The Complace service center repairs switching power supplies in a wide variety of devices.
Switching power supply circuit
Switching power supplies are used in 90% of electronic devices. But to do this you need to know the basic principles of circuit design. Therefore, we present a diagram of a typical switching power supply.
Switching power supply operation
Primary circuit of a switching power supply
The primary circuit of the power supply circuit is located before the pulse ferrite transformer.
There is a fuse at the input of the unit.
Next comes the CLC filter. The coil, by the way, is used to suppress common-mode interference. Following the filter is a rectifier based on a diode bridge and an electrolytic capacitor. To protect against short high-voltage pulses, a varistor is installed after the fuse in parallel with the input capacitor. The resistance of the varistor drops sharply at increased voltage. Therefore, all excess current goes through it to the fuse, which burns, turning off the input circuit.
The protective diode D0 is needed in order to protect the power supply circuit if the diode bridge burns out. The diode will not allow negative voltage to pass into the main circuit. Because the fuse will open and burn.
Behind the diode there is a 4-5 ohm varistor to smooth out sudden jumps in current consumption at the moment of switching on. And also for the initial charging of capacitor C1.
The active elements of the primary circuit are as follows. Switching transistor Q1 and PWM (pulse width modulator) controller. The transistor converts the rectified DC voltage of 310V into alternating voltage. It is converted by transformer T1 on the secondary winding into a reduced output.
And one more thing - to power the PWM regulator, the rectified voltage taken from the additional winding of the transformer is used.
Operation of the secondary circuit of the switching power supply
In the output circuit after the transformer there is either a diode bridge or 1 diode and a CLC filter. It consists of electrolytic capacitors and a choke.
Optical feedback is used to stabilize the output voltage. It allows you to galvanically decouple the output and input voltages. Optocoupler OC1 and integrated stabilizer TL431 are used as feedback actuators. If the output voltage after rectification exceeds the voltage of the TL431 stabilizer, the photodiode is turned on. It includes a phototransistor that controls the PWM driver. The TL431 regulator reduces the duty cycle of the pulses or stops altogether. Until the voltage drops to the threshold.
Repair of switching power supplies
Malfunctions of switching power supplies, repair
Based on the circuit diagram of the switching power supply, we will move on to its repair. Possible malfunctions:
- If the varistor and the fuse at the input or VCR1 have burned out, then we look further. Because they just don't burn.
- The diode bridge burned out. Usually this is a microcircuit. If there is a protective diode, then it usually lights up. They need to be replaced.
- The 400V capacitor C1 is damaged. Rarely, but it happens. Often its malfunction can be identified by its appearance. But not always. Sometimes a seemingly good capacitor turns out to be bad. For example, by internal resistance.
- If the switching transistor burns out, then unsolder it and check it. If faulty, replacement is required.
- If the PWM regulator burns out, then replace it.
- Short circuit, as well as breakage of the transformer windings. The chances of repair are minimal.
- An optocoupler malfunction is an extremely rare case.
- Malfunction of the TL431 stabilizer. For diagnostics, we measure the resistance.
- If there is a short circuit in the capacitors at the output of the power supply, then we unsolder it and diagnose it with a tester.
Examples of repair of switching power supplies
For example, consider the repair of a switching power supply for several voltages.
The malfunction was the absence of output voltage at the output of the block.
For example, in one power supply two capacitors 1 and 2 in the primary circuit were faulty. But they weren't swollen.
On the second one the PWM controller did not work.
All the capacitors in the picture appear to be working, but the internal resistance turned out to be high. Moreover, the internal ESR resistance of capacitor 2 in the circle was several times higher than the nominal one. This capacitor is in the PWM regulator circuit, so the regulator did not work. The functionality of the power supply was restored only after replacing this capacitor. Because PWM worked.
Repair of computer power supplies
An example of repairing a computer power supply. An expensive 800 W power supply came in for repair. When it was turned on, the circuit breaker was knocked out.
It turned out that the short circuit was caused by a burnt-out transistor in the primary power circuit. The repair price was 3,000 rubles.
It makes sense to repair only high-quality, expensive computer power supplies. Because repairing a power supply may be more expensive than a new one.
Prices for repair of pulsed power supplies
Prices for repairing switching power supplies vary greatly. The fact is that there are a lot of electrical circuits for switching power supplies. There are especially many differences in circuits with PFC (Power Factor Correction). ZAS increases efficiency.
But the most important thing is whether there is a circuit diagram for the burnt-out power supply. If so electrical diagram is available, repairing the power supply is greatly simplified.
The repair price ranges from 1000 rubles for simple power supplies. But it reaches 10,000 rubles for complex expensive power supplies. The price is determined by the complexity of the power supply. And also how many elements burned in it. If all new power supplies are the same, then all faults are different.
