Hello. Today I’ll go over a well-worn topic, so the article will be useful for those who have not yet learned how to determine the parameters of an unknown transformer. I have long wanted to write an article about this, but there was no more or less decent transformer. Today I removed a transformer from a microwave oven from the times of the USSR, I will determine what voltages are on it and show you.
Well, let's start with the fact that it is generally accepted to test the windings for resistance and where the resistance is greater is the network one. This method has the right to life, but not for all transformers. Anode filament is difficult to determine where the network is, and it is also difficult to determine if there are two symmetrical windings of 110V or 127V. How to deal with a transformer like my hero of the article in the photo, which has 14 inputs

At the time of writing this article, I will forget where I removed the transformer from, I will forget where everything was turned on. I’ll take a multimeter in ohmmeter mode at a limit of 200 Ohms and start measuring and immediately recording which windings are connected and what their resistance is. For convenience, I will mark the windings on paper.

As a result, I have a table of resistances (I did not take into account the resistance of the multimeter probes, so the readings are not accurate) and a transformer diagram. As if already from the diagram it is clear that the network winding is between contacts 1-2, but how to determine if there were also windings with high resistance, say 20 Ohm or 30 Ohm.

Everything is simple here, the network winding is usually wound first. But it's worth being safe. I take a 220V 40W light bulb and connect it in series with the windings, as described in the article. You need to start with the winding with the highest resistance, and move towards decreasing resistance. If the lamp begins to illuminate specifically, it means that the XX current has begun to exceed the norm.

I select the previous winding and now connect the transformer through the fuse. I leave it for an hour and see how it warms up. If the trance is slightly warm, then the winding is selected correctly. On this winding the transformer must produce the rated rated power, in my case it must pull 180-200W

And finally, it remains to measure the voltage on the remaining windings. Winding 13-14 is a tap on the other side wound with a thick wire of at least 2.5 squares. The remaining windings are wound with 0.51mm kV wire, which means that each winding will withstand about 1A

The voltages for my tasks are not entirely standard, but perhaps it will be useful somewhere without rewinding
That's all for now. I hope it was useful and interesting. If you like my articles, I recommend subscribing to updates Contact or Odnoklassniki so as not to miss something new
With uv. Edward

A transformer is a simple electrical device and is used to convert voltage and current. The input and one or more output windings are wound on a common magnetic core. An alternating voltage applied to the primary winding induces a magnetic field, which causes an alternating voltage of the same frequency to appear in the secondary windings. Depending on the ratio of the number of turns, the transmission coefficient changes.

To check for transformer malfunctions, you first need to determine the terminals of all its windings. This can be done using it, where the pin numbers and type designation are indicated (then you can use reference books); if the size is large enough, there are even drawings. If the transformer is directly in some electronic device, then all this will be clarified by the fundamental electrical diagram on the device and specification.

Having identified all the terminals, you can use a multimeter to check two defects: a break in the winding and a short circuit to the housing or another winding.

To determine a break, you need to “ring” each winding in turn using an ohmmeter; the absence of readings (“infinite” resistance) indicates a break. The DMM may give unreliable readings when testing windings with a high number of turns due to their high inductance.

To search for a short circuit to the housing, one multimeter probe is connected to the winding terminal, and the second probe alternately touches the terminals of other windings (either one of the two is enough) and the housing (the contact area must be cleaned of paint and varnish). There should not be a short circuit; it is necessary to check each pin.

Transformer interturn short circuit: how to determine

Another common defect in transformers is an interturn short circuit; it is almost impossible to recognize it only with a multimeter. Attentiveness, keen vision and sense of smell can help here. The wire is insulated only due to its varnish coating; if the insulation breaks down between adjacent turns, the resistance still remains, which leads to local heating. Upon visual inspection, a serviceable transformer should not show blackening, drips or swelling of the fill, charring of paper, or a burning smell.

If the type of transformer is determined, then from the reference book you can find out the resistance of its windings. To do this, use a multimeter in megohmmeter mode. After measuring the insulation resistance of the transformer windings, we compare it with the reference: differences of more than 50% indicate a winding malfunction. If the resistance of the transformer windings is not indicated, then the number of turns and the type of wire are always given and theoretically, if desired, it can be calculated.

