Determination of winding terminals. How to determine the primary winding of a transformer by resistance

Transformers are widely used in radio electronics. They are AC voltage converters and, unlike other radio elements, rarely fail. To determine their serviceability, you need to know how to test a transformer with a multimeter. This method is quite simple, and you need to understand the principle of operation of the transformer and its main characteristics.

Basic information about transformers

To convert alternating voltage ratings, special electrical machines are used - transformers.

A transformer is an electromagnetic device designed to convert alternating voltage and current of one magnitude into alternating current and voltage of another magnitude.

Device and principle of operation

It is used in all consumer power supply schemes, as well as for transmitting electricity over long distances. The transformer design is quite primitive:

1. The ferromagnetic core is made of a ferromagnetic material and is called a magnetic core. Ferromagnets are substances that have spontaneous magnetization; the parameters (atoms have constant spin or orbital magnetic moments) vary greatly due to the magnetic field and temperature.
2. Windings: primary (mains voltage is connected) and secondary (power supply to a consumer or group of consumers). There can be more than 2 secondary windings.
3. Additional components are used for power transformers: coolers, gas relays, temperature indicators, moisture absorbers, current transformers, protection systems and continuous oil regeneration.

The principle of operation is based on the conductor being in an alternating electric field. When a conductor moves, for example, a solenoid (coil with a core), a voltage can be removed at its terminals, which depends directly proportionally to the number of turns. This approach is implemented in a transformer, but it is not the conductor that moves, but the electric field generated by alternating current. It moves along a magnetic circuit made of ferromagnet. Ferromagnetic is a special alloy ideal for... Basic materials for cores:

1. Electrical steel contains a large mass fraction of silicon (Si) and is combined under high temperature with carbon, the mass fraction of which is no more than 1%. Ferromagnetic properties are not clearly expressed, and eddy current losses (Foucault currents) occur. Losses increase directly proportionally with increasing frequency. To solve this problem, Si is added to carbon steel (E42, E43, E320, E330, E340, E350, E360). The abbreviation E42 stands for: E - electrical steel containing 4% - Si with 2% magnetic losses.
2. Permalloy is a type of alloy and its constituent parts are nickel and iron. This species is characterized by a high value of magnetic permeability. Used in low-power transformers.

When current flows through the primary winding (I), a magnetic flux F is formed in its turns, which propagates along the magnetic circuit to winding II, as a result of which an EMF (electromotive force) is formed in it. The device can operate in 2 modes: load and idle.

Transformation coefficient and its calculation

Transformation ratio (k) is a very important characteristic. Thanks to it, you can identify malfunctions. The transformation ratio is a value showing the ratio of the number of turns of winding I to the number of turns of winding II. According to k, transformers are:

1. Decreasing (k > 1).
2. Raising (k< 1).

It is easy to find, and to do this you need to find out the voltage ratio of each of the windings. If there are more than 2 windings, the calculation is made for each of them. To accurately determine k, you need to use 2 voltmeters, since the network voltage can change, and these changes need to be monitored. You only need to apply the voltage specified in the specifications. k is determined in several ways:

According to the passport, which indicates all the parameters of the device (supply voltage, transformation ratio, wire cross-section on the windings, number of turns, type of magnetic circuit, dimensions).

1. Calculation method.
2. Using the Schering Bridge.
3. Using special equipment (for example, UIKT-3).

Calculating k is not difficult, and there are a number of formulas that allow you to do this. There is no need to take into account the losses of the magnetic circuit used during manufacturing at the factory. Research has shown the relationship between the magnetic core (iron ore) and k. To improve the efficiency of a transformer you need reduce magnetic losses:

1. Use of special alloys for the magnetic core (thickness reduction and special processing).
2. Reducing the number of turns when using thick wire, and at high frequencies, a large cross-section provides space for the creation of eddy currents.

Amorphous steel is used for these purposes. But it also has a limitation called magnetostriction (a change in the geometric dimensions of a material under the influence of an electromagnetic field). Using this technology, it is possible to obtain sheets for iron ore with a thickness of hundredths of millimeters.

Calculation formulas

In the absence of appropriate documentation, you need to make calculations yourself. In each specific case, the calculation methods are different. Basic formulas for calculating k:

1. Without taking into account possible errors: k = U1 / U2 = n1 / n2, where U1 and U2 are U on windings I and II, n1 and n2 are the number of turns on windings I and II.
2. When taking into account errors: k = U1 / U2 = (e * n1 + I1 * R1) / (e * n2 + I2 * R2), where U1 and U2 are the voltages on windings I and II; n1 and n2 - number of turns on windings I and II; e - EMF (electromotive force) in each of the turns of the windings; I1 and I2 - current strengths of windings I and II; R1 and R2 - resistances for I and II.
3. According to the known powers when windings are connected in parallel: kz = Z1 / Z2 = ku * ku, where kz - k in power, Z1 and Z2 - power on the primary and secondary windings, ku - k in voltage (k = U1 / U2).
4. For currents when windings are connected in series: k = I1 / I2 = n2 / n1. When taking into account the resulting no-load current (loss current Io): I1 * n1 = I2 * n2 + Io.

