How to distinguish the primary winding from the secondary. How to identify an unknown transformer. Checking household step-down devices

10.07.2021

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 AC 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 obtain 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.

An electrical transformer is a fairly common device used in everyday life to solve a number of problems.

And breakdowns can occur in it, which can be identified by a device for measuring electric current parameters - a multimeter.

From this article you will learn how to test a current transformer with a multimeter (ring), and what rules should be followed when doing this.

As you know, any transformer consists of the following components:

primary and secondary coils (there may be several secondary ones);
core or magnetic circuit;
frame.

Thus, the list of possible breakdowns is quite limited:

The core is damaged.
A wire has burned out in one of the windings.
The insulation is broken, resulting in electrical contact between the turns in the coil (turn-to-turn short circuit) or between the coil and the housing.
Coil terminals or contacts are worn.

Current transformer T-0.66 150/5a

Some of the defects are determined visually, so the transformer must first be carefully inspected. Here's what you should pay attention to:

cracks, chips of insulation or its absence;
condition of bolted connections and terminals;
swelling of the fill or its leakage;
blackening on visible surfaces;
charred paper;
characteristic smell of burnt material.

If there is no obvious damage, you should check the device for functionality using instruments. To do this, you need to know which windings all its conclusions belong to. On converters large sizes this information can be presented in the form of a graphic image.

If there is none, you can use a reference book in which you can find your transformer by marking. If it is part of an electrical appliance, the data source may be a specification or a circuit diagram.

Methods for checking a transformer with a multimeter

First of all, you should check the insulation condition of the transformer. To do this, the multimeter must be switched to megger mode. After this, measure the resistance:

between the housing and each of the windings;
between the windings in pairs.

The voltage at which such a test should be carried out is indicated in the technical documentation for the transformer. For example, for most high-voltage models, insulation resistance measurements are prescribed to be carried out at a voltage of 1 kV.

Checking the device with a multimeter

The required resistance value can be found in the technical documentation or in the reference book. For example, for the same high-voltage transformers it is at least 1 mOhm.

This test is not capable of detecting interturn short circuits, as well as changes in the properties of wire and core materials. Therefore, it is imperative to check the performance characteristics of the transformer, for which the following methods are used:

Not all devices perceive a voltage of 220 volts. Reduces voltage to allow the use of electrical appliances.

How to check a varistor with a multimeter and what a varistor is needed for, read.

You can familiarize yourself with the rules for checking the voltage in an outlet with a multimeter.

Direct method (testing the circuit under load)

This is the one that first comes to mind: you need to measure the currents in the primary and secondary windings of a working device, and then, by dividing them by each other, determine the actual transformation ratio. If it corresponds to the passport, the transformer is working, if not, you need to look for a defect. This coefficient can be calculated independently if you know the voltage that the device should produce.

For example, if it says 220V/12V, then we have a step-down transformer, therefore, the current in the secondary winding should be 220/12 = 18.3 times higher than in the primary (the term “step-down” refers to voltage).

Scheme for testing a single-phase transformer by direct measurement of primary and secondary voltages using a standard transformer

The load must be connected to the secondary winding so that currents flow in the windings at least 20% of the rated values. When you turn it on, be on your guard: if you hear a crackling sound, there is a burning smell, or you see smoke or sparking, the device must be turned off immediately.

If the transformer under test has several secondary windings, then those that are not connected to the load should be short-circuited. In an open secondary coil when connecting the primary to an alternating current source, high voltage, which can not only damage equipment, but also kill a person.

Serial connection of transformer windings using a battery and a multimeter

If we are talking about a high-voltage transformer, then before turning it on you need to check whether its core needs to be grounded. This is indicated by the presence of a special terminal marked with the letter “Z” or a special icon.

The direct method of checking a transformer allows you to fully assess the condition of the latter. However, it is not always possible to turn on the transformer with a load and make all the necessary measurements.

If due to safety requirements or other reasons this cannot be done, the condition of the device is checked indirectly.

Indirect method

Part this method includes several tests, each of which displays the state of the device in one aspect. Therefore, it is advisable to carry out all these tests together.

