 240 V with a multimeter?
• How to count ohms on a multimeter
• How to test a coil with a multimeter

Recommendations

(1) website - https://www.computerhope.com/jargon/w/website.htm

(2) power line - https://www.sciencedirect.com/topics/engineering/power-line