Is it possible to test household step-down transformers?

You can try using a multimeter to check the common classic step-down transformers used in power supplies for various devices with an input voltage of 220 volts and a constant output voltage of 5 to 30 volts. Carefully, avoiding the possibility of touching the bare wires, apply 220 volts to the primary winding. If there is a smell, smoke, or crackling noise, you must turn it off immediately, the experiment is unsuccessful, the primary winding is faulty.
If everything is normal, then by touching only the tester probes, the voltage on the secondary windings is measured. A difference from the expected value by more than 20% to a lesser extent indicates a malfunction of this winding.

To weld at home, you need a functional and productive device, the purchase of which is now too expensive. It is quite possible to assemble from scrap materials after first studying the corresponding diagram.

What's happened solar panels and how to use them to create a home energy supply system, he will talk about this topic.

A multimeter can also help if you have the same but known-to-be-good transformer. The winding resistances are compared, a spread of less than 20% is normal, but we must remember that for values ​​less than 10 ohms, not every tester will be able to give correct readings.

The multimeter did everything it could. For further testing, you will also need an oscilloscope.

Detailed instructions: how to test a transformer with a multimeter on video

12.12.2017

It is often necessary to familiarize yourself in advance with the question of how to test a transformer. After all, if it fails or unstable work It will be difficult to find the cause of equipment failure. This simple electrical device can be diagnosed with a conventional multimeter. Let's look at how to do this.

What is the equipment?

How to check a transformer if we don’t know its design? Let's look at the principle of operation and types of simple equipment. Turns of copper wire of a certain cross-section are applied to the magnetic core so that leads remain for the supply winding and the secondary winding.

Energy is transferred to the secondary winding in a non-contact manner. At this point it becomes almost clear how to check the transformer. The usual inductance is measured in the same way with an ohmmeter. The turns form a resistance that can be measured. However, this method is applicable when the specified value is known. After all, the resistance can change up or down as a result of heating. This is called an interturn short circuit.

Such a device will no longer produce reference voltage and current. The ohmmeter will only show an open circuit or a complete short circuit. For additional diagnostics, use the same ohmmeter to check the short circuit to the housing. How to test a transformer without knowing the winding terminals?

Kinds

Transformers are divided into the following groups:

• Down and up.
• Power ones often serve to reduce the supply voltage.
• Current transformers for supplying a constant amount of current to the consumer and maintaining it in a given range.
• Single and multiphase.
• Welding purposes.
• Pulse.

Depending on the purpose of the equipment, the principle of approach to the question of how to check the transformer windings also changes. Only small-sized devices can be dialed with a multimeter. Power machines already require a different approach to fault diagnosis.

Dialing method

The ohmmeter diagnostic method will help with the question of how to check the power transformer. The resistance between the terminals of one winding begins to ring. This is how the integrity of the conductor is established. Before this, the housing is inspected for the absence of deposits and deposits as a result of heating of the equipment.

Next, the current values ​​in Ohms are measured and compared with the passport values. If there are none, then additional diagnostics under voltage will be required. It is recommended to ring each terminal relative to the metal body of the device where the ground is connected.

Before taking measurements, all ends of the transformer should be disconnected. It is recommended to disconnect them from the circuit for your own safety. They also check for the presence of an electronic circuit, which is often present in modern models nutrition. It should also be desoldered before testing.

Infinite resistance speaks of complete isolation. Values ​​of several kilo-ohms already raise suspicions about a breakdown in the housing. This may also be due to accumulated dirt, dust or moisture in the air gaps of the device.

Live

Tests with power applied are carried out when the question is how to test a transformer for an interturn short circuit. If we know the value of the supply voltage of the device for which the transformer is intended, then measure the no-load value with a voltmeter. That is, the output wires are in the air.

If the voltage value differs from the nominal value, then conclusions are drawn about an interturn short circuit in the windings. If you hear a crackling or sparking sound when the device is operating, then it is better to turn off such a transformer immediately. It's faulty. There are permissible deviations in measurements:

• For voltage, values ​​may differ by 20%.
• For resistance, the norm is a spread of values ​​of 50% from the passport values.

