How to check a pulse transformer with a multimeter
The main element of the power supply of digital instruments is a device for converting current and voltage. Therefore, when equipment breaks down, suspicion often falls on it. The easiest way to check the pulse transformer is with a multimeter. There are several measurement methods. Which one to choose depends on the situation and the expected damage. At the same time, it is not difficult to independently check any of them.
Before proceeding directly to testing a pulse transformer (IT), it is advisable to know how it works, understand the principle of operation and distinguish between existing types. Such a pulse device is used not only as part of the power supply, it is used in the construction of protection against short circuits in idle mode and as a stabilizing element.
A pulse transformer is used to convert the magnitude of current and voltage without changing their shape. That is, it can change the amplitude and polarity of various kinds of pulses, coordinate various electronic cascades with each other, and create a reliable and stable feedback. Therefore, the main requirement for it is the preservation of the pulse shape.
The magnetic circuit in the transformer is made of electrical steel plates, except for the toroidal shape, in which it is made of rolled or ferromagnetic material. Coil frames are placed on insulators, and only copper wires are used. The thickness of the plates is selected depending on the frequency.
The arrangement of the windings can be made in a spiral, conical and cylindrical form. A feature of the first type is the use not of wire, but of a wide thin foil tape. The second - are performed with different insulation thickness, affecting the voltage between the primary and secondary windings. The third type is a structure with a wire wound on a rod in a spiral.
The principle of operation of IT is based on the occurrence of electromagnetic induction. So, if voltage is applied to the primary winding, then alternating current will begin to flow through it. Its appearance will lead to the emergence of a non-constant magnetic flux. Thus, this coil is a kind of magnetic field source. This flux is transmitted through a short-circuited core to the secondary winding, inducing an electromotive force (EMF) on it.
The output voltage depends on the ratio of the number of turns between the primary winding and the secondary, and the maximum current strength depends on the cross section of the wire used. When a powerful load is connected to the output, the current consumption increases, which, with a small wire cross section, leads the transformer to overheat, damage the insulation and burn out.
The operation of IT also depends on the frequency of the signal that is applied to the primary winding. The higher this frequency is, the less losses will occur during the transformation of energy. Therefore, at a high speed of the applied pulses, the dimensions of the device can be smaller. This is achieved by the operation of the magnetic circuit in saturation mode, and a small air gap is used to reduce the residual induction. This principle is used in the construction of IT, to which a signal with a duration of only a few microseconds is applied.
To test the performance of a pulse transformer, you can use both an analog multimeter and a digital one. The use of the second is preferable because of the convenience of its use. The essence of preparing a digital tester comes down to checking the battery and test leads. At the same time, the pointer-type device, in addition to this, is further adjusted.
The analog device is tuned by switching the operating mode to the area of measurement of the lowest possible resistance. After that, two wires are inserted into the sockets of the tester and short-circuited. With a special construction handle, the position of the arrow is set opposite to zero. If the arrow cannot be set to zero, then this indicates a discharged battery that will need to be replaced.
It's easier with a digital multimeter. Its design uses an analyzer that monitors the condition of the battery and, if its parameters deteriorate, displays a message on the tester's screen about the need to replace it.
When checking the parameters of the transformer, two fundamentally different approaches are used. The first is to assess the health directly in the circuit, and the second - autonomously from it. But it is important to understand that if IT is not removed from the circuit, or at least not disconnected from a number of pins, then the measurement error can be very large. This is due to other radio elements that shunt the input and output of the device.
An important step in checking the transformer with a multimeter is to determine the windings. However, their direction does not play a significant role. This can be done by marking applied to the device. Usually a certain code is indicated on the transformer.
In some cases, the layout of the windings can be applied to IT or even their conclusions are signed. If the transformer is installed in the device, then the circuit diagram or specification will help in finding the pinout. Also, often the designations of the windings, namely the voltage and the common output, are signed on the PCB itself near the connectors to which the device is connected.
After the conclusions are determined, you can proceed directly to the test of the transformer. The list of malfunctions that may occur in the device is limited to four points:
The verification sequence is reduced to the initial external inspection of the transformer. It is carefully checked for blackening, chips, and smell. If no obvious damage is detected, then proceed to the measurement with a multimeter.