For example, in one complex power supply, 10 elements and 3 tracks burned out. Nevertheless, it was possible to restore it, and the cost of repair was 8,000 rubles. By the way, the device itself costs about 1,000,000 rubles. Such power supplies are not sold in Russia.
Chinese device chargers for laptops described .
Industrial power supplies often fail, sometimes even high-quality and expensive ones. In this case, an ordinary person most often throws it away and buys a new one, but the cause of the breakdown may be insignificant, and for a radio amateur such devices are of considerable interest in terms of study and the possibility of returning them to functionality. Despite the fact that devices that cost a lot of money are often thrown away.
We invite users to consider a simple repair of a stabilized switching-type power supply based on a flyback generator with current and voltage feedback, which, in addition to stabilization, also allows for overload protection. The unit is powered by an alternating current network with a voltage of 100 to 240 Volts, frequency 50/60 Hertz, and produces a constant voltage of 12 Volts 2 Amperes.
The malfunction described here is quite common in power supplies of this type and has the following symptoms: the output voltage periodically appears and disappears with a certain frequency, which is visually observed as flashing and extinguishing of the output power indicator LED:
If the indicator LED is not installed, then a similar symptom can be detected with a dial voltmeter by connecting it to the output of the power supply. In this case, the voltmeter needle will periodically deviate to a certain value and return back (maybe not all the way). This phenomenon is observed due to the device’s protection triggering when the voltage or current at certain points exceeds the permissible limit.
This can happen both during a short circuit and when the circuit breaks. A short circuit most often occurs during the breakdown of capacitors or semiconductor radioelements, such as diodes or transistors. A break can occur in both semiconductors and resistors. In any case, first of all, you should visually inspect the printed circuit board and the radio elements installed on it.
Diagnosis of the power supply before repair
It is best to carry out visual diagnostics using a magnifying glass:
A burnt resistor with position number R18 was found on the board, and when tested, it was revealed that it was broken and the contact was broken:
Repairing the power supply step by step with photos
The resistor could burn out if its rated power dissipation was exceeded for a long time. The burnt resistor was soldered out, and its seat was cleaned:
To replace a resistor you need to find out its value. To do this, a known-good power supply was disassembled. The indicated resistor turned out to have a resistance of 1 ohm:
Further along the circuit of this resistor, a broken capacitor with position number C6 was discovered, the continuity of which showed its low resistance, and therefore unsuitability for further use:
It was precisely the breakdown of this capacitor that could cause the resistor to burn out and further inoperability of the entire device as a whole. This capacitor has also been removed from its place, you can compare how small it is:
The broken capacitor is comparable to the head of a match; such a small part caused the breakdown of the power supply. Next to it on the board, parallel to it, there is a second similar capacitor, which has survived. Unfortunately, there was no replacement capacitor and all hopes rested on the remaining second capacitor. But in place of the burnt resistor, a resistor with the required resistance of 1 Ohm was selected, but not surface mounted:
This resistor was installed on the seat of the burnt one, the soldering areas were cleaned of flux residues, and the seat of the broken capacitor was varnished for better insulation and to eliminate the possibility of air breakdown of this place:
After a test run, the power supply started working in normal mode and the indicator LED stopped blinking:
Subsequently, the installed resistor was replaced with a surface-mount resistor and a second layer of varnish was applied in place of the removed capacitor:
Of course, it would be ideal to install a second capacitor, but even without it, the power supply works fine, without extraneous noise and flickering of the LED:
After connecting the adapter to the network, the output voltage was measured, it turned out to be within the normal range, namely 11.9 Volts:
At this point, the repair of the device can be considered complete, since its functionality has been restored and it can continue to be used for its intended purpose. It is worth noting that the block is made according to a very good design, which, unfortunately, it was not possible to sketch.
At the moment, a quick external inspection reveals a good mains and output filter, well-thought-out control circuitry power transistor and good output voltage stabilization. The physical design of the device is also high level, the installation is tight and smooth, the soldering is clean, precision radio elements are used. All this allows you to get the device High Quality with precisely specified parameters and characteristics.
- Read more about
Semiconductors and capacitors in high-voltage circuits are especially dangerous, which in the event of a breakdown can lead to irreversible consequences for the entire device if it is turned on repeatedly without detection full list damaged components. With correct and careful diagnostics, in most cases everything ends well and the breakdown can be eliminated by replacing the damaged parts with the same serviceable ones or those that are similar in nominal value and parameters.
Video instructions for repairing a switching power supply:
Switching power supplies are the most unreliable component in modern radio devices. This is understandable - huge currents, high voltages. All power consumed by the device passes through the UPS. At the same time, let’s not forget that the amount of power supplied by the UPS to the load can change tens of times, which cannot have a beneficial effect on its operation.
Most manufacturers use simple switching power supply circuits, and this is understandable. The presence of several levels of protection often only complicates repairs and has virtually no effect on reliability, since the increase in reliability due to an additional protection loop is compensated by the unreliability of additional elements, and during repairs it takes a long time to figure out what these parts are and why they are needed.