Measurement with an ammeter

Let's figure out how to check a current transformer. It is included in a chain: standard or self-made. It is important that the current value is not less than the rated value. Measurements with an ammeter are carried out in the primary circuit and in the secondary circuit.

The current in the primary circuit is compared with the secondary readings. More precisely, they divide the first values ​​by those measured in the secondary winding. The transformation coefficient should be taken from the reference book and compared with the calculations obtained. The results should be the same.

The current transformer cannot be measured at idle. In this case, too high a voltage may form on the secondary winding, which can damage the insulation. You should also observe the polarity of the connection, which will affect the operation of the entire connected circuit.

Typical faults

Before checking the microwave transformer, we list common types of breakdowns that can be repaired without a multimeter. Often power supplies fail due to a short circuit. It is installed by inspecting circuit boards, connectors, and connections. Mechanical damage to the transformer housing and its core occurs less frequently.

Mechanical wear of transformer terminal connections occurs on moving machines. Large supply windings require constant cooling. In its absence, overheating and melting of the insulation is possible.

TDKS

Let's figure out how to check a pulse transformer. An ohmmeter can only establish the integrity of the windings. The functionality of the device is established when connected to a circuit that involves a capacitor, load and sound generator.

A pulse signal in the range from 20 to 100 kHz is applied to the primary winding. On the secondary winding, measurements are made with an oscilloscope. Determine the presence of pulse distortion. If they are missing, conclusions are drawn about a working device.

Distortions in the oscillogram indicate damaged windings. It is not recommended to repair such devices yourself. They are set up in laboratory conditions. There are other schemes for testing pulse transformers, which examine the presence of resonance on the windings. Its absence indicates a faulty device.

You can also compare the shape of the pulses supplied to the primary winding and those output from the secondary. A deviation in shape also indicates a malfunction of the transformer.

Multiple windings

To measure resistance, the ends are freed from electrical connections. Select any output and measure all resistances relative to the others. It is recommended to record the values ​​and label the tested ends.

This way we can determine the type of connection of the windings: with middle terminals, without them, with a common connection point. More common with separate connection windings The measurement can only be done with one of all the wires.

If there is a common point, then we measure the resistance between all existing conductors. Two windings with a middle terminal will only have value between the three wires. Several terminals are found in transformers designed to operate in several networks rated 110 or 220 Volts.

Diagnostic nuances

A hum when a transformer is operating is normal if these are specific devices. Only sparking and crackling indicate a malfunction. Often, heating of the windings is normal operation of the transformer. This is most often observed with step-down devices.

Resonance can be created when the transformer housing vibrates. Then you just need to secure it with insulating material. The operation of the windings changes significantly if the contacts are loose or dirty. Most problems can be solved by cleaning the metal to a shine and re-covering the terminals.

When measuring voltage and current values, one should take into account the ambient temperature, the size and nature of the load. Control of the supply voltage is also necessary. Checking the frequency connection is mandatory. Asian and American technology is designed for 60 Hz, which leads to lower output values.

Incorrect connection of the transformer can lead to device malfunction. Under no circumstances should direct voltage be connected to the windings. The coils will quickly melt otherwise. Accuracy in measurements and proper connection will help not only find the cause of the breakdown, but also, possibly, eliminate it in a painless way.

IN modern technology transformers are used quite often. These devices are used to increase or decrease the parameters of alternating electric current. The transformer consists of an input and several (or at least one) output windings on a magnetic core. These are its main components. It happens that the device fails and there is a need to repair or replace it. You can determine whether the transformer is working properly using a home multimeter on your own. So, how to test a transformer with a multimeter?

Basics and operating principle

The transformer itself is an elementary device, and its operating principle is based on the two-way transformation of the excited magnetic field. What is characteristic is that a magnetic field can be induced exclusively using alternating current. If you have to work with a constant, you must first transform it.

A primary winding is wound around the core of the device, to which an external alternating voltage with certain characteristics is supplied. Next come it or several secondary windings in which an alternating voltage is induced. The transmission coefficient depends on the difference in the number of turns and the properties of the core.