Determining the reliability of winding terminal markings

To carry out this test, the multimeter must be switched to ohmmeter mode. Next, you need to “ring” all available conclusions in pairs. Between those of them that belong to different coils, the resistance will be equal to infinity. If the multimeter shows a specific value, then the terminals belong to the same coil.

You can immediately compare the measured resistance with that given in the reference book. If there is a discrepancy of more than 50%, then an interturn short circuit or partial destruction of the wire has occurred.

Connecting a transformer to a multimeter

Please note that on coils with high inductance, that is, consisting of a significant number of turns, the digital multimeter may erroneously show an overestimated resistance. In such cases, it is advisable to use an analog device.

The windings should be checked with direct current, which the transformer cannot transform. When using an alternating voltage, an EMF will be induced in other coils and it is quite possible that it will be quite high. So, if an alternating voltage of only 20 V is applied to the secondary coil of a 220/12 V step-down transformer, then a voltage of 367 V will appear at the primary terminals and if they are accidentally touched, the user will receive a strong electric shock.

Next, you need to determine which terminals should be connected to the current source and which to the load. If it is known that the transformer is a step-down transformer, then the coil with the largest number of turns and the highest resistance must be connected to the current source. With a step-up transformer the opposite is true.

All methods of measuring force electric current

But there are models that have both step-down and step-up coils among the secondary coils. Then the primary coil can, with a certain degree of probability, be recognized by the following characteristics: its terminals are usually attached away from the rest, and the coil can also be located on the frame in a separate section.

The development of the Internet has made this method possible: you need to take a photo of the transformer and write a request with the attached photo and all available information (brand, etc.) to one of the online thematic forums.

Perhaps one of its participants has dealt with such devices and can tell you in detail how it needs to be connected.

If the secondary coil has intermediate taps, it is necessary to recognize its beginning and end. To do this, you need to determine the polarity of the terminals.

Determining the polarity of winding terminals

As a meter, you should use a magnetoelectric ammeter or voltmeter, the polarity of the terminals of which is known. The device must be connected to a secondary coil. It is most convenient to use those models in which the “zero” is located in the middle of the scale, but in the absence of one, the classic one with the “zero” location on the left will do.

If there are several secondary coils, the others need to be bypassed.

Checking the polarity of phase windings of AC electrical machines

A small direct current must be passed through the primary coil. Suitable as a source regular battery, in this case, a resistor must be included in the circuit between it and the coil so that a short circuit does not occur. An incandescent lamp can serve as such a resistor.

There is no need to install a switch in the primary coil circuit: just follow the multimeter needle to close the circuit by touching the wire from the lamp to the coil output, and immediately open it.

If the same poles from the battery and the multimeter are connected to the terminals of the coils, that is, the polarity is the same, then the arrow on the device will move to the right.

For a multi-polar connection - to the left.

At the moment the power is turned off, the opposite picture will be observed: with a unipolar connection, the arrow will move to the left, with a multi-polar connection - to the right.

On a device with a “zero” at the beginning of the scale, the movement of the needle to the left is more difficult to notice, since it almost immediately bounces off the limiter. Therefore you need to watch carefully.

Using the same scheme, the polarities of all other coils are checked.

A multimeter is a very necessary device for measuring current strength, which is used to identify malfunctions of certain devices. – read useful tips optionally.

Instructions for checking diodes with a multimeter are presented.

Removing the magnetization characteristic

To be able to use this method, you need to prepare ahead of time: while the transformer is new and known to be in good working order, its so-called current-voltage characteristic (volt-ampere characteristic) is measured. This is a graph showing the dependence of the voltage at the terminals of the secondary coils on the magnitude of the magnetizing current flowing through them.

Schemes for measuring magnetization characteristics

Having opened the circuit of the primary coil (so that the results are not distorted by interference from nearby power equipment), alternating current of varying strength is passed through the secondary, measuring the voltage at its input each time.

The power of the power supply used for this must be sufficient to saturate the magnetic circuit, which is accompanied by a decrease in the slope of the saturation curve to zero (horizontal position).

Measuring instruments must belong to an electrodynamic or electromagnetic system.

Before and after the test, the magnetic circuit must be demagnetized by increasing the current in the winding in several steps and then reducing it to zero.