Of course, everyone pulse block power supply has its own characteristics, differing in the power supplied to the load, the stability of the output voltages, the range of operating network voltages and other parameters that play a role during repairs only when it is necessary to choose a replacement for a missing part.
It is clear that when making repairs it is advisable to have a diagram. Well, if it’s not there, simple TVs can be repaired without it. The operating principle of all switching power supplies is almost the same, the only difference is in the circuit designs and types of parts used.
- How to fix ?
So, step-by-step instruction repair of a switching power supply:
- We turn on the TV, make sure that it is not working, that the standby indicator is not lit. If it lights up, then the problem is most likely not in the power supply. Just in case, you will need to check the horizontal scan supply voltage.
- Turn off the TV and disassemble it.
- We carry out an external inspection of the TV board, especially the area where the power supply is located. Sometimes swollen capacitors, burnt resistors, and more can be found. We'll have to check them out in the future.
- We carefully look at the soldering, especially the transformer, key transistor/microcircuit, and chokes.
- We check the power circuit: we call the power cord, fuse, power switch (if there is one), chokes in the power circuit, rectifier bridge. Often, with a faulty UPS, the fuse does not blow - it simply does not have time. If the key transistor breaks through, the ballast resistance is more likely to burn out than the fuse. It happens that the fuse burns due to a malfunction of the posistor, which controls the demagnetizing device (demagnetization loop). Be sure to check for a short circuit the terminals of the mains power filter capacitor without desoldering it, since in this way you can often check for breakdown the collector-emitter terminals of a key transistor or microcircuit if a power switch is built into it. Sometimes power is supplied to the circuit from the filter capacitor through ballast resistors, and if they break, it is necessary to check for breakdown directly at the switch electrodes.
- We check the remaining parts of the block - diodes, transistors, some resistors. First, we check without desoldering the part; desoldering it only when there is a suspicion that the part may be faulty. In most cases, such a check is sufficient. Ballast resistors often break. Ballast resistances have a small value (tenths of an Ohm, units of Ohm) and are designed to limit pulse currents, as well as for protection as fuses.
- We look to see if there are any short circuits in the secondary power circuits - to do this, we check the terminals of the capacitors of the corresponding filters at the outputs of the rectifiers for a short circuit.
Turn it on. At this stage there are three options:
- The light flashed brightly, then went out, and a raster appeared. Or the standby mode indicator lights up. In both cases, you need to measure the voltage supplying the horizontal scan - it varies for different TVs, but not more than 125 Volts. Often its value is written on the printed circuit board, sometimes near the rectifier, sometimes near the TDKS. If it is raised to 150–160 Volts, and the TV is in standby mode, then switch it to operating mode. Some TVs allow overvoltage at idle (when horizontal scanning does not work). If the voltage is too high in operating mode, check the electrolytic capacitors in the power supply only by replacing them with a known good one. The fact is that often electrolytic capacitors in a UPS lose their frequency properties and cease to perform their functions at the generation frequency, despite the fact that when tested by a tester using the charge-discharge method, the capacitor seems to be in good order. The optocoupler (if present) or the optocoupler control circuit may also be faulty. Check whether the output voltage is regulated by internal regulation (if any). If it is not adjustable, then you need to continue searching for faulty parts.
- The light flashed brightly and went out. Neither the raster nor the standby mode indication appeared. This indicates that the switching power supply does not start. It is necessary to measure the voltage on the surge protector capacitor; it should be 280–300 Volts. If it is not there, sometimes they put a ballast resistor between the mains rectifier bridge and the capacitor. Check the power supply and rectifier circuits again. If the voltage is too low, one of the diodes of the mains rectifier bridge may be broken or, more often, the mains power filter capacitor has lost capacity. If the voltage is normal, then you need to once again check the rectifiers of the secondary power supplies, as well as the starting circuit. The triggering circuit for simple TVs consists of several resistors connected in series. When testing a circuit, you need to measure the voltage drop across each of them, measuring the voltage directly at the terminals of each resistor.
- The light is on at full brightness. Turn off the TV immediately. Recheck all items. And remember - there are no miracles in radio engineering, it means you missed something somewhere, you didn’t check everything.
- Step by step
If you have repaired a UPS, then you have probably encountered this situation: all faulty elements have been replaced, the remaining ones seem to have been checked, but you turn on the TV and... bam... and everything has to start all over again! There are no miracles in radio engineering, and if something doesn’t work, there’s a reason for it! Our task is to find her!
The UPS is the most unreliable component in modern radio devices. This is understandable - huge currents, high voltages - after all, all the power consumed by the device passes through the UPS. At the same time, let’s not forget that the amount of power supplied by the UPS to the load can change tens of times, which cannot have a beneficial effect on its operation.