Varieties

Today you can find many types of transformer on the market. Depending on the design chosen by the manufacturer, a variety of materials can be used. As for the shape, it is selected solely for the convenience of placing the device in the body of the electrical appliance. The design power is affected only by the configuration and material of the core. In this case, the direction of the turns does not affect anything - the windings are wound both towards and away from each other. The only exception is the identical choice of direction if several secondary windings are used.

To check such a device, a conventional multimeter is sufficient, which will be used as a current transformer tester. No special devices are required.

Check procedure

Testing a transformer begins with identifying the windings. This can be done using markings on the device. Pin numbers, as well as their type designations, should be indicated, which allows you to establish more information in reference books. In some cases there are even explanatory drawings. If the transformer is installed in some kind of electronic device, then the electronic circuit diagram of this device, as well as a detailed specification, can clarify the situation.

So, when all the conclusions are determined, it’s the tester’s turn. With its help, you can identify the two most common faults - a short circuit (to the housing or an adjacent winding) and a winding break. In the latter case, in ohmmeter mode (resistance measurement), all windings are called back one by one. If any of the measurements shows one, that is, infinite resistance, then there is a break.

There is an important nuance here. It is better to check on an analog device, since a digital one can give distorted readings due to high induction, which is especially typical for windings with a large number of turns.

When checking a short circuit to the housing, one of the probes is connected to the terminal of the winding, while the second probe rings the terminals of all other windings and the housing itself. To check the latter, you will need to first clean the contact area from varnish and paint.

Determination of interturn short circuit

Another common failure of transformers is interturn short circuit. It is almost impossible to check a pulse transformer for such a malfunction with just a multimeter. However, if you attract your sense of smell, attentiveness and sharp vision, the problem can well be solved.

A little theory. The wire on the transformer is insulated exclusively with its own varnish coating. If an insulation breakdown occurs, the resistance between adjacent turns remains, as a result of which the contact area heats up. That is why the first step is to carefully inspect the device for streaks, blackening, burnt paper, swelling and a burning smell.

Next, we try to determine the type of transformer. Once this is achieved, you can look at the resistance of its windings using specialized reference books. Next, switch the tester to megohmmeter mode and begin measuring the insulation resistance of the windings. In this case, the pulse transformer tester is a regular multimeter.

Each measurement should be compared with that indicated in the reference book. If there is a discrepancy of more than 50%, then the winding is faulty.

If the resistance of the windings is not indicated for one reason or another, the reference book must provide other data: the type and cross-section of the wire, as well as the number of turns. With their help, you can calculate the desired indicator yourself.

Checking household step-down devices

It is worth noting the moment of checking classic step-down transformers with a multimeter tester. They can be found in almost all power supplies that reduce the input voltage from 220 Volts to the output voltage of 5-30 Volts.

The first step is to check the primary winding, which is supplied with a voltage of 220 Volts. Signs of a primary winding malfunction:

• the slightest visibility of smoke;
• the smell of burning;
• crack.

In this case, the experiment should be stopped immediately.

If everything is normal, you can proceed to measurements on the secondary windings. You can touch them only with the tester contacts (probes). If the results obtained are less than the control ones by at least 20%, then the winding is faulty.

Unfortunately, such a current block can be tested only in cases where there is a completely similar and guaranteed working block, since it is from it that the control data will be collected. It should also be remembered that when working with indicators of the order of 10 ohms, some testers may distort the results.

No-load current measurement

If all tests have shown that the transformer is fully operational, it would not be amiss to conduct another diagnostic - for the no-load current of the transformer. Most often it is equal to 0.1-0.15 of the nominal value, that is, the current under load.

To carry out the test, the measuring device is switched to ammeter mode. Important point! The multimeter should be connected to the transformer under test in a short-circuited manner.

This is important because when electricity is supplied to the transformer winding, the current increases to several hundred times the rated current. After this, the tester probes open and the indicators are displayed on the screen. It is they that display the value of the current without load, the no-load current. In a similar way, indicators are measured on the secondary windings.

To measure voltage, a rheostat is most often connected to the transformer. If you don’t have it at hand, a tungsten spiral or a series of light bulbs can be used.