As you use the device, you need to take the current-voltage characteristic at certain intervals and compare it with the original one. A decrease in its steepness will indicate the appearance of an interturn short circuit.

Video on the topic

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 high resistance primary windings. 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 with a large number of turns is tested. The thing is that the more turns, the higher the inductance.

The closure is checked like this:

One multimeter probe is connected to the output end of the winding.
The second probe is alternately connected to the other ends.
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 one by half or more, then this is a direct confirmation of the 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 measuring device switch 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.

If the transformer has two windings, four outputs, it costs nothing to ring. The problem is due to significant differences in actual designs. The transformer is equipped with multiple secondary winding terminals to obtain the required voltage ratings. The entry side is not easy. Two separate transformers can be wound on one magnetic core. How to assess suitability for use? Let's see how to test a transformer.

Checking the transformer with a Chinese tester

Not every transformer is made to be powered by a 220 volt network with a frequency of 50 Hz. In industry, measurement industry, higher education other devices are used. Observing unsuitable characteristics, it would be a bad idea to use the devices in industrial circuits. Therefore, first, we pay attention to labeling. Conducted in accordance with GOST. The problem appears: an individual document has been issued for each type of transformer.

Symbols of power (GOST 52719-2007) transformers

Logo of the manufacturing company. There is such an icon; you can certainly find a lot of useful information on the official website of the plant. The problem is limited to the cessation of existence of the enterprise. You understand the vitality of the issue for a collapsing country. The second stage concerns the search for a short digital marking, we will puzzle the search engine: Yandex, Google. There is a high chance of immediately finding the characteristics, as well as the electrical circuit of the device. Then nothing is easier than ringing the transformer, determining the presence of a breakdown and the integrity of the windings. We remind you that the insulation resistance (on a magnetic circuit, for example) is at least 20 MOhm according to existing standards. Affects any adjacent, electrically isolated windings. Having purchased a Chinese tester, amateurs can take measurements with their own hands.
We consider the name of the product to be a key factor. You need to understand: different classes are intended for their own purposes. You can, of course, use an input transformer, forming galvanic isolation, while simultaneously understanding the resulting result. In devices, voltage is usually not standardized separately; the operation is meaningless. The secondary winding of the current transformer is connected to the corresponding coil of the control and measurement device. The voltage is assessed separately if necessary. The marking may contain the words “transformer”, “autotransformer”. Let's immediately understand the meaning. Yandex will help. For example, an autotransformer is characterized by the absence of galvanic isolation between the primary and secondary windings. In fact, when electric trains are moving, it is convenient to place autotransformers at intervals and relieve voltage using the typical method. The current trajectory will significantly reduce losses. The distance between the source and grounding (through the rails) is reduced. There are many other types of transformers. The type is determined, we find the GOST of the corresponding class of device, and then we move on, equipped with reliable information support. Regarding of this class We find devices: marking is carried out in accordance with GOST 11677-75. It differs from GOST, according to which the consideration began, due to the different scope of action. GOST 11677 is international. Therefore, you need to know: even on the same class of products, the tags are not the same.
The serial number will help you obtain technical support. We know for sure that there are specialists who speak English in Taiwan and China; we strongly recommend that you try to contact us if problems arise. For Soviet products, the information will most likely be useless.
The type symbol will help you understand design features. For example, let's meet TZRL. According to GOST 7746-2001, there are tables (2 and 3) that lead to decoding. As for the first letter, it characterizes the word “transformer”. Bad luck - the sign does not decipher the letter Z. Give up? We visit Yandex and soon find: Z means “protective”. Then it’s simple: the letter O according to the table is “support”, L characterizes the cast type of insulation. We find climatic version U2. Decoding is carried out in accordance with GOST 15150, placement category type 2 GOST 15150. Having the information in hand, you can find the distinctive features of the transformer. Regarding the future placement, we decided to check the transformer for a reason. Surely a warm place has been prepared that meets the specified standards.
We consider information regarding regulatory documentation useful. The standard according to which the transformer is manufactured is indicated by the nameplate. All that remains is to open the document and decipher the inscription. In each specific case, there may be slight deviations in the designations; a search engine (Yandex, Google) will help you figure it out.