Functionality check

Transformers are mainly used in power supplies. Winding and manufacturing the transformer itself from scratch is a difficult task and not everyone can do it. Therefore, a ready-made one is taken as a basis and modernized by changing the number of turns of the secondary winding. Main transformer malfunctions:

1. Breakage of conclusions.
2. Damage to the magnetic circuit.
3. Insulation failure.
4. Combustion during short circuit.

Diagnosis begins with a visual examination. Initial diagnostics include inspection of the transformer terminals, its coils for charring, and the integrity of the magnetic circuit.

If the leads are worn out, it is necessary to clean them, and in some cases, if they are broken, to disassemble the transformer, solder them and ring them with a tester.

If the magnetic circuit is damaged, you need to replace it or find out from reference books about something similar for a specific model, since it cannot be repaired. Individual plates can be replaced.

In the event of a short circuit, it is necessary to carry out diagnostics for operability using measuring instruments (checking the transformer with a multimeter).

When the insulation is broken, contact occurs between the turns of the windings or on the housing. It is quite difficult to determine this malfunction. For this you need to do the following:

1. Turn the device into resistance measurement mode.
2. One probe should be on the housing, and the other should be connected to each terminal of the transformer in turn.
3. In all cases, the device should show infinity, which indicates the absence of a short circuit on the housing.
4. Regardless of the instrument readings, a breakdown in the housing exists, and it is necessary to completely disassemble the transformer and even unwind its windings to determine the cause.

To search for short-circuited turns, you need to determine where winding I (input) is and where winding II (output) is on an unknown transformer. For this You should use the following algorithm:

1. Find out the resistance of the primary winding of a 220 volt transformer using multimeter measurements in the “resistance” mode. It is necessary to record the readings of the device. Select the winding with the highest resistance.
2. Take a 50 W light bulb and connect it in series with this winding.
3. Plug in for 5-7 seconds.

After this, turn off and check the windings for heating. If there is no noticeable temperature rise, then start looking for short-circuited turns. How to check a transformer for an interturn short circuit: you need to use a megohmmeter at a voltage of 1000 V. When measuring insulation breakdown, you need to ring the housing and winding terminals, as well as windings that are independent of each other, for example, terminals I and II.

It is necessary to determine the transformation coefficient and compare it with the document. If they match, the transformer is working.

There are two more verification methods:

1. Direct - involves testing under load. To implement it, it is necessary to assemble the power circuit of windings I and II. By measuring the current values ​​in the windings, and then using formulas (4), determine k and compare it with the passport data.
2. Indirect methods. Include: checking the polarity of winding terminals, determining magnetization characteristics (rarely used). The polarity is determined using a voltmeter or ammeter of magnetoelectric design with determination of the polarity at the output. When the arrow deviates to the right, the polarities coincide.

Checking a pulse transformer is quite complicated, and can only be done by an experienced radio amateur. There are many ways to check the serviceability of pulse generators.

Thus, the transformer can be easily checked with a multimeter, knowing the main features and testing algorithm. To do this, you need to find out the type of transformer, find documentation on it and calculate the transformation ratio. In addition, it is necessary to carry out a visual inspection of the device.

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 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.

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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

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:

1. The core is damaged.
2. A wire in one of the windings has burned out.
3. 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.
4. 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 larger transducers, this information can be presented in graphical form.

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.

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 the primary coil is connected to an alternating current source, high voltage may appear, which can not only damage the 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

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.

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 for measuring 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. An ordinary battery can serve as a source, but a resistor must be included in the circuit between it and the coil to prevent a short circuit. 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.

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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

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.

For an answer to the question of what a transformer is, let’s turn to the well-known Wikipedia. It says - a transformer or converter is an electromagnetic device that has two, often more, windings and is used to convert alternating current voltages into one or more systems using electromagnetic induction, without changing the frequency of the current.

The main use of a transformer is to change the characteristics of electricity and voltage. Despite the fact that this device makes very difficult transformations, its design is extremely simple. It consists of a core, a number of coils of copper wire are wound around it. Among them, one is input (or in other words primary), the rest of the coils are called secondary or output.