Most manufacturers use simple UPS designs. This is understandable. The presence of several levels of protection can often only complicate repairs and have virtually no effect on reliability, since the increase in reliability due to an additional protection loop is compensated by the unreliability of additional elements, and during repairs we have to spend a long time figuring out what these parts are and why they are needed. Of course, each UPS has its own characteristics, differing in the power supplied to the load, the stability of the output voltages, the range of operating mains voltages and other characteristics that play a role during repairs only when you need to choose a replacement for a missing part.
It is clear that when making repairs it is advisable to have a diagram. Well, if it’s not there, simple TVs can be repaired without it. The operating principle of all UPSs is almost the same, the only difference is in the circuit designs and types of parts used.
I use a technique developed over many years of repair experience. More precisely, this is not a technique, but a set of mandatory actions for repairs, proven by practice.
The proposed method assumes that you are at least somewhat familiar with how a TV works. For repairs, you need a tester (avometer) and, preferably, but not necessarily, an oscilloscope.
So, let's repair the power supply.
They brought you a TV or yours got damaged.
Turn on the TV, make sure that it is not working, that the standby indicator is not on. If it is on, then the problem is most likely not with the UPS. Just in case, you will need to check the horizontal scan supply voltage.
Turn off the TV and take it apart.
External inspection of the TV board, especially the area where the UPS is located. Sometimes swollen capacitors, burnt resistors, etc. can be found.
We'll have to check them out in the future.
Carefully examine the soldering, especially the transformer, key transistor/chip, and chokes.
Check the power circuit: check the power cord, fuse, power switch - if there is one, chokes in the power circuit, rectifier bridge.
Often, with a faulty UPS, the fuse does not blow - it simply does not have time. If the key transistor breaks through, the ballast resistance is more likely to burn out than the fuse. It happens that the fuse burns due to a malfunction of the posistor, which controls the demagnetizing device (demagnetization loop). Be sure to check for a short circuit the terminals of the mains power filter capacitor without desoldering it, since in this way you can often check for breakdown the collector-emitter terminals of a key transistor or microcircuit if a power switch is built into it. Sometimes power is supplied to the circuit from the filter capacitor through ballast resistors, and if they break, it is necessary to check for breakdown directly at the switch electrodes.
Take a moment to check the remaining parts of the unit - diodes, transistors, some resistors. First, we check without desoldering the part; desoldering it only when there is a suspicion that the part may be faulty. In most cases, such a check is sufficient. Ballast resistors often break. Ballast resistances have a small value (tenths of an Ohm, units of Ohm) and are designed to limit pulse currents, as well as for protection as fuses.
We need to see if there are any short circuits in the secondary power circuits - to do this, we check the terminals of the capacitors of the corresponding filters at the outputs of the rectifiers for short circuits.
After completing all checks and replacing faulty parts, you can perform a live test. To do this, instead of the mains fuse, we connect a 150-200 Watt 220 Volt light bulb. This is necessary so that the light bulb protects the UPS in case the malfunction is not resolved. Disconnect the degaussing device.
Turn it on. There are three options:
- The light flashed brightly, then went out, and a raster appeared. Or the standby mode indicator lights up. In both cases, you need to measure the voltage supplying the horizontal scan - it varies for different TVs, but not more than 125 Volts. Often its value is written on the printed circuit board, sometimes near the rectifier, sometimes near the TDKS. If it is too high to 150-160 Volts, and the TV is in standby mode, then switch it to operating mode; some TVs allow the voltage to be too high at idle (when the horizontal scan does not work). If the voltage is too high in operating mode, check the electrolytic capacitors in the power supply only by replacing them with a known good one. The fact is that often electrolytic capacitors in a UPS lose their frequency properties and cease to perform their functions at the generation frequency, despite the fact that when tested by a tester using the charge-discharge method, the capacitor seems to be in good order. The optocoupler (if there is one) or the optocoupler control circuit may also be faulty. Check whether the output voltage is regulated by internal regulation (if any). If it is not adjustable, then you need to continue searching for faulty parts.
- The light flashed brightly and went out. Neither the raster nor the standby mode indication appeared. This indicates that the UPS does not start. It is necessary to measure the voltage on the surge protector capacitor, it should be 280-300 Volts. If it is not there, sometimes they put a ballast resistor between the mains rectifier bridge and the capacitor. Check the power supply and rectifier circuits again. If the voltage is too low, one of the diodes of the mains rectifier bridge may be broken or, what is more common, the mains power filter capacitor may have lost capacity. If the voltage is normal, then you need to once again check the rectifiers of the secondary power supplies, as well as the starting circuit. The triggering circuit for simple TVs consists of several resistors connected in series. When testing a circuit, you need to measure the voltage drop across each of them, measuring the voltage directly at the terminals of each resistor.