The date of manufacture is indicated by the soft aluminum plate. The information will be useful to those who wish to contact the service. technical support manufacturer.
The nameplate provides a drawn electrical diagram winding connections, pin numbers (colors, other symbols). According to the information, nothing is easier than finding faults in transformers. Even if the nameplate is half erased, you can probably find a nameplate for a similar device. Then you can redraw and print necessary information. On specialized forums, amateurs willingly share such information. Stop being discouraged. Finally, we can learn a lot from reference books. You will find it using Yandex. Search electronic versions books, network resources suffer from low accuracy. The search bar contains file extensions: djvu, pdf, torrent. Don't worry about copyright, the book is downloaded for review. We looked and deleted it. The information received cannot be passed on, of course. I came across a brochure developed by ABS Electro, which provides the necessary information on the product. Inside some devices there are thermal relays and some other elements. Therefore, ringing a transformer is ten times more difficult for an ordinary person. In consumer electronics, there is often a 135 degree Celsius fuse hidden in the turns of the primary and secondary windings; a truly complex product will present a surprise to experienced researchers. By the way, thermal fuses sometimes decorate the magnetic circuit, the tester showed a break in the winding, look for the protective elements.
The nominal frequency Hz may be absent if the network corresponds to a standard (industrial) one. A high-frequency transformer should not be used instead of a regular one. The resistance of the windings will be completely different, the characteristics will change. The transformer will not work correctly and will become hotter.
The characteristics of the operating mode are indicated if the nature of the transformer’s operation is beyond the scope of the term “continuous”. According to accepted standards, the device can operate indefinitely. Otherwise, the operating cycle is given. After a certain period of activity, the transformer will need to rest. Otherwise, it will burn out, the protection (relays, fuses) will trip, or the winding will fail due to overheating.
Rated apparent power kVA is specified for significant windings. Good to know: LV means low, VN high voltage. It is easy to understand by studying the welding machine transformer. The electrode current is high, the voltage is low. The turns are formed by a thick wire, the resistance is small. The rated apparent power will allow the source to be coordinated with the consumer. Let’s say you have low-voltage equipment and need to quickly select a transformer. Avoiding racking your brains, you should compare the power: consumption, permissible secondary winding of the transformer. Aspects will become clearer. Maximum power equipment consumption is below the operating (nominal) secondary winding of the transformer.
Current transformer label
The voltage rating of the main secondary winding is a characteristic by which one can understand whether the transformer is working. It is enough to ensure the absence short circuit, turn on primary winding to the network. We will take measurements using a tester (designed for the specified range). Much more reliable than measuring resistance and trying to calculate the transmission coefficient.
Voltage stabilizers often use transformers with a variable number of turns. A special slider bypasses the secondary winding, removing the required voltage. The markings of some transformers contain voltage change limits. Of course, this is taken into account by the inspector. By the way, most often this is where the malfunction of transformers lies. Either it closes adjacent turns, or bad contact runner. We will fix any damage found.
The rated currents of the windings will sometimes allow you to select the components of the network without looking. For example, a circuit breaker. Many devices provide maximum current ratings. It is useful to measure the value with an ammeter; you will need to connect the consumer. It's clear, short circuit the secondary winding should not be made.
The short circuit voltage of the secondary winding is indicated as a percentage of the nominal value. It is clear that, unlike the ideal energy source studied by physics class teachers, real instruments are powerless to provide indicators. Therefore, with a sharp increase in current, the voltage rapidly drops. Percentages are given relative to nominal value. Specific meaning You can do the math yourself with the help of the Windows OS calculator. It’s difficult to say whether it’s worth trying to organize a short circuit with your own hands. Risky: the plugs will be blown out, the transformer is at risk.

We hope we have talked enough about ways to troubleshoot transformers. The main thing is to discover the cause, then everyone rotates around their own axis. The simplest (often the only) solution to the problem is to rewind the faulty coil. It is made with wire bought on the market; counting the number of turns is a separate art. It's easier to make a request to the forum. The answer will probably be:

link to a specialized computer program;
share their experience;
will advise.

Please note that the symbols and list of parameters are determined by the type of transformer. They will not necessarily be identical to the review of the VashTekhnik portal.