To increase the load, increase the number of bulbs or reduce the number of turns of the spiral.

As you can see, you don’t even need any special tester to check. A completely ordinary multimeter will do. It is highly desirable to have at least an approximate understanding of the operating principles and structure of transformers, but for successful measurements it is enough just to be able to switch the device to ohmmeter mode.

How to check a transformer?

The transformer, which translates as “Converter,” has entered our lives and is used everywhere in everyday life and industry. That is why it is necessary to be able to check the transformer for operability and serviceability in order to prevent breakdown in the event of a failure. After all, the transformer is not so cheap. However, not every person knows how to check a current transformer on their own and often prefers to take it to a specialist, although the matter is not at all difficult.

Let's take a closer look at how you can check the transformer yourself.

How to test a transformer with a multimeter

The transformer works on a simple principle. In one of its circuits a magnetic field is created due to alternating current, and in the second circuit an electric current is created due to a magnetic field. This allows the two currents inside the transformer to be isolated. To test a transformer you must:

1. Find out whether the transformer is externally damaged. Carefully inspect the transformer shell for dents, cracks, holes or other damage. Often the transformer deteriorates due to overheating. Perhaps you will see traces of melting or swelling on the body, then there is no point in looking at the transformer further and it is better to have it repaired.
2. Inspect the transformer windings. There must be clearly printed labels. It won’t hurt to have a diagram of the transformer with you, where you can see how it is connected and other details. The scheme should always be present in documents or, at least, on the developer’s page on the Internet.
3. Also find the input and output of the transformer. The voltage of the winding that creates the magnetic field must be marked on it and in the documents on the diagram. It should also be noted on the second winding, where the current and voltage are generated.
4. Find the filtering at the output where power is converted from AC to DC. Diodes and capacitors must be connected to the secondary winding, which perform filtering. They are indicated on the diagram, but not on the transformer.
5. Prepare a multimeter to measure the line voltage measurement. If the panel cover prevents access to the network, remove it while checking. You can always buy a multimeter in a store.
6. Connect the input circuit to the source. Use the multimeter in AC mode and measure the primary voltage. If the voltage drops below 80% of the expected value, then the primary winding is likely to be faulty. Then simply disconnect the primary winding and check the voltage. If it rises, then the winding is faulty. If it does not rise, then there is a malfunction in the primary input circuit.
7. Also measure the output voltage. If there is filtering, then the measurement is carried out in constant current mode. If not, then in AC mode. If the voltage is incorrect, then you need to check the entire unit one by one. If all the parts are in order, then the transformer itself is faulty.

It is common to hear a humming or hissing sound from the transformer. This means that the transformer is about to burn out and it must be urgently turned off and sent for repairs.

In addition, the windings often have different ground potentials, which affects the voltage calculation.

A transformer is a simple electrical device and is used to convert voltage and current. The input and one or more output windings are wound on a common magnetic core. An alternating voltage applied to the primary winding induces a magnetic field, which causes an alternating voltage of the same frequency to appear in the secondary windings. Depending on the ratio of the number of turns, the transmission coefficient changes.

To check for transformer malfunctions, you first need to determine the terminals of all its windings. This can be done according to it, where the pin numbers and type designation are indicated (then you can use reference books); if the size is large enough, there are even drawings. If the transformer is directly in some kind of electronic device, then all this will be clarified by the circuit diagram of the device and the specification.

Having identified all the terminals, you can use a multimeter to check two defects: a break in the winding and a short circuit to the housing or another winding.

To determine a break, you need to “ring” each winding in turn using an ohmmeter; the absence of readings (“infinite” resistance) indicates a break.

The DMM may give unreliable readings when testing windings with a high number of turns due to their high inductance.

To search for a short circuit to the housing, one multimeter probe is connected to the winding terminal, and the second probe alternately touches the terminals of other windings (either one of the two is enough) and the housing (the contact area must be cleaned of paint and varnish). There should not be a short circuit; it is necessary to check each pin.

