Frequency range of "sweep":
LF power transformers: 40-60 Hz.
Switching power supply transformers: 8-40 kHz.
Separation transformers, TDKS: 13-17 kHz.
Separation transformers, TDKS monitors (for PC):
CGA: 13-17 kHz.
EGA: 13-25 kHz.
VGA: 25-50 kHz.
If you take a pulse power transformer, for example a horizontal scanning transformer, connect it according to Fig. 1, apply U = 5 - 10V F = 10 - 100 kHz sinusoid to winding I through C = 0.1 - 1.0 μF, then on winding II using an oscilloscope we observe the shape of the output voltage.
Rice. 1. Connection diagram for method 1
Having “run” the AF generator at frequencies from 10 kHz to 100 kHz, you need to get a pure sinusoid in some section (Fig. 2 on the left) without emissions and “humps” (Fig. 2 in the center). The presence of diagrams in the entire range (Fig. 2. on the right) indicates interturn short circuits in the windings, etc. and so on.
This technique, with a certain degree of probability, allows you to reject power transformers, various isolation transformers, and partially line transformers. It is only important to choose the frequency range.
Rice. 2. Shapes of observed signals
Method 2
Necessary equipment: Low frequency generator, Oscilloscope.
Principle of operation:
The operating principle is based on the phenomenon of resonance. An increase (2 times or more) in the amplitude of oscillations from the low-frequency generator indicates that the frequency of the external generator corresponds to the frequency of the internal oscillations of the LC circuit.
To check, short-circuit winding II of the transformer. The oscillation in the LC circuit will disappear. It follows from this that short-circuited turns disrupt resonance phenomena in the LC circuit, which is what we wanted.
The presence of short-circuited turns in the coil will also make it impossible to observe resonance phenomena in the LC circuit.
Rice. 3. Connection diagram for method 2
We add that to test pulse transformers of power supplies, capacitor C had a nominal value of 0.01 µF - 1 µF. The generation frequency is selected experimentally.
Method 3
Necessary equipment: Low frequency generator, Oscilloscope.
Principle of operation:
The principle of operation is the same as in the second case, only a version of a series oscillatory circuit is used.
Rice. 4. Connection diagram for method 3
The absence (disruption) of oscillations (quite sharp) when the frequency of the low-frequency generator changes indicates resonance of the LC circuit. Everything else, as in the second method, does not lead to a sharp interruption of oscillations on the monitoring device (oscilloscope, AC millivoltmeter).
N. Tyunin
Testing pulse transformers (IT) used in power supplies and output stages of horizontal scanning (TDKS) of modern televisions using an ohmmeter (even a digital one) does not give positive results. The reason is that the IT windings, with the exception of the high-voltage TDKS windings, have very low active resistance. The simplest (but not the most accessible) way is to measure the inductances of the windings and compare them with the passport data, if any. Another method, proposed in, is to check the IT using a low-frequency generator operating at the resonant frequency of the circuit formed by the external capacitor C1 and the IT winding T1 (Fig. 1).
The proposed method for checking IT does not require a separate generator, but uses the calibrator available in almost every oscilloscope. As a rule, this is a generator of rectangular pulses with a frequency of 1... 2 kHz. The transformer being tested is connected to an oscilloscope according to the circuit shown in Fig. 2. Oscillogram 1 (Fig. 3) corresponds to the shape of the output signal of the calibrator when it is not connected to IT, and oscillogram 2 corresponds to the shape of the signal at the CT control point (see Fig. 2) after connecting the calibrator to the primary winding T1. If differentiated pulses are present at the test point and the amplitude of the signal Um2 approximately corresponds to the amplitude of the output signal of the calibrator Um1, then the tested IT can be considered serviceable. If there are no pulses, then we can clearly conclude that one of the IT windings has a short circuit. It is possible that the signal has the shape shown in oscillogram 3 (see Fig. 3) and its amplitude is greatly underestimated. This indicates that there are short-circuited turns in one of the IT windings.
The proposed verification method can be successfully applied without removing IT from the circuit. In this case, disconnect one of the terminals of the primary winding from the circuit and connect it to the output of the calibrator (see Fig. 2) and check the IT in the above sequence. The signal shape on a working IT should correspond to oscillogram 2 (see Fig. 3). If one of the diodes of the secondary rectifiers in the circuit is faulty or there are short-circuited turns in one of the IT windings, then the signal shape will correspond to oscillogram 3.