To check the integrity of the windings, it is best to use a digital tester, but you can also examine them using a switch. In the first case, the diode continuity mode is used, indicated on the multimeter by the symbol -|>| —))). To determine the breakage, test leads are connected to the digital instrument. One plugs into the jacks labeled V/Ω and the other plugs into COM. The galet switch is transferred to the continuity area. Measuring probes sequentially touch each winding, red - to one of its conclusions, and black - to the other. With its integrity, the multimeter will beep.
An analog tester checks in the resistance measurement mode. To do this, the tester selects the smallest resistance measurement range. This can be implemented via buttons or a switch. The probes of the device, as in the case of a digital multimeter, touch the beginning and end of the winding. If it is damaged, the arrow will remain in place and will not deviate.
In the same way, a short circuit test is performed. A short circuit may occur due to insulation breakdown. As a result, the resistance of the winding will decrease, which will lead to a redistribution of the magnetic flux in the device. For testing, the multimeter switches to resistance test mode. By touching the probes to the windings, they look at the result on a digital display or on a scale (arrow deviation). This result should not be less than 10 ohms.
To make sure that there is no short circuit to the magnetic circuit, they touch the “iron” of the transformer with one probe, and with the second - in series with each winding. Deviation of the arrow or the appearance of a sound signal should not be. It is worth noting that it is possible to ring the interturn circuit with a tester only in an approximate form, since the error of the device is quite high.
If a transformer malfunction is suspected, testing can be carried out without completely disconnecting it from the circuit. This test method is called direct, but is associated with the risk of electric shock. The essence of actions in measuring current is to perform the following steps:
When voltage is applied, if the transformer is operational, a current will begin to flow through it, the value of which can be seen on the tester screen. If IT has several secondary windings, then the current strength is checked on each of them.
The voltage measurement is as follows. The circuit with the installed transformer is connected to the power supply, and then the tester switches to the ACV (variable signal) area. Wire plugs plug into V/Ω and COM jacks and touch the beginning and end of the winding. If the IT is correct, the result will be displayed on the screen.
To be able to check the transformer with a multimeter in this way, its current-voltage characteristic is necessary. This graph displays the relationship between the potential difference at the terminals of the secondary windings and the current strength, leading to their magnetization.
The essence of the method is as follows: the transformer is removed from the circuit, pulses of different sizes are applied to its secondary winding using a generator. The power supplied to the coil should be sufficient to saturate the magnetic circuit. Each time the pulse changes, the current in the coil and the voltage at the source output are measured, and the magnetic circuit is demagnetized. To do this, after removing the voltage, the current in the winding increases in several approaches, after which it decreases to zero.
As the CVC is removed, its real characteristic is compared with the reference one. A decrease in its steepness indicates the appearance of an interturn short circuit in the transformer. It is important to note that in order to build the current-voltage characteristic, it is necessary to use a multimeter with an electrodynamic head (pointer).
Thus, using a conventional multimeter, you can most likely determine the health of IT, but for this it is best to perform a set of measurements. Although for a correct interpretation of the result, one should understand the principle of operation of the device and imagine what processes take place in it, but in principle, for a successful measurement, it is enough just to be able to switch the device to different modes.
In this video of the Soldering Iron TV channel, we will look at the simplest ways to check the windings and how to get it from a conventional transformer. The best option is to have two identical windings. In this case, each has an amplitude voltage of 12 volts, and their resistance is 100 milliohms.
It is very important to make the right connection here. The windings are connected to each other by those ends whose phases are opposite, that is, shifted by 180 degrees. And then at the other two ends, the sum of the voltages of both windings is obtained. These ends are connected to the inputs of a conventional diode bridge, and the outputs of the bridge are connected to 2 smoothing capacitors, which are connected so that one of them is charged through the upper diodes with a positive voltage from the ends of the windings relative to the ground, and the other is negative through the lower diodes. And the ground, which is the middle point here, is connected to other contacts. Two resistors are used as a load here. Separately for plus and minus power.
Now let's see this circuit in action.
We will establish a special observation for the positive and negative voltages at the output. Without load, the indicators very quickly reached the level of plus and minus 12 volts and there are no ripples. And after connecting the load, ripples appeared and the voltage dipped a little.
Let's now load and minus a bipolar power supply and see how changes in load resistance will affect the ripple. So, the latter is reduced by several times and the pulsations have increased significantly from this. Now let's reduce the current consumption, returning the previous resistance, and take a closer look at the ripples on the plus side of the supply.