Initially, the current flows to the input coil, on which a voltage arises as a result of the induction of a magnetic field. The final of the secondary coils creates an alternating type current, equal in its characteristics to the current in the primary coil. If there are different numbers of turns wound on the input and output windings, then accordingly the current characteristics will be different. As they say, everything ingenious is simple. But this device quite often fails, and its defects are usually not invisible to the naked eye. It is because of this that the question increasingly comes up: how to test a converter with a multimeter or other measuring device?

It should be noted that various testers, including a multimeter, will be needed even if you have a transformer with unmarked and unfamiliar parameters. It will also be possible to find them out with a multimeter.

Before starting work, you first need to get your bearings with the coils. It will be necessary to take all the ends of the windings out, move them apart and check with a multimeter, with this we will find the beginning and end of each of the coils. We number the input and output of each coil.

The simplest case is when you have only four endings, so you get two for each winding. However, you often come across devices that have more than four ends. It may be that some of them will not ring through, but this does not mean that a break has occurred somewhere. Most likely, this is a shielding winding, which is usually located between the input and output windings and is usually connected to ground.

Transformer structure and its purpose

All converters are divided into single-phase and three-phase. What's behind this? If electricity flows through three wires, then we have three phase wires and a neutral wire - this means three-phase. But if there are only two wires, then we have single-phase electricity. To turn three phases into one, you just need to use one three-phase wire and its zero. All apartments and houses use single-phase current. The outlet where the TV is plugged in receives single-phase alternating current.

Power transformer

Similar types of transformers are installed on electrical networks and in various installations for receiving and transforming electric current. It got its name from the fact that it serves to supply and receive energy to and from power lines and operates with voltages up to 1150 kV.

By their design, power type transformers contain two, sometimes three or more coils mounted on a core. They work both at substations and at various power plants. Three-phase converters are the most common because they have 15 percent less loss than using three single-phase ones.

Network transformer

Transformers of this type, in Soviet times, were found in almost every device. It is they who convert the mains voltage from the standard 220 volts to what is needed for a particular device.

Typically, these converters are equipped with several output coils in order to be able to use several power sources to power different sections of the electrical circuit. Nowadays they are often found in devices that contain radio tubes.

Autotransformer

This is one of the types of low frequency converters in which the output coil is part of the input coil or vice versa. In such a converter, the coils are connected not only magnetically, but also electrically. Several leads extend from one coil and allow different voltages to be output from one single winding.

One of the advantages is the cost, which is much lower, but the disadvantage is the absence of galvanic isolation on the coils. They are used in various automatic control devices and high voltage networks.

Laboratory autotransformer

This type of transformer is a special rather than a typical case. It is designed to smoothly regulate the voltage supplied to a particular device. Its configuration looks like a ring-shaped transformer with a single coil.

Current transformer

A current transformer is a device whose input coil is powered from a power source, and its output coil is fed to measuring diagnostic devices with a low self-resistance. The most common type of converter of this type is the measuring current transformer.

How to identify windings

As you know, transformers are designed to change the incoming current to the desired value. A standard converter usually has two windings, primary and secondary. The current flows into the primary circuit and the load is supplied to the secondary circuit. But more often, modern converters are equipped with several coils, which complicates their correct identification.

By carefully examining the outer layer of the transformer, you can find an image on the insulation of the structure diagram or digital designations of the coils; old Soviet transformers have a code indicated by which you can find all the information in the reference book.

If, during external inspection, no markings are found, the thickness of the wire will help to suggest the purpose of certain turns. If the transformer is a step-down transformer, then the turns of the primary winding are always thinner than the turns of the secondary coils.

If you consider the sequence of winding the turns of the coils in the converter, you will notice that the primary winding is wound first, and then the secondary windings are wound on top of it.

In some transformer models, most often network ones, determining the purpose of the coils is not difficult at all. The turns of the primary and secondary windings are located on a plastic base and separated by a partition.

No-load current measurement

When, as a result of testing, it turns out that the converter is in working condition, it is recommended to also check its no-load current. As a rule, if the device is working properly, then this parameter is within 10-15% of the nameplate value. The rated value should be considered the current under load.

Before checking the idle value, the multimeter is switched to the ammeter position. It should be taken into account that when electricity enters the winding, the inrush current significantly exceeds the rating value, so the tester is connected to the device being tested in a short-circuited manner.

How to check household step-down transformers

Using a multimeter, you can also test the most common voltage-stepping transformers in most household electrical appliances, which are used in power supplies with an incoming voltage of 220 volts and an outgoing voltage of 5 to 30. Avoiding the possibility of touching bare wires, apply a voltage of 220 volts to the input coil. If everything went without consequences, then press the multimeter probes and measure the voltage value on the secondary coils. If the readings differ from normal by more than 20 percent, then this is evidence of a malfunction of this coil.

A multimeter will not be able to help us in any way; now we will need a generator and an oscilloscope.