Literature
A. Rodin, N. Tyunin. Repair of imported TVs. Repair, Issue 9. Moscow: Solon, 2000.
[email protected]
Due to the widespread use of switching power supplies in various technologies, in the event of a breakdown, it is required to be able to independently repair them. All this, starting from low-power smartphone chargers with voltage stabilization, power supplies for digital set-top boxes, LCD and LED TVs and monitors, to the same powerful computer power supplies, ATX format, the simplest cases of repair of which, we have already considered earlier, that’s all will be .
Photo - switching power supply
It was also said earlier that for us to carry out most measurements, a regular digital multimeter is sufficient. But there is one important nuance here: when checking, for example, by measuring resistance, or in audio testing mode, we can only determine a conditionally non-working part by the low resistance between its legs. Usually it is somewhere from zero to 40-50 Ohms, or a break, but then to do this you need to know what resistance there should be between the legs of the working part, which is not always possible to check. But when checking the functionality of a PWM controller, this is usually not enough. You need either an oscilloscope or a determination of its performance based on indirect evidence.
Cheap multimeter DT
The resistance between the legs may be higher than these limits, but the microcircuit may in fact not be working. But recently I came across this case: the connector of the power cable going from the power supply to the scaler had access from above for measurement only to the upper one, of the two rows of contacts on the connector, the lower one was hidden by the case, and access to it was only available from the back of the board, which makes repairs very difficult. Even a simple measurement of voltage at the connectors can be difficult in such a situation. You need a second person who agrees to hold the board, on the connector of which you will measure the voltage at the terminals on the back side of the board, and some of the parts there are under mains voltage, and the board itself is suspended. This is not always possible, often people whom you ask to hold the board are simply afraid to pick it up, especially if these are power boards. On the one hand, they do the right thing, precautions with untrained personnel should always be more strict.
PWM controller - microcircuit
So what should we do? How can you quickly and without problems conditionally check the operation of the PWM controller, and to be more precise, the power circuits, and at the same time the pulse transformer, step-up transformer that powers the backlight lamps? And it’s very simple... Recently I found one interesting method on YouTube, for masters, the author explained everything very clearly. I'll start from afar.
Transformer
What is, simply put, a regular transformer? These are two or more windings on one core. But there is one nuance here that we will take advantage of: the core, like the windings themselves, in theory can be separate and simply be nearby, close to each other. The parameters will deteriorate greatly, but for our purposes this will be more than enough. So, around each transformer, or inductor, with a significant number of turns, after turning on the power to the circuit, there is a magnetic field, and it is greater, the more turns the winding of the transformer, or inductor, has. What will happen if we apply another inductor, for example with an inductance of 470 μH, to the winding of a transformer or inductor connected to the device’s network, and for our probe we need just such a one, loaded with an LED? For example, like the one in the photo below:
In other words, the magnetic field of the inductor or transformer will penetrate the turns of our inductor, and a voltage will appear at its terminals, which can be used, in our case, to indicate the operability of the power supply circuit. Of course, you need to bring the probe as close as possible to the part being tested, and with the throttle down. What do the parts on the board that we need to touch with our probe look like?
The pulse transformer is circled in red on the board, and the backlight transformer is circled in green. If the circuit is working properly, when you bring the probe close to them, the LED should light up. This means that power is supplied to our, figuratively speaking, inductance being tested. Let's look at it in practice. If the output transistor is broken, the pulse transformer will not work.
In the diagram it is again highlighted in red. If the Schottky diode is broken, at the output, after the transformer, there will be no indication on the filter choke. But there is one nuance here: if the inductor on the board has a small number of turns, the glow will be either barely noticeable or absent altogether. Likewise, if, for example, transistor switches or diode assemblies are broken, through which power is supplied to the step-up transformer, for backlight lamps, LCD monitor or TV, there will be no indication when checking on this transformer.
The cost of this choke in a radio store is only 30 rubles; they are also sometimes found in ATX power supplies, a regular LED, or 5 rubles in a glass flask. As a result, we have a simple, cheap, and very useful device for repairs, which allows us to carry out preliminary diagnostics of a switching power supply within literally one minute. Relatively speaking, with this probe you can check whether there is voltage on all the parts shown in the following photo.
I have been using this probe for only 3-4 days so far, but I already believe that I can recommend it for use to all beginning radio amateurs - repairmen who do not yet have an oscilloscope in their home workshop. Also, this sample may be useful for those who travel abroad. Happy repairs to everyone - AKV.
Due to the widespread use of switching power supplies in various technologies, in the event of a breakdown, it is required to be able to independently repair them. All this, starting from low-power smartphone chargers with voltage stabilization, power supplies for digital set-top boxes, LCD and LED TVs and monitors, to the same powerful computer power supplies, ATX format, the simplest cases of repair of which, we have already considered earlier, that’s all will be .