- The light is on at full brightness. Turn off the TV immediately. Recheck all items. And remember - there are no miracles in radio engineering, it means you missed something somewhere, you didn’t check everything.
95% of the malfunctions fit into this diagram, but there are more complex malfunctions when you have to rack your brains. For such cases, you cannot write methods and you cannot create instructions.
They have always been important elements of any electronic devices. These devices are used in amplifiers and receivers. The main function of power supplies is considered to be to reduce the maximum voltage that comes from the network. The first models appeared only after the AC coil was invented.
Additionally, the development of power supplies was influenced by the introduction of transformers into the device circuit. The peculiarity of pulse models is that they use rectifiers. Thus, voltage stabilization in the network is carried out in a slightly different way than in conventional devices where a converter is used.
Power supply device
If we consider a conventional power supply, which is used in radio receivers, then it consists of a frequency transformer, a transistor, and several diodes. Additionally, the circuit contains a choke. Capacitors are installed with different capacities and their parameters can vary greatly. Rectifiers are usually used of the capacitor type. They belong to the high-voltage category.
Operation of modern blocks
Initially, the voltage is supplied to the bridge rectifier. At this stage, the peak current limiter is activated. This is necessary so that the fuse in the power supply does not burn out. Next, the current passes through the circuit through special filters, where it is converted. Several capacitors are needed to charge the resistors. The unit starts up only after a breakdown of the dinistor. Then the transistor is unlocked in the power supply. This makes it possible to significantly reduce self-oscillations.
When voltage generation occurs, the diodes in the circuit are activated. They are connected to each other using cathodes. A negative potential in the system makes it possible to lock the dinistor. The rectifier start-up is facilitated after the transistor is turned off. In addition, two fuses are provided to prevent saturation of the transistors. They operate in the circuit only after a breakdown. To start feedback, a transformer is required. It is fed by pulsed diodes in the power supply. At the exit alternating current passes through capacitors.
Features of laboratory blocks
The operating principle of switching power supplies of this type is based on active current conversion. Bridge rectifier in standard scheme one is provided. In order to remove all interference, filters are used at the beginning and also at the end of the circuit. The pulsed laboratory power supply has conventional capacitors. Saturation of transistors occurs gradually, and this has a positive effect on diodes. Voltage adjustment is provided in many models. The protection system is designed to save blocks from short circuits. Cables for them are usually used in a non-modular series. In this case, the power of the model can reach up to 500 W.
The power supply connectors in the system are most often installed as ATX 20 type. To cool the unit, a fan is mounted in the case. The speed of rotation of the blades must be adjusted in this case. A laboratory-type unit should be able to withstand the maximum load at 23 A. At the same time, the resistance parameter is maintained on average at 3 ohms. The maximum frequency that a switching laboratory power supply has is 5 Hz.
How to repair devices?
Most often, power supplies suffer due to blown fuses. They are located next to the capacitors. Repair of switching power supplies should begin by removing the protective cover. Next, it is important to inspect the integrity of the microcircuit. If no defects are visible on it, it can be checked using a tester. To remove fuses, you must first disconnect the capacitors. After this they can be removed without any problems.
To check the integrity of this device, inspect its base. Burnt fuses have a dark spot at the bottom, which indicates damage to the module. To replace this element, you need to pay attention to its markings. Then you can purchase a similar product in a radio electronics store. Installation of the fuse is carried out only after fixing the condensates. Another common problem in power supplies is considered to be faults with transformers. They are boxes in which coils are installed.
When very high voltage is applied to the device, they cannot withstand it. As a result, the integrity of the winding is compromised. It is impossible to repair switching power supplies with such a breakdown. In this case, the transformer, like the fuse, can only be replaced.
Network power supplies
The operating principle of network-type switching power supplies is based on a low-frequency reduction in the amplitude of interference. This happens thanks to the use of high-voltage diodes. Thus, it is more effective to control the limiting frequency. Additionally, it should be noted that transistors are used at medium power. The load on the fuses is minimal.
Resistors are used quite rarely in a standard circuit. This is largely due to the fact that the capacitor is capable of participating in current conversion. The main problem with this type of power supply is the electromagnetic field. If capacitors are used with low capacitance, then the transformer is at risk. In this case, you should be very careful about the power of the device. The network switching power supply has limiters for peak current, and they are located immediately above the rectifiers. Their main task is to control the operating frequency to stabilize the amplitude.
Diodes in this system partially serve as fuses. Only transistors are used to drive the rectifier. The locking process, in turn, is necessary to activate the filters. Capacitors can also be used as isolation type in the system. In this case, the transformer will start up much faster.
Application of microcircuits
A wide variety of microcircuits are used in power supplies. In this situation, much depends on the number of active elements. If more than two diodes are used, the board must be designed for input and output filters. Transformers are also produced in different capacities, and their dimensions are quite different.