It turns out the amplitude of the pulsation is approximately 700 millivolts. We will remember this result for comparison with other options. And now it's time to apply this circuit to a real transformer.
Suppose there is a transformer without identification marks. It is necessary to check its performance, how many windings are here and at what voltage. The easiest way to do this is to plug in 220 or 110 volts, depending on the input voltage for which it is designed. And measure it on the secondary windings. Since there is a risk of shorting them out during measurement, we will use that. what comes our way. In our case, this is heat shrink. First, put it on the conclusions of the secondary windings. Let's set the measurement mode in this case to two hundred volts. The next step is to turn it on. But since this is a known working transformer, let's turn it on not through a light bulb. If this is an unknown transformer and we do not know its performance, it is best to turn it on through a light bulb, that is, we connect it to a break in one of the wires.
Now let's measure in pairs. Most often in transformers it is pair windings that are brought out side by side.
It's about 9 volts. We have defined one of the windings. These are the first two - 9 volts. We measure the second two. Also 9 volts.
That is, we found the second winding. The third and fourth pairs are also 9 volts each. It remains to check that they are not connected.
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Often you need to familiarize yourself in advance with the question of how to test the transformer. After all, if it fails or becomes unstable, it will be difficult to look for the cause of equipment failure. This simple electrical device can be diagnosed with a conventional multimeter. Let's see how to do it.
How to check the transformer if we do not know its design? Consider the principle of operation and varieties of simple equipment. Coils of copper wire of a certain section are applied to the magnetic core so that there are leads for the supply winding and the secondary. 
The transfer of energy to the secondary winding is carried out in a non-contact way. Here it becomes almost clear how to check the transformer. Similarly, the usual inductance is called with an ohmmeter. The turns form a resistance that can be measured. However, this method is applicable when the target value is known. After all, the resistance can change up or down as a result of heating. This is called interturn short circuit.
Such a device will no longer produce a reference voltage and current. The ohmmeter will only show an open circuit or a complete short circuit. For additional diagnostics, a short circuit test to the case is used with the same ohmmeter. How to test a transformer without knowing the winding leads?
This is determined by the thickness of the outgoing wires. If the transformer is step-down, then the lead wires will be thicker than the lead wires. And, accordingly, on the contrary: the booster wires are thicker. If two windings are output, then the thickness may be the same, this should be remembered. The surest way to look at the markings and find the technical specifications of the equipment.
Transformers are divided into the following groups:
Depending on the purpose of the equipment, the principle of approach to the question of how to check the transformer windings also changes. A multimeter can only ring small devices. Power machines already require a different approach to troubleshooting.
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 establishes the integrity of the conductor. Before this, the body is inspected for the absence of carbon deposits, sagging as a result of equipment heating. 
Next, the current values \u200b\u200bare measured in Ohms and compared with the passport ones. If there are none, then additional diagnostics under voltage will be required. It is recommended to ring each output relative to the metal case of the device, where the ground is connected.
Before taking measurements, disconnect all ends of the transformer. It is also recommended to disconnect them from the circuit for your own safety. They also check for the presence of an electronic circuit, which is often present in modern power models. It should also be soldered before testing.
Infinite resistance speaks of a whole isolation. Values of several kilo-ohms already raise suspicions about a breakdown on the case. It can also be due to accumulated dirt, dust or moisture in the air gaps of the device.
Energized tests are performed when the question is how to test a transformer for turn-to-turn faults. If we know the magnitude of the supply voltage of the device for which the transformer is intended, then measure the idle 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 the interturn circuit in the windings. If crackling, sparking is heard during operation of the device, then it is better to turn off such a transformer immediately. He is defective. There are permissible deviations in measurements:
Let's figure out how to check the current transformer. It is included in the chain: regular or actually made. It is important that the current value is not less than the nominal value. Measurements with an ammeter are carried out in the primary circuit and in the secondary. 
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 ratio should be taken from the reference book and compared with the calculations obtained. The results should be the same.
The current transformer must not be measured at no load. In this case, too high 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.
Before checking the microwave transformer, we will give frequent types of breakdowns that can be fixed without a multimeter. Often, power supplies fail due to a short circuit. It is established by inspecting circuit boards, connectors, connections. Less often, mechanical damage to the transformer case and its core occurs. 
Mechanical wear of the connections of the transformer leads occurs on moving machines. Large supply windings require constant cooling. In its absence, overheating and melting of the insulation is possible.