Photo - switching power supply
It was also said earlier that for us to carry out most measurements, a regular digital multimeter is sufficient. But there is one important nuance here: when checking, for example, by measuring resistance, or in audio testing mode, we can only determine a conditionally non-working part by the low resistance between its legs. Usually it is somewhere from zero to 40-50 Ohms, or a break, but then to do this you need to know what resistance there should be between the legs of the working part, which is not always possible to check. But when checking the functionality of a PWM controller, this is usually not enough. You need either an oscilloscope or a determination of its performance based on indirect evidence.
Cheap multimeter DT
The resistance between the legs may be higher than these limits, but the microcircuit may in fact not be working. But recently I came across this case: the connector of the power cable going from the power supply to the scaler had access from above for measurement only to the upper one, of the two rows of contacts on the connector, the lower one was hidden by the case, and access to it was only available from the back of the board, which makes repairs very difficult. Even a simple measurement of voltage at the connectors can be difficult in such a situation. You need a second person who agrees to hold the board, on the connector of which you will measure the voltage at the terminals on the back side of the board, and some of the parts there are under mains voltage, and the board itself is suspended. This is not always possible, often people whom you ask to hold the board are simply afraid to pick it up, especially if these are power boards. On the one hand, they do the right thing, precautions with untrained personnel should always be more strict.
PWM controller - microcircuit
So what should we do? How can you quickly and without problems conditionally check the operation of the PWM controller, and to be more precise, the power circuits, and at the same time the pulse transformer, step-up transformer that powers the backlight lamps? And it’s very simple... Recently I found one interesting method on YouTube, for masters, the author explained everything very clearly. I'll start from afar.
Transformer
What is, simply put, a regular transformer? These are two or more windings on one core. But there is one nuance here that we will take advantage of: the core, like the windings themselves, in theory can be separate and simply be nearby, close to each other. The parameters will deteriorate greatly, but for our purposes this will be more than enough. So, around each transformer, or inductor, with a significant number of turns, after turning on the power to the circuit, there is a magnetic field, and it is greater, the more turns the winding of the transformer, or inductor, has. What will happen if we apply another inductor, for example with an inductance of 470 μH, to the winding of a transformer or inductor connected to the device’s network, and for our probe we need just such a one, loaded with an LED? For example, like the one in the photo below:
In other words, the magnetic field of the inductor or transformer will penetrate the turns of our inductor, and a voltage will appear at its terminals, which can be used, in our case, to indicate the operability of the power supply circuit. Of course, you need to bring the probe as close as possible to the part being tested, and with the throttle down. What do the parts on the board that we need to touch with our probe look like?
The pulse transformer is circled in red on the board, and the backlight transformer is circled in green. If the circuit is working properly, when you bring the probe close to them, the LED should light up. This means that power is supplied to our, figuratively speaking, inductance being tested. Let's look at it in practice. If the output transistor is broken, the pulse transformer will not work.
In the diagram it is again highlighted in red. If the Schottky diode is broken, at the output, after the transformer, there will be no indication on the filter choke. But there is one nuance here: if the inductor on the board has a small number of turns, the glow will be either barely noticeable or absent altogether. Likewise, if, for example, transistor switches or diode assemblies are broken, through which power is supplied to the step-up transformer, for backlight lamps, LCD monitor or TV, there will be no indication when checking on this transformer.
The cost of this choke in a radio store is only 30 rubles; they are also sometimes found in ATX power supplies, a regular LED, or 5 rubles in a glass flask. As a result, we have a simple, cheap, and very useful device for repairs, which allows us to carry out preliminary diagnostics of a switching power supply within literally one minute. Relatively speaking, with this probe you can check whether there is voltage on all the parts shown in the following photo.
I have been using this probe for only 3-4 days so far, but I already believe that I can recommend it for use to all beginning radio amateurs - repairmen who do not yet have an oscilloscope in their home workshop. Also, this sample may be useful for those who travel abroad. Happy repairs to everyone - AKV.
If you take a pulse power transformer, for example a horizontal scanning transformer, connect it according to Fig. 1, apply U = 5 - 10V F = 10 - 100 kHz sinusoid to winding I through C = 0.1 - 1.0 µF, then on winding II using an oscilloscope we observe the shape of the output voltage.
Rice. 1. Connection diagram for method 1
Having “run” the AF generator at frequencies from 10 kHz to 100 kHz, you need to get a pure sinusoid in some section (Fig. 2 on the left) without emissions and “humps” (Fig. 2 in the center). The presence of diagrams in the entire range (Fig. 2. on the right) indicates interturn short circuits in the windings, etc. and so on.