You can solder microcircuits yourself. In this case, you need to calculate the maximum resistance of the resistors taking into account the power of the device. To create an adjustable model, special blocks are used. This type of system is made with double tracks. Ripple inside the board will occur much faster.
Benefits of Regulated Power Supplies
The principle of operation of switching power supplies with regulators is the use of a special controller. This element in the circuit can change throughput transistors. Thus, the limiting frequency at the input and output is significantly different. The switching power supply can be configured in different ways. Voltage adjustment is carried out taking into account the type of transformer. Conventional coolers are used to cool the device. The problem with these devices is usually excess current. In order to solve this, protective filters are used.
The power of devices on average fluctuates around 300 W. Only non-modular cables are used in the system. In this way, short circuits can be avoided. Power supply connectors for connecting devices are usually installed in the ATX 14 series. The standard model has two outputs. Rectifiers are used with higher voltage. They can withstand resistance at 3 ohms. In turn, the maximum load of the switching regulated power supply is up to 12 A.
Operation of 12 volt units
Pulse includes two diodes. In this case, filters are installed with a small capacity. In this case, the pulsation process occurs extremely slowly. The average frequency fluctuates around 2 Hz. The efficiency of many models does not exceed 78%. These blocks are also distinguished by their compactness. This is due to the fact that transformers are installed with low power. They do not require refrigeration.
The 12V switching power supply circuit additionally involves the use of resistors marked P23. They can withstand only 2 ohms of resistance, but this is enough power for a device. A 12V switching power supply is used most often for lamps.
How does the TV box work?
The operating principle of switching power supplies of this type is the use of film filters. These devices are able to cope with interference of various amplitudes. Their choke winding is synthetic. Thus, high-quality protection of important components is ensured. All gaskets in the power supply are insulated on all sides.
The transformer, in turn, has a separate cooler for cooling. For ease of use, it is usually set to silent. These devices can withstand maximum temperatures of up to 60 degrees. The operating frequency of the TV switching power supply is maintained at 33 Hz. At sub-zero temperatures These devices can also be used, but much in this situation depends on the type of condensates used and the cross-section of the magnetic circuit.
Models of 24 volt devices
In 24-volt models, low-frequency rectifiers are used. Only two diodes can successfully cope with interference. The efficiency of such devices can reach up to 60%. Regulators are rarely installed on power supplies. The operating frequency of the models does not exceed 23 Hz on average. Resistors can only withstand 2 ohms. Transistors in models are installed with the marking PR2.
To stabilize the voltage, resistors are not used in the circuit. The 24V switching power supply filters are of the capacitor type. In some cases, dividing species can be found. They are necessary to limit the maximum frequency of the current. To quickly start a rectifier, dinistors are used quite rarely. The negative potential of the device is removed using the cathode. At the output, the current is stabilized by blocking the rectifier.
Power sides on diagram DA1
Power supplies of this type differ from other devices in that they can withstand heavy loads. There is only one capacitor in the standard circuit. For normal operation of the power supply, the regulator is used. The controller is installed directly next to the resistor. No more than three diodes can be found in the circuit.
The direct reverse conversion process begins in the dinistor. To start the unlocking mechanism, a special throttle is provided in the system. Waves with large amplitude are damped by the capacitor. It is usually installed of the dividing type. Fuses are rarely found in a standard circuit. This is justified by the fact that the maximum temperature in the transformer does not exceed 50 degrees. Thus, the ballast choke copes with its tasks independently.
Models of devices with DA2 chips
Switching power supply microcircuits of this type are distinguished from other devices by their increased resistance. They are used mainly for measuring instruments. An example is an oscilloscope that shows fluctuations. Voltage stabilization is very important for him. As a result, the device's readings will be more accurate.
Many models are not equipped with regulators. Filters are mainly double-sided. At the output of the circuit, transistors are installed as usual. All this makes it possible to withstand a maximum load of 30 A. In turn, the maximum frequency indicator is at around 23 Hz.
Blocks with installed DA3 chips
This microcircuit allows you to install not only a regulator, but also a controller that monitors fluctuations in the network. The resistance of the transistors in the device can withstand approximately 3 ohms. The powerful switching power supply DA3 can handle a load of 4 A. You can connect fans to cool the rectifiers. As a result, the devices can be used at any temperature. Another advantage is the presence of three filters.
Two of them are installed at the input under the capacitors. One separating type filter is available at the output and stabilizes the voltage that comes from the resistor. There are no more than two diodes in a standard circuit. However, a lot depends on the manufacturer, and this should be taken into account. The main problem with power supplies of this type is that they are not able to cope with low-frequency interference. As a result, it is impractical to install them on measuring instruments.
How does the VD1 diode block work?
These blocks are designed to support up to three devices. They have three-way regulators. Communication cables are installed only non-modular ones. Thus, current conversion occurs quickly. Rectifiers in many models are installed in the KKT2 series.