Let's figure out how to check a pulse transformer. An ohmmeter can only establish the integrity of the windings. The operability of the device is established when connected to a circuit where a capacitor, a load and a sound generator are involved. 
A pulse signal is sent to the primary winding in the range from 20 to 100 kHz. On the secondary winding, measurements are made with an oscilloscope. Establish the presence of pulse distortion. If they are absent, draw conclusions about a serviceable device.
Oscillogram distortions indicate damaged windings. It is not recommended to repair such devices yourself. They are set up in the laboratory. There are other schemes for checking pulse transformers, where the presence of resonance on the windings is examined. Its absence indicates a faulty device.
You can also compare the shape of the pulses applied to the primary winding and exited from the secondary. The shape deviation also indicates a transformer malfunction.
For resistance measurements, the ends are freed from electrical connections. Choose any output and measure all resistances relative to the rest. It is recommended to write down the values and mark the tested ends. 
So we can determine the type of connection of the windings: with middle conclusions, without them, with a common connection point. More often found with a separate connection of the windings. Measurement can be done with only one of all the wires.
If there is a common point, then we measure the resistance between all available conductors. Two windings with a middle terminal will only make sense between the three wires. Several conclusions are found in transformers designed to work in several networks with a nominal value of 110 or 220 Volts.
The hum during transformer operation is normal if these are specific devices. Only sparking and crackling indicate a malfunction. Often the heating of the windings is the normal operation of the transformer. This is most often seen with step-down devices.
Resonance can be created when the transformer case vibrates. Then you just need to fix it with insulating material. The operation of the windings changes significantly with loose or dirty contacts. Most of the problems are solved by cleaning the metal to a shine and a new close-fitting of the conclusions.
When measuring voltage and current values, the ambient temperature, the magnitude and nature of the load should be taken into account. Supply voltage control is also required. Checking the frequency connection is mandatory. Asian and American appliances are rated at 60 Hz, resulting in lower output values.
Improper connection of the transformer may lead to a malfunction of the device. Under no circumstances should direct voltage be connected to the windings. The coils will melt quickly otherwise. Accuracy in measurements and competent connection will help not only to find the cause of the breakdown, but also, possibly, to eliminate it in a painless way.
In modern technology, transformers are used quite often. These devices are used to increase or decrease the parameters of alternating electric current. The transformer consists of 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 it becomes necessary to repair or replace it. To determine whether the transformer is working, you can use a home multimeter on your own. So, how to check the transformer with a multimeter?
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. Tellingly, a magnetic field can only be induced using alternating current. If you have to work with a constant, you must first convert it.
A primary winding is wound on the core of the device, to which an external alternating voltage with certain characteristics is supplied. It is followed by 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.
There are many types of transformers on the market today. Depending on the design chosen by the manufacturer, a variety of materials can be used. As for the shape, it is chosen solely from the convenience of placing the device in the appliance case. The design power is only affected by the configuration and material of the core. At the same time, 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 multiple secondary windings are used.
To test such a device, a conventional multimeter is sufficient, which will be used as a current transformer tester. No special devices are required.
The transformer test begins with the definition of the windings. This can be done by marking on the device. The pin numbers should be indicated, as well as their type designations, which allows you to establish more information from the directories. In some cases, there are even explanatory drawings. If the transformer is installed in some kind of electronic device, then the electronic circuit diagram of this device, as well as a detailed specification, will be able to clarify the situation.
So, when all the conclusions are determined, the tester's turn comes. With it, you can install the two most common malfunctions - a short circuit (to the case or an adjacent winding) and a winding break. In the latter case, in the ohmmeter mode (resistance measurement), all windings call back in turn. If any of the measurements shows one, that is, infinite resistance, then there is a break.
There is an important nuance here. It is better to check on an analog device, since a digital one can give distorted readings due to high induction, which is especially true for windings with a large number of turns.
When a short circuit to the case is being checked, one of the probes is connected to the winding terminal, while the second leads to the conclusions of all other windings and the case itself. To check the latter, you will first need to clean the place of contact from varnish and paint.
Another common transformer failure is interturn short circuit. It is almost impossible to check a pulse transformer for such a malfunction with only a multimeter. However, if you involve the sense of smell, attentiveness and sharp vision, the problem may well be solved.