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 according to 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:

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.
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, in extreme cases, on the developer’s page on the Internet.
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.
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.
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.
Connect the input circuit to the source. Use the multimeter in mode alternating current and measure the voltage of the primary winding. 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.
Also measure the output voltage. If there is filtration, then the measurement is carried out in the mode direct current. 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.

IN modern technology transformers are used quite often. These devices are used to increase or decrease the parameters of a variable 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 belongs to the elementary devices, and the principle of its operation is based on the two-way transformation of the excited magnetic field. Typically, 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 winding is supplied. AC voltage with certain characteristics. 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.

For check similar device An ordinary 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 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 important nuance. 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 frequent breakdown transformers is an 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 tester pulse transformers- This 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 contacts of the tester (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 row 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.

It is often necessary to familiarize yourself in advance with the question of how to test a transformer. After all, if it fails or is unstable, it will be difficult to find the cause of the 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 supply to consumers constant value current and its retention 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 measured 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 the availability electronic circuit, which is often present in modern nutritional patterns. 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.

Under voltage

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 we 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, the first values are divided 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 often they are found with separate winding connections. 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 the heating of the windings is normal operation 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, temperature should be taken into account environment, magnitude 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 they be connected to the windings. constant pressure. 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.

To use what is in stock power transformer, it is necessary to know its key characteristics as accurately as possible. There are almost never any difficulties in solving this problem if the markings are preserved on the product. The required parameters can be easily found on the Internet by simply entering the letters and numbers stamped on the transformer into the search bar.
However, quite often there are no markings - the inscriptions are erased, destroyed by corrosion, and so on. Many modern products (especially cheap ones) are not marked at all. Of course, you shouldn’t throw away the transformer in such cases. After all, its price on the market can be quite decent.

The most important parameters of power transformers

What do you need to know about a transformer in order to use it correctly and, most importantly, safely for your purposes? Most often this is some kind of repair household appliances or making your own low-voltage crafts. And you need to know the following about the transformer lying in front of us:

To which terminals should mains power be supplied (230 volts)?
From which terminals should the undervoltage be removed?
What will it be (12 volts, 24 or others)?
How much power can the transformer produce?
How not to get confused if there are several windings, and, accordingly, paired terminals?

It is quite possible to calculate all these characteristics even when there is absolutely no information about the brand and model of the power transformer.
To complete the work you will need the simplest tools and consumables:

multimeter with ohmmeter and voltmeter functions;
soldering iron;
electrical tape or heat shrink tubing;
mains plug with wire;
a pair of ordinary wires;
incandescent lamp;
calipers;
calculator.

You will also need some kind of wire stripping tool and a minimum soldering kit - solder and rosin.

Definition of primary and secondary windings

The primary winding of the step-down transformer is designed to supply mains power. That is, it is to it that you need to connect 230 volts, which are in a regular household outlet. In the most simple options the primary winding may have only two terminals. However, there are also those in which there are, for example, four conclusions. This means that the product is designed to operate on both 230 V and 110 V. We will consider a simpler option.
So, how to determine the terminals of the primary winding of a transformer? To solve this problem you will need a multimeter with an ohmmeter function. With its help you need to measure the resistance between all available terminals. Where it will be the most, there is the primary winding. It is advisable to immediately mark the findings found, for example, with a marker.

The primary winding can be determined in another way. To do this, the wound wire inside the transformer must be clearly visible. In modern versions this is most often the case. In old products, the insides may be filled with paint, which precludes the use of the described method. The winding whose wire diameter is smaller is visually highlighted. It is primary. It needs to be supplied with mains power.
It remains to calculate the secondary winding from which the reduced voltage is removed. Many have already guessed how to do this. Firstly, the resistance of the secondary winding will be much less than that of the primary. Secondly, the diameter of the wire with which it is wound will be larger.