They differ in that they can transfer energy from the capacitor to the winding. As a result, the load from the filters is partially removed. The performance of such devices is quite high. At temperatures above 50 degrees they can also be used.
Bravis LED-16E96B TV after a voltage drop.
The power supply is assembled on a PWM controller SW2658a.
The microcircuit is rare, but strangely enough there is a datasheet available. And nothing more.
SW2658-type-diagram. PSU adapter for Chinese TV.
The power adapter, as expected, died with special effects.
The TV itself was not damaged, I checked it using a working power supply.
The adapter is opened using a non-sharp screwdriver and a hammer. Light blows on the seam. Then, using a wide screwdriver, it is peeled further.
Visually, one of the network capacitors 15 uF x 400 volts was swollen.
Naturally the fuse is broken. It made a good noise, and in some places the board had to be washed with alcohol.
At first I didn’t even realize why the board was so smoked. Later, under the silicone, a torn inductor L1 wound on a ferrite core rang. I wired it with the same wire.
I had to throw out only 15 centimeters of wire. It was wound turn to turn. I didn’t wind it so neatly, the first layers were even, and then how it turned out. This did not affect performance in any way.
Choke in power supply with burnt wire
I had to remember the old technology)
I took my favorite PWM controller, which used to be installed in DVDs, receivers, adapters... 5H0165R.
Transformer ( 1
The electronic circuit serves to rectify the alternating voltage (converting it into direct voltage) and stabilize the output voltage at 12 V.
On the circuit diagram T1 – transformer. Typical transformer malfunctions are burnout or breakage of the wire of the primary winding, less often the secondary winding. As a rule, the primary, network winding is faulty ( 1 ).
The cause of a break or burnout is a thin wire that cannot withstand network surges and overloads. Let's say thank you to the Chinese, they are frugal guys, they don't want to run thicker wires...
The resistance of the primary winding should be several kilo-ohms (1 kOhm = 1000 Ohms), the secondary winding should be several tens of Ohms.
25.5 Ohm , which is also normal.
C1 (100uF 16V) 470 µF (25V)
It is not necessary to know the pinout (location and purpose) of the terminals of the 78L12 stabilizer; you must either remember, sketch or photograph the location of the faulty chip on the electronic board and solder the working part as soon as you find it. This simple operation will save your time if you soldered a faulty microcircuit, but a replacement was not found in time and you forgot how the microcircuit was soldered.
Network power adapters - miniature power supplies for various electronic household equipment are used to power antenna amplifiers, radiotelephones, chargers. Despite the active introduction of switching power supplies, transformer power supplies are still actively used and are used in the user’s everyday life.
It is not uncommon for these transformer units to fail or break.
If the adapter breaks, you can replace it with a new one, the cost is low. But why give your hard-earned money if in most cases you can fix the problem yourself within 15 to 30 minutes and save yourself from searching for a replacement and wasting money?
So, let’s look at the composition of a conventional low-power power supply and its repair.
An adapter for 12V and a current of 100mA with a power of 3.6 Watts from the antenna amplifier came to the repair table.
The photo shows the adapter after the repair.
What parts does a typical transformer adapter consist of?
If we disassemble the adapter, we will find a transformer inside ( 1 ) and a small electronic circuit ( 2 ).
Transformer ( 1 ) serves to reduce the 220V alternating mains voltage to the level of 13 - 15 V.
The electronic circuit serves to rectify the alternating voltage (converting it into direct voltage) and stabilize the output voltage at 12 V.
It's simple. What can break in such a simple device?
Let's take a look at the circuit diagram of this adapter.
On schematic diagram T1 – transformer. Typical transformer malfunctions are burnout or breakage of the wire of the primary winding, less often the secondary winding. As a rule, the primary, network winding is faulty ( 1 ).
The cause of a break or burnout is a thin wire that cannot withstand network surges and overloads. Let's say thank you to the Chinese, they are frugal guys, they don't want to run thicker wires...
Checking the health of the transformer is quite simple. It is necessary to measure the resistance of the primary and secondary windings. The resistance of the primary winding should be several kilo-ohms (1 kOhm = 1000 Ohms), the secondary winding should be several tens of Ohms.
When checking the adapter transformer for the primary winding, the resistance turned out to be 1.8 kOhm, which indicates that the primary winding is working properly.
For the secondary winding the resistance was 25.5 Ohm , which is also normal.
If there is no indication, you should measure the resistance of the primary winding of the transformer. This is easy to do; you don’t even have to disassemble the power supply, but measure the winding resistance through the contacts of the power plug.
We disassemble the power supply and perform an external inspection. Please pay attention to the darkened areas around the radio components, chips and cracks on the housings of the power stabilizer (78L12), and swelling of the filter capacitors.