A bit of theory. The wire on the transformer is insulated exclusively with its own varnish coating. If there is an insulation breakdown, the resistance between adjacent turns remains, as a result of which the contact point heats up. That is why the first step is to carefully inspect the device for the appearance of streaks, blackening, burnt paper, swelling and burning smell.
Next, we try to determine the type of transformer. As soon as this is obtained, according to specialized reference books, you can see the resistance of its windings. Next, we switch the tester to the megohmmeter mode and begin to measure the insulation resistance of the windings. In this case, the pulse transformer tester is a regular multimeter.
Each measurement should be compared with that specified in the handbook. 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, other data must be given in the reference book: 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.
It should be noted the moment of checking classic step-down transformers with a tester-multimeter. You can find them in almost all power supplies that lower 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 failure:
In this case, you should immediately stop the experiment.
If everything is fine, you can proceed to the measurement on the secondary windings. You can only touch them 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, it is possible to test such a current block 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.
If all tests have shown that the transformer is fully functional, it will not be superfluous to conduct another diagnosis - for the current of the idle 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 the ammeter mode. Important point! The multimeter should be connected short-circuited to the transformer under test.
This is important because during the supply of electricity to the transformer winding, the current strength increases up to several hundred times in comparison with the nominal one. After that, the tester probes open, and the indicators are displayed on the screen. It is they who display the value of the current without load, the no-load current. In a similar way, the indicators are measured on the secondary windings.
To measure the voltage, a rheostat is most often connected to the transformer. If it is not at hand, a tungsten spiral or a row of light bulbs can be used.
To increase the load, increase the number of light bulbs or reduce the number of turns of the spiral.
As you can see, no special tester is even required for verification. A normal multimeter will do. It is highly desirable to have at least an approximate understanding of the principles of operation and the design of transformers, but for a successful measurement, it is enough just to be able to switch the device to ohmmeter mode.
Often you need to familiarize yourself in advance with the question of how to test the transformer. After all, if it fails or becomes unstable, it will be difficult to look for the cause of equipment failure. This simple electrical device can be diagnosed with a conventional multimeter. Let's see how to do it.
How to check the transformer if we do not know its design? Consider the principle of operation and varieties of simple equipment. Coils of copper wire of a certain section are applied to the magnetic core so that there are leads for the supply winding and the secondary.
The transfer of energy to the secondary winding is carried out in a non-contact way. Here it becomes almost clear how to check the transformer. Similarly, the usual inductance is called with an ohmmeter. The turns form a resistance that can be measured. However, this method is applicable when the target value is known. After all, the resistance can change up or down as a result of heating. This is called interturn short circuit.
Such a device will no longer produce a reference voltage and current. The ohmmeter will only show an open circuit or a complete short circuit. For additional diagnostics, a short circuit test to the case is used with the same ohmmeter. How to test a transformer without knowing the winding leads?
This is determined by the thickness of the outgoing wires. If the transformer is step-down, then the lead wires will be thicker than the lead wires. And, accordingly, on the contrary: the booster wires are thicker. If two windings are output, then the thickness may be the same, this should be remembered. The surest way to look at the markings and find the technical specifications of the equipment.
Transformers are divided into the following groups:
Depending on the purpose of the equipment, the principle of approach to the question of how to check the transformer windings also changes. A multimeter can only ring small devices. Power machines already require a different approach to troubleshooting.
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 establishes the integrity of the conductor. Before this, the body is inspected for the absence of carbon deposits, sagging as a result of equipment heating.
Next, the current values \u200b\u200bare measured in Ohms and compared with the passport ones. If there are none, then additional diagnostics under voltage will be required. It is recommended to ring each output relative to the metal case of the device, where the ground is connected.
Before taking measurements, disconnect all ends of the transformer. It is also recommended to disconnect them from the circuit for your own safety. They also check for the presence of an electronic circuit, which is often present in modern power models. It should also be soldered before testing.
Infinite resistance speaks of a whole isolation. Values of several kilo-ohms already raise suspicions about a breakdown on the case. It can also be due to accumulated dirt, dust or moisture in the air gaps of the device.
Power-on tests are carried out when the question is how to check the transformer for. If we know the magnitude of the supply voltage of the device for which the transformer is intended, then measure the idle 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 the interturn circuit in the windings. If crackling, sparking is heard during operation of the device, then it is better to turn off such a transformer immediately. He is defective. There are permissible deviations in measurements:
Let's figure out how to check the current transformer. It is included in the chain: regular or actually made. It is important that the current value is not less than the nominal value. Measurements with an ammeter are carried out in the primary circuit and in the secondary.