The task becomes a little more complicated if the transformer has several windings. This option is especially scary for beginners. However, the method for identifying them is also very simple, and is similar to that described above. First of all, you need to find the primary winding. Her resistance will be many times greater than that of the others.
To conclude the topic on transformer windings, it is worth saying a few words about why the resistance of the primary winding is greater than that of the secondary, but with the diameter of the wire everything is exactly the opposite. This will help beginners understand the issue in more detail, which is very important when working with high voltage.
A mains voltage of 220 V is supplied to the primary winding of the transformer. This means that with a power of, for example, 50 W, a current of about 0.2 A will flow through it (we divide the power by the voltage). Accordingly, a large cross-section of wire is not needed here. This is, of course, a very simplified explanation, but for beginners (and the solution to the problem posed above) it will be enough.
More significant currents flow in the secondary winding. Let's take the most common transformer, which produces 12 V. With the same power of 50 W, the current flowing through the secondary winding will be about 4 A. This is already quite a large value, because the conductor through which such current will pass must be thicker. Accordingly, the larger the cross-section of the wire, the less its resistance will be.
Using this theory and a simple ohmmeter, you can easily calculate which winding is on a step-down transformer without markings.

Determination of secondary winding voltage

The next step in identifying the “unnamed” transformer will be to determine the voltage on its secondary winding. This will allow us to determine whether the product is suitable for our purposes. For example, you are assembling a 24 V power supply, but the transformer produces only 12 V. Accordingly, you will have to look for another option.

To determine the voltage that can be removed from the secondary winding, the transformer will have to be supplied with mains power. This is already a rather dangerous operation. Through carelessness or ignorance, you can get a strong electric shock, get burned, damage the wiring in the house, or burn the transformer itself. Therefore, it would be a good idea to stock up on a few safety recommendations.
Firstly, when testing, the transformer should be connected to the network through an incandescent lamp. It is connected in series, into the break of one of the wires going to the plug. The light bulb will serve as a fuse in case you do something wrong, or the transformer under test is faulty (shorted, burnt out, wet, etc.). If it glows, something has gone wrong. There is a short circuit in the transformer, so it is better to immediately pull out the plug from the socket. If the lamp does not glow, nothing stinks or smokes, you can continue working.
Secondly, all connections between the outputs and the plug must be carefully insulated. Do not neglect this recommendation. You won’t even notice how, when looking at the readings of a multimeter, for example, if you start straightening twisted wires, you will receive a good electric shock. This is dangerous not only for health, but also for life. For insulation, use electrical tape or heat-shrink tubing of the appropriate diameter.
Now the process itself. A regular plug with wires is soldered to the terminals of the primary winding. As stated above, an incandescent lamp is added to the circuit. All connections are isolated. A multimeter in voltmeter mode is connected to the terminals of the secondary winding. Make sure that it is turned on to measure AC voltage. Beginners often make a mistake here. By setting the multimeter handle to measure DC voltage, you will not burn anything, however, you will not get any sane and useful readings on the display.

Now you can insert the plug into the socket. If everything is in working order, the device will show you the reduced voltage generated by the transformer. Similarly, you can measure the voltage on other windings, if there are several of them.

Simple ways to calculate the power of a power transformer

With the power of a step-down transformer, things are a little more complicated, but there are still some simple techniques. The most accessible way to determine this characteristic is to measure the diameter of the wire in the secondary winding. To do this you will need a caliper, a calculator and the information below.
First, the diameter of the wire is measured. For example, let's take a value of 1.5 mm. Now you need to calculate the cross-section of the wire. To do this, you need to square half the diameter (radius) and multiply by the number “pi”. For our example, the cross-section will be about 1.76 square millimeters.
Next, for the calculation you will need the generally accepted value of current density per square millimeter of conductor. For household step-down transformers, this is 2.5 amperes per square millimeter. Accordingly, a current of about 4.3 A can flow “painlessly” through the second winding of our sample.
Now we take the previously calculated voltage of the secondary winding and multiply it by the resulting current. As a result, we get the approximate value of the power of our transformer. At 12 V and 4.3 A, this parameter will be around 50 W.
The power of an “unnamed” transformer can be determined in several other ways, however, they are more complex. Those interested can find information about them on the Internet. Power is determined by the cross-section of the transformer windows, using calculation programs, as well as by the nominal operating temperature.

Conclusion

From all of the above, we can conclude that determining the characteristics of a transformer without markings is a fairly simple task. The main thing is to follow safety rules and be extremely careful when working with high voltage.