Transformer interturn short circuit: how to determine

Another common defect in transformers is an interturn short circuit; it is almost impossible to recognize it only with a multimeter. Attentiveness, keen vision and sense of smell can help here. The wire is insulated only due to its varnish coating; if the insulation breaks down between adjacent turns, the resistance still remains, which leads to local heating. Upon visual inspection, a serviceable transformer should not show blackening, drips or swelling of the fill, charring of paper, or a burning smell.

If the type of transformer is determined, then from the reference book you can find out the resistance of its windings. To do this, use a multimeter in megohmmeter mode. After measuring the insulation resistance of the transformer windings, we compare it with the reference: differences of more than 50% indicate a winding malfunction. If the resistance of the transformer windings is not indicated, then the number of turns and the type of wire are always given and theoretically, if desired, it can be calculated.

Is it possible to test household step-down transformers?

You can try using a multimeter to check the common classic step-down transformers used in power supplies for various devices with an input voltage of 220 volts and a constant output voltage of 5 to 30 volts. Carefully, avoiding the possibility of touching the bare wires, apply 220 volts to the primary winding.

If there is a smell, smoke, or crackling noise, you must turn it off immediately, the experiment is unsuccessful, the primary winding is faulty.

If everything is normal, then by touching only the tester probes, the voltage on the secondary windings is measured. A difference from the expected value by more than 20% to a lesser extent indicates a malfunction of this winding.

To weld at home, you need a functional and productive device, the purchase of which is now too expensive. It is quite possible to assemble from scrap materials after first studying the corresponding diagram.

He will talk about what solar panels are and how to use them to create a home energy supply system.

A multimeter can also help if you have the same but known-to-be-good transformer. The winding resistances are compared, a spread of less than 20% is normal, but we must remember that for values ​​less than 10 ohms, not every tester will be able to give correct readings.

The multimeter did everything it could. For further testing, you will also need an oscilloscope.

Detailed instructions: how to test a transformer with a multimeter on video

The main purpose of a transformer is to convert current and voltage. And although this device performs quite complex transformations, it itself has a simple design. This is a core around which several coils of wire are wound. One of them is the input winding (called the primary winding), the other is the output winding (secondary). Electric current is applied to the primary coil, where the voltage induces a magnetic field. The latter in the secondary windings generates an alternating current of exactly the same voltage and frequency as in the input winding. If the number of turns in the two coils is different, then the current at the input and output will be different. Everything is quite simple. True, this device often fails, and its defects are not always visible, so many consumers have a question: how to check a transformer with a multimeter or other device?

It should be noted that a multimeter is also useful if you have a transformer with unknown parameters in front of you. So they can also be determined using this device. Therefore, when starting to work with it, you must first deal with the windings. To do this, you will have to pull out all the ends of the coils separately and ring them, thereby looking for paired connections. In this case, it is recommended to number the ends, determining which winding they belong to.

The simplest option is four ends, two for each coil. More often there are devices that have more than four ends. It may turn out that some of them “do not ring through,” but this does not mean that a break has occurred in them. These may be so-called shielding windings, which are located between the primary and secondary windings; they are usually connected to ground.

This is why it is so important to pay attention to resistance when dialing. For the network primary winding, it is determined by tens or hundreds of Ohms. Please note that small transformers have higher primary winding resistance. It's all about the larger number of turns and small diameter of the copper wire. The resistance of the secondary windings is usually close to zero.

Transformer check

So, the windings were determined using a multimeter. Now you can move directly to the question of how to test a transformer using the same device. We are talking about defects. There are usually two of them:

• break;
• wear of the insulation, which leads to a short circuit to another winding or to the device body.

It’s easy to determine a break, that is, each coil is checked for resistance. The multimeter is set to ohmmeter mode, the two ends are connected to the device with probes. And if the display shows no resistance (readings), then this is guaranteed to be a break. Testing with a digital multimeter may not be reliable if the winding is tested with big amount turns. The thing is that the more turns, the higher the inductance.

The closure is checked like this:

1. One multimeter probe is connected to the output end of the winding.
2. The second probe is alternately connected to the other ends.
3. In the case of a short to housing, the second probe is connected to the transformer housing.