This technique, with a certain degree of probability, allows you to reject power transformers, various isolation transformers, and partially line transformers. It is only important to choose the frequency range.
Rice. 2. Shapes of observed signals Method 2
Necessary equipment:
- LF generator,
- Oscilloscope
Principle of operation:
The operating principle is based on the phenomenon of resonance. An increase (2 times or more) in the amplitude of oscillations from the low-frequency generator indicates that the frequency of the external generator corresponds to the frequency of the internal oscillations of the LC circuit.
To check, short-circuit winding II of the transformer. The oscillation in the LC circuit will disappear. It follows from this that short-circuited turns disrupt resonance phenomena in the LC circuit, which is what we wanted.
The presence of short-circuited turns in the coil will also make it impossible to observe resonance phenomena in the LC circuit.
We add that to test pulse transformers of power supplies, capacitor C had a nominal value of 0.01 µF - 1 µF. The generation frequency is selected experimentally.
Method 3
Necessary equipment: Low frequency generator, Oscilloscope.
Principle of operation:
The principle of operation is the same as in the second case, only a version of a series oscillatory circuit is used.
Rice. 4. Connection diagram for method 3 The absence (disruption) of oscillations (quite sharp) when the frequency of the low-frequency generator changes indicates resonance of the LC circuit. Everything else, as in the second method, does not lead to a sharp interruption of oscillations on the monitoring device (oscilloscope, AC millivoltmeter).
To check the operation of a pulse transformer, you can use both an analog and digital multimeter. The use of the second is preferable due to its ease of 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 is additionally adjusted to this.
The analog device is configured by switching the operating mode to the area of measuring the minimum possible resistance. Afterwards, two wires are inserted into the tester sockets and short-circuited. Using a special construction handle, the position of the arrow is set opposite zero. If the arrow cannot be set to zero, then this indicates discharged batteries that will need to be replaced
How to test a pulse transformer with a multimeter
To check the pulse transformer, you can use both an analog device and a digital multimeter. The use of the second is preferable due to its ease of 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 is additionally adjusted to this.
Method of testing with an analogue (pointer) measuring device
- The analog device is configured by switching the operating mode to the area of measuring the minimum possible resistance.
- Afterwards, two wires are inserted into the tester sockets and short-circuited.
- Using a special construction handle, the position of the arrow is set opposite zero. If the arrow cannot be set to zero, then this indicates discharged batteries that will need to be replaced.
Procedure for identifying defects
An important step in checking a transformer with a multimeter is identifying the windings. However, their direction does not play a significant role. This can be done using the markings on the device. Usually a certain code is indicated on the transformer.
In some cases, the IT may be marked with a diagram of the location of the windings or even their conclusions may be labeled. If the transformer is installed in the device, then a circuit diagram or specification will help in finding the pinout. Also often the designations of the windings, namely the voltage and the common terminal, are signed on the PCB itself near the connectors to which the device is connected.
Once the conclusions have been determined, you can proceed directly to testing the transformer. The list of malfunctions that may occur in the device is limited to four points:
- core damage;
- burnt out contact;
- insulation breakdown leading to an interturn or frame short circuit;
- wire break.
The check sequence is reduced to an initial external inspection of the transformer. It is carefully checked for blackening, chips, and odor. If no obvious damage is detected, then proceed to measurement with a multimeter.
How to check a pulse transformer for interturn short circuit and open circuit
To check the integrity of the windings, it is best to use a digital tester, but you can also examine them using a pointer tester.
In the first case, the diode testing mode is used, indicated on the multimeter by the diode designation symbol in the diagram.
- To determine a break, test leads are connected to the digital device.
- One is inserted into the connectors marked V/Ω, and the second is inserted into COM.
- The roller switch is moved to the dialing area.
- The measuring probes are sequentially touched to each winding, red to one of its terminals, and black to the other. If it is intact, the multimeter will beep.
An analog tester performs the test in resistance measurement mode. To do this, the tester selects the smallest resistance measurement range. This can be implemented through 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, interturn and short circuits are checked.
A short circuit may occur due to an insulation breakdown. As a result, the winding resistance will decrease, which will lead to redistribution of the magnetic flux in the device.
To carry out testing, the multimeter switches to resistance testing mode.
By touching the windings with probes, they look at the result on a digital display or on a scale (arrow deflection).
This result should not be less than 10 ohms.
To make sure that there is no short circuit on the magnetic circuit, touch the “hardware” of the transformer with one probe, and touch the second one sequentially to each winding. There should be no deviation of the arrow or appearance of a sound signal. It is worth noting that the interturn short circuit can only be measured with a tester in an approximate form, since the error of the device is quite high.
Video: How to check a pulse transformer?