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 ratio should be taken from the reference book and compared with the calculations obtained. The results should be the same.
The current transformer must not be measured at no load. On the secondary winding, in this case, too much damage to the insulation may be formed. You should also observe the polarity of the connection, which will affect the operation of the entire connected circuit.
Before checking the microwave transformer, we will give frequent types of breakdowns that can be fixed without a multimeter. Often, power supplies fail due to a short circuit. It is established by inspecting circuit boards, connectors, connections. Less often, mechanical damage to the transformer case and its core occurs.
Mechanical wear of the connections of the transformer leads occurs on moving machines. Large supply windings require constant cooling. In its absence, overheating and melting of the insulation is possible.
Let's figure out how to check a pulse transformer. An ohmmeter can only establish the integrity of the windings. The operability of the device is established when connected to a circuit where a capacitor, a load and a sound generator are involved.
A pulse signal is sent to the primary winding in the range from 20 to 100 kHz. On the secondary winding, measurements are made with an oscilloscope. Establish the presence of pulse distortion. If they are absent, draw conclusions about a serviceable device.
Oscillogram distortions indicate damaged windings. It is not recommended to repair such devices yourself. They are set up in the laboratory. There are other schemes for checking pulse transformers, where the presence of resonance on the windings is examined. Its absence indicates a faulty device.
You can also compare the shape of the pulses applied to the primary winding and exited from the secondary. The shape deviation also indicates a transformer malfunction.
For resistance measurements, the ends are freed from electrical connections. Choose any output and measure all resistances relative to the rest. It is recommended to write down the values and mark the tested ends.
So we can determine the type of connection of the windings: with middle conclusions, without them, with a common connection point. More often found with a separate connection of the windings. Measurement can be done with only one of all the wires.
If there is a common point, then we measure the resistance between all available conductors. Two windings with a middle terminal will only make sense between the three wires. Several conclusions are found in transformers designed to work in several networks with a nominal value of 110 or 220 Volts.
The hum during transformer operation is normal if these are specific devices. Only sparking and crackling indicate a malfunction. Often the heating of the windings is the normal operation of the transformer. This is most often seen with step-down devices.
Resonance can be created when the transformer case vibrates. Then you just need to fix it with insulating material. The operation of the windings changes significantly with loose or dirty contacts. Most of the problems are solved by cleaning the metal to a shine and a new close-fitting of the conclusions.
When measuring voltage and current values, the ambient temperature, the magnitude and nature of the load should be taken into account. Supply voltage control is also required. Checking the frequency connection is mandatory. Asian and American appliances are rated at 60 Hz, resulting in lower output values.
Improper connection of the transformer may lead to a malfunction of the device. Under no circumstances should direct voltage be connected to the windings. The coils will melt quickly otherwise. Accuracy in measurements and competent connection will help not only to find the cause of the breakdown, but also, possibly, to eliminate it in a painless way.
It is very useful for beginner radio amateurs to be able and know how to check the transformer. Such knowledge is useful for the reason that it saves time and money. In most linear power supplies, the lion's share of the cost is the transformer. Therefore, if a transformer with unknown parameters is in your hands, do not rush to throw it away. Better take a multimeter. Also, for some experiments, we need an incandescent lamp with a cartridge.
In order to more consciously perform further experiments and experiments, one should understand how the transformer transformer is arranged and works. Let's look at it here in a simplified form.
The simplest transformer consists of two windings wound on a core or magnetic circuit. Each winding is a conductor isolated from each other. And the core is recruited from thin sheets of special electrical steel isolated from each other. Voltage is applied to one of the windings, called the primary, and voltage is removed from the second, called the secondary.
When an alternating voltage is applied to the primary winding, since the electrical circuit is closed, a bullet is created in it for the flow of an alternating electric current. An alternating magnetic field is always formed around a conductor with alternating current. The magnetic field is closed and amplified by the core of the magnetic circuit and induces a variable electromotive force EMF in the secondary winding. When a load is connected to the secondary winding, an alternating current flows in it i 2 .
This knowledge is not yet enough to fully understand how to test a transformer with a multimeter. So let's take a look at a few more points.