There is another frequently encountered defect - the so-called interturn short circuit. It occurs when the insulation of two adjacent turns wears out. In this case, the resistance of the wire remains, so overheating occurs in the place where there is no insulating varnish. Usually, this produces a burning smell, blackening of the winding and paper appears, and the filling swells. This defect can also be detected with a multimeter. In this case, you will have to find out from the reference book what resistance the windings of a given transformer should have (we will assume that its brand is known). By comparing the actual indicator with the reference one, you can accurately say whether there is a flaw or not. If the actual parameter differs from the reference value by half or more, then this is a direct confirmation of an interturn short circuit.

Attention! When checking the transformer windings for resistance, it does not matter which probe is connected to which end. In this case, polarity does not play any role.

No-load current measurement

If the transformer, after testing with a multimeter, turns out to be in good condition, then experts recommend checking it for such a parameter as no-load current. Typically, for a working device it is 10-15% of the nominal value. In this case, the nominal refers to the current under load.

For example, a transformer brand TPP-281. Its input voltage is 220 volts, and the no-load current is 0.07-0.1 A, that is, it should not exceed one hundred milliamps. Before checking the transformer for the no-load current parameter, it is necessary to switch the measuring device to ammeter mode. Please note that when electricity is supplied to the windings, the inrush current can exceed the rated current by several hundred times, so the measuring device is connected to the device under test in a short-circuited manner.

After that, you need to open the terminals of the measuring device, and the numbers will appear on its display. This is the current without load, that is, no-load. Next, the voltage is measured without load on the secondary windings, then under load. A 10-15% reduction in voltage should result in current readings that do not exceed one ampere.

To change the voltage, you need to connect a rheostat to the transformer; if there is none, you can connect several light bulbs or a spiral of tungsten wire. To increase the load, you must either increase the number of bulbs or shorten the spiral.

Conclusion on the topic

Before checking a transformer (step-down or step-up) with a multimeter, you need to understand how this device is designed, how it works, and what nuances need to be taken into account when carrying out the test. In principle, there is nothing complicated about this process No. The main thing is to know how to switch the measuring device itself to ohmmeter mode.

Related posts:

Transformers are used in almost all electrical appliances, both industrial and household.

Let's leave transformers used by energy companies outside the scope of this article and consider voltage conversion devices used in power supplies for household electrical appliances.

How does a transformer work and what is it for?

A transformer is one of the elementary electrical devices. The principle of its operation is based on the excitation of a magnetic field and its two-way transformation.

Important! A magnetic field can only be induced on the core using alternating current. Therefore, there are no transformers operating on direct current. If it is necessary to convert direct voltage, it is first made alternating or pulsed. For example, using master oscillators.

A primary winding is wound around a single magnetic core, to which an alternating voltage with primary characteristics is supplied. An alternating voltage is induced on the remaining windings wound on the same core. The difference in the number of turns in relation to the primary determines the transmission coefficient.

How to calculate the winding of a transformer?

For example, the primary consists of 2200 turns and is supplied with 220 volts of alternating voltage. For every 10 turns of such a transformer there is 1 volt. Accordingly, to obtain the required voltage value on the secondary windings, it is necessary to multiply it by 10, and we will get the number of turns of the secondary.

To get 24 volts, we need 240 turns of the secondary winding. If you need to take several values ​​from one transformer, you can wind several windings.
How to check a transformer and determine its windings?

The end of one winding is often connected to the beginning of the next. For example, we have two secondary 240 and 200 turns connected in series. Then on winding I there will be 24 volts, on winding II – 20 volts. And if you remove the voltage from the extreme terminals, you get 44 volts.


The next value is the maximum load power. This is a constant value. If the primary is designed for a power of 220W, then a current of 1A can be passed through it. Accordingly, at a voltage of 20 volts on the secondary winding, the operating current can reach 11A.

Based on the required power, the cross-section of the magnetic circuit (core) and the cross-section of the conductor from which the windings are wound are calculated.

To understand the principle of calculating the magnetic circuit, take a look at the attached table:


This is a typical calculation for the W-shaped core used in most household transformers. The magnetic core is assembled from plates made of electrical steel or iron-based alloys with the addition of nickel. This material does an excellent job of maintaining a stable magnetic field.

The first thing to do is take a piece of paper, a pencil and a multimeter. Using all this, ring the windings of the transformer and draw a diagram on paper. This should look something very similar to Figure 1.