During the repair of the antenna adapter, it turned out that the 78L12 stabilizer chip was faulty. The electrolytic capacitor was also replaced C1 (100uF 16V) to a capacitor with a larger capacity - 470 µF (25V) . When replacing a capacitor, the polarity of the capacitor must be taken into account.
It is not necessary to know the pinout (location and purpose) of the terminals of the 78L12 stabilizer; you must either remember, sketch or photograph the location of the faulty chip on the electronic board and solder the working part as soon as you find it. This simple operation will save your time if you soldered a faulty microcircuit, but a replacement was not found in time and you forgot how the microcircuit was soldered.
A switching power supply is built into most household appliances. As practice shows, this particular unit quite often fails, requiring replacement.
The high voltage constantly passing through the power supply does not have the best effect on its elements. And the point here is not the mistakes of the manufacturers. By increasing the service life by installing additional protection, you can achieve the reliability of the protected parts, but lose it on newly installed ones. In addition, additional elements complicate repairs - it becomes difficult to understand all the intricacies of the resulting circuit.
Manufacturers solved this problem radically by reducing the cost of the UPS and making it monolithic and non-separable. Such disposable devices are becoming more common. But, if you are lucky - the collapsible unit has failed, independent repair is quite possible.
The operating principle of all UPSs is the same. The differences relate only to the diagrams and types of parts. Therefore, it is quite simple to understand the breakdown, having basic knowledge of electrical engineering.
For repairs you will need a voltmeter.
It is used to measure the voltage across an electrolytic capacitor. It is highlighted in the photo. If the voltage is 300 V, the fuse is intact and all other elements associated with it (surge filter, power cable, input) are in good order.
There are models with two small capacitors. In this case, the normal functioning of the mentioned elements is indicated by a constant voltage of 150 V on each of the capacitors.
If there is no voltage, you need to ring the diodes of the rectifier bridge, the capacitor, the fuse itself, and so on. The tricky thing about fuses is that, once they fail, they look no different from working samples. A fault can only be detected through a continuity test - a blown fuse will show high resistance.
Having discovered a faulty fuse, you should carefully inspect the board, since it often fails simultaneously with other elements.
A damaged capacitor is easy to notice with the naked eye - it will be destroyed or swollen.
In this case, he does not need to be called, but simply disappears. The following elements are also soldered and ringed:
- power or rectifier bridge (looks like a monolithic block or can consist of four diodes);
- filter capacitor (looks like a large block or several blocks connected in parallel or in series), located in the high-voltage part of the block;
- transistors installed on the radiator (these are power switches).
Important. All parts are desoldered and replaced at the same time! Replacing one at a time will lead to burnout of the power unit each time.
Burnt out elements must be replaced with new ones. The radio market offers a wide range of parts for power supplies. It’s quite easy to find good options at minimal prices.
On a note. The fuse can be successfully replaced with a piece of copper wire. A wire thickness of 0.11 millimeters corresponds to a 3 Ampere fuse.
Causes of failure:- voltage fluctuations;
- lack of protection (there is space for it, but the element itself is not installed - this is how manufacturers save money).
Solution this malfunction of switching power supplies:
- install protection (it is not always possible to select the right part);
- or use a mains voltage filter with good protective elements (not jumpers!).
What to do if there is no output voltage?
Another common cause of power supply failure has nothing to do with the fuse. We are talking about the absence of output voltage when such an element is fully operational.
Solution:
- Swollen capacitor - requires desoldering and replacement.
- A failed inductor - it is necessary to remove the element and change the winding. The damaged wire is unwound. At the same time, the turns are counted. Then a new suitable wire is wound at the same number of turns. The part is returned to its place.
- Deformed bridge diodes are replaced with new ones.
- If necessary, parts are checked by a tester (if no damage is visually detected).
Before that, it is necessary to study the rules for the safe use of such a tool. You should not shine such a device into reflective surfaces, as this can damage your eyes.
It is quite possible to build it yourself. A fan is used as a blower, and a coil is used as a heater. The best option is a circuit with a thyristor.
Causes of failure:
- poor ventilation.
Solution:
- do not cover ventilation openings;
- ensure optimal temperature conditions - cooling and ventilation.
What you need to remember:
- The first connection of the unit is made to a 25 Watt lamp. This is especially important after replacing diodes or transistors! If an error is made somewhere or a malfunction is not noticed, the passing current will not damage the entire device as a whole.
- When starting work, do not forget that a residual discharge remains on electrolytic capacitors for a long time. Before soldering the parts, it is necessary to short-circuit the capacitor leads. You can't do this directly. It is necessary to short-circuit through a resistance rated higher than 0.5 V.
If the entire UPS has been thoroughly checked, but still does not work, you can contact a repair shop. Perhaps your case relates to a complex breakdown that is still fixable.
According to statistics, about 5% of breakdowns require unit replacement. Fortunately, this device is always available. In stores you can find a rich assortment in different price categories.