Without delving into the details, which are useless here, we note that the EMF, like the voltage, is determined by the number of turns of the winding, other things being equal
E~w.
The more turns, the higher the EMF (or voltage) value of the winding. In most cases, we are dealing with step-down transformers. A high voltage of 220 V is supplied to their primary winding (230 V according to the new GOST), and a low voltage is removed from the secondary winding: 9 V, 12 V, 24 V, etc. Accordingly, the number of turns will also be different. In the first case it is higher, and in the second it is lower.
Because
E 1 > E 2,
That
w1 > w2.
Also, without giving justification, we note that the powers of both windings are always equal:
S1 = S2.
And since power is the product of current i and voltage u
S = u∙i,
That
S 1 \u003d u 1 ∙ i 1;S 2 \u003d u 2 ∙ i 2.
From where we get a simple equation:
u 1 ∙i 1 = u 2 ∙i 2.
The last expression is of great practical interest for us, which is as follows. To maintain the balance of the powers of the primary and secondary windings, as the voltage increases, the current must be reduced. Therefore, less current flows in a winding with a higher voltage and vice versa. Simply put, since the voltage in the primary winding is higher than in the secondary, the current in it is less than in the secondary. This keeps the proportion. For example, if the voltage is 10 times higher, then the current is 10 times lower.
The ratio of the number of turns or the ratio of the EMF of the primary winding to the secondary winding is called the transformation ratio:
k t \u003d w 1 / w 2 \u003d E 1 / E 2.
From the above, we can draw the most important conclusion that will help us understand how to test a transformer with a multimeter.
The conclusion is as follows. Since the primary winding of the transformer is designed for a higher voltage (220 V, 230 V) relative to the secondary (12 V, 24 V, etc.), it is wound with a large number of turns. But at the same time, less current flows in it, so a thinner wire of greater length is used. Hence it follows that primary winding step-down transformer has great resistance , how secondary .
Therefore, using a multimeter, it is already possible to determine which conclusions are the conclusions of the primary winding, and which are the secondary ones, by measuring and comparing their resistances.
How to determine the windings of a transformer
By measuring the resistance of the windings, we learned which of them is designed for a higher voltage. But we still don’t know if it’s possible to supply 220 V to it. After all, a higher voltage still doesn’t mean 220 V. Sometimes transformers come across that are designed to operate on AC 110 V and 127 V or a lower value. Therefore, if such a transformer is connected to a 220 V network, it will simply burn out.
In this case, experienced electricians do this. They take an incandescent lamp and connect it in series with the intended primary winding. Next, one output of the winding and the output of the light bulb are connected to a 220 V network. If the transformer is designed for 220 V, then the lamp will not light up , since the applied voltage of 220 V is completely balanced by the EMF of the winding self-induction. EMF and applied voltage are directed oppositely. Therefore, a small current will flow through the incandescent lamp - the no-load current of the transformer. The magnitude of this current is insufficient to heat the filament of an incandescent lamp. For this reason, the lamp does not light up.
If the lamp lights up even at full heat, then 220 V cannot be supplied to such a transformer; it's not designed for that kind of pressure.
Very often you can find a transformer that has many leads. This means that it has several secondary windings. You can find out the voltage of each of them as follows.
Earlier, we looked at how to test a transformer with a multimeter and determine the primary winding by the resistance ratio. Also, using an incandescent lamp, you can make sure that it is designed for 220 V (230 V).
Now the matter is small. We supply 220 V to the primary winding and measure the alternating voltage at the terminals of the remaining windings using a multimeter.
Connection of transformer windings
The secondary windings of the transformer are connected in series and rarely in parallel. When connected in series, the windings can be connected in accordance with and counter.
Consistent connection of the transformer windings is used in order to obtain a greater voltage value than one of the windings gives. With a consonant connection, the beginning of one winding, indicated in the drawings of electrical circuits by a dot or a cross, is connected to the end of the previous one. It should be remembered here that the maximum current of all connected windings should not exceed the value of the one that is designed for the smallest current.
In a back-to-back connection, the beginnings or ends of the windings are connected together. In a back-to-back connection, the EMFs are directed in the opposite direction. The difference in EMF is obtained at the conclusions: a smaller value is subtracted from a larger value. If two windings with equal EMF values are connected in opposite directions, then the outputs will be zero.
Now we know how, how to check the transformer with a multimeter, and we can also find the primary and secondary windings.