The winding terminals in the picture should be numbered. It is possible that there will be much fewer outputs, in the simplest case there are only four: two outputs of the primary (network) winding and two outputs of the secondary winding. But this does not always happen; more often there are several more windings.

Some conclusions, although they exist, may not “ring” with anything. Are these windings broken? Not at all, most likely these are shielding windings located between other windings. These ends are usually connected to a common wire - the “ground” of the circuit.

Therefore, it is advisable to write down the resistance of the windings on the resulting diagram, since the main goal of the study is to determine network winding. Its resistance, as a rule, is greater than that of other windings, tens and hundreds of Ohms. Moreover, the smaller the transformer, the greater the resistance of the primary winding: the small diameter of the wire affects and a large number of turns. The resistance of the step-down secondary windings is almost zero - a small number of turns and a thick wire.

Rice. 1. Diagram of transformer windings (example)

Let's assume that we managed to find the winding with the highest resistance, and we can consider it a network winding. But you don’t need to plug it into the network right away. To avoid explosions and other unpleasant consequences, it is best to perform a test run by connecting in series with the winding a 220V light bulb with a power of 60...100W, which will limit the current through the winding to 0.27...0.45A.

The power of the light bulb should approximately correspond to the overall power of the transformer. If the winding is determined correctly, then the light bulb does not light up; in extreme cases, the filament glows slightly. In this case, you can almost safely connect the winding to the network; for starters, it’s better to use a fuse for a current of no more than 1...2A.

If the light bulb burns brightly enough, then this may be a 110...127V winding. In this case, you should ring the transformer again and find the second half of the winding. After this, connect the halves of the windings in series and turn them on again. If the light goes out, then the windings are connected correctly. Otherwise, swap the ends of one of the found half-windings.

So, we will assume that the primary winding has been found and the transformer has been connected to the network. The next thing you need to do is measure the no-load current of the primary winding. For a working transformer it is no more than 10...15% of the rated current under load. So for the transformer, the data of which is shown in Figure 2, when powered from a 220V network, the no-load current should be in the range of 0.07...0.1A, i.e. no more than one hundred milliamps.

Rice. 2. Transformer TPP-281

How to measure the no-load current of a transformer

The no-load current should be measured with an AC ammeter. In this case, at the moment of connection to the network, the ammeter leads must be short-circuited, since the current when the transformer is turned on can be a hundred or more times higher than the rated one. Otherwise, the ammeter may simply burn out. Next, open the ammeter leads and look at the result. During this test, let the transformer operate for 15...30 minutes and make sure that no noticeable heating of the winding occurs.

The next step is to measure the voltage on the secondary windings without load - open circuit voltage. Let's assume that the transformer has two secondary windings, and the voltage of each is 24V. Almost what is needed for the amplifier discussed above. Next, we check the load capacity of each winding.

To do this, you need to connect a load to each winding, ideally a laboratory rheostat, and by changing its resistance, ensure that the voltage on the winding drops by 10-15%. This can be considered the optimal load for a given winding.

Along with the voltage measurement, the current is measured. If the indicated voltage reduction occurs at a current of, for example, 1A, then this is the rated current for the winding under test. Measurements should begin by setting the rheostat slider R1 to the right position according to the diagram.

Figure 3. Test circuit for transformer secondary winding

Instead of a rheostat, you can use light bulbs or a piece of a spiral from an electric stove as a load. You should start measuring with a long piece of a spiral or by connecting one light bulb. To increase the load, you can gradually shorten the spiral by touching it with a wire at different points, or increasing the number of connected lamps one by one.

To power the amplifier, one winding with a midpoint is required (see article). We connect two secondary windings in series and measure the voltage. It should be 48V, the connection point of the windings will be the midpoint. If, as a result of the measurement, the voltage at the ends of the windings connected in series is zero, then the ends of one of the windings should be swapped.

In this example, everything worked out almost successfully. But more often it happens that the transformer has to be rewound, leaving only the primary winding, which is almost half the battle. How to calculate a transformer is a topic for another article; here we only talked about how to determine the parameters of an unknown transformer.