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 output, 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. There is one bridge rectifier in the standard circuit. 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 the throughput of 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 subzero 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.
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, a well-thought-out circuitry for controlling the power transistor and good stabilization of the output voltage. The physical design of the device is also at a high level, the installation is rigid and smooth, the soldering is clean, and precision radio elements are used. All this allows you to obtain a high-quality device 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 identifying a complete list of 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, each switching power supply 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 parameters 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 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 instructions for repairing 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
In the modern world, the development and obsolescence of personal computer components occurs very quickly. At the same time, one of the main components of a PC - the ATX form factor - is practically has not changed its design for the last 15 years.
Consequently, the power supply of both an ultra-modern gaming computer and an old office PC work on the same principle and have common methods for diagnosing faults.
The material presented in this article can be applied to any personal computer power supply with a minimum of nuances.
A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on a TL494 PWM controller, triggered by a PS-ON (Power Switch On) signal from the motherboard. The rest of the time, until the PS-ON pin is pulled to ground, only the Standby Supply with a voltage of +5 V at the output is active.
Let's take a closer look at the structure of the ATX power supply. Its first element is
:
Its task is to convert alternating current from the mains to direct current to power the PWM controller and standby power supply. Structurally, it consists of the following elements:
- Fuse F1 protects the wiring and the power supply itself from overload in the event of a power supply failure, leading to a sharp increase in current consumption and, as a consequence, to a critical increase in temperature that can lead to a fire.
- A protective thermistor is installed in the neutral circuit, which reduces the current surge when the power supply is connected to the network.
- Next, a noise filter is installed, consisting of several chokes ( L1, L2), capacitors ( C1, C2, C3, C4) and counter-wound choke Tr1. The need for such a filter is due to the significant level of interference that the pulse unit transmits to the power supply network - this interference is not only picked up by television and radio receivers, but in some cases can lead to the malfunction of sensitive equipment.
- A diode bridge is installed behind the filter, converting alternating current into pulsating direct current. Ripple is smoothed out by a capacitive-inductive filter.
Standby power supply is a low-power independent pulse converter based on the T11 transistor, which generates pulses through an isolation transformer and a half-wave rectifier on the D24 diode, powering a low-power integrated voltage stabilizer on the 7805 chip. Although this circuit is, as they say, time-tested, its significant drawback is high voltage drop across the 7805 stabilizer, which leads to overheating under heavy load. For this reason, damage in the circuits powered from the standby source can lead to its failure and subsequent inability to turn on the computer.
The basis of the pulse converter is PWM controller. This abbreviation has already been mentioned several times, but has not been deciphered. PWM is pulse width modulation, that is, changing the duration of voltage pulses at their constant amplitude and frequency. The task of the PWM unit, based on a specialized TL494 microcircuit or its functional analogues, is to convert DC voltage into pulses of the appropriate frequency, which, after an isolation transformer, are smoothed by output filters. Voltage stabilization at the output of the pulse converter is carried out by adjusting the duration of the pulses generated by the PWM controller.
Any modern TV has a switching power supply.
A power supply is a whole unit designed to provide the TV with supply voltages of a certain power necessary for the normal functioning of the electrical appliance.
When the pulse unit is faulty, all sorts of problems with the television receiver are observed, including the fact that it does not work at all or stops turning on.
Possible power supply malfunctions
When VseRemont24 technicians come to a client’s home, they most often encounter a faulty power supply. This is the most common malfunction of televisions of all models, brands and types.
The power supply can be in the general circuit of the TV or in the form of a separate module.
Power supplies are unique in each TV, each has its own circuit. But their performance is equally negatively affected by:
- violation by the owner of operating rules (especially temperature conditions),
- relatively simple circuits,
- unprofessional equipment repair.
Malfunctions typical for most power supplies:
- Fuse blown.
- The power supply does not start, there is voltage on the rectifier, the key elements are working.
- The power supply does not start because the protection is triggered.
- The power (key) transistor burns out.
- Under- or over-voltage in primary or secondary circuits.
Obviously, only an experienced TV technician can figure out the breakdown and repair the TV. Self-repair is highly undesirable, however, it is possible.
Checking and repairing the power supply
If you have some experience, all the necessary knowledge and tools (in particular, a multimeter and a soldering iron), try to repair a television receiver.
Algorithm of actions when checking the TV power supply:
- Turn off the TV (unplug from the socket).
- Discharge the high voltage capacitor.
- Remove the board from the TV case.
- Inspect the board (visual diagnostics).
- Check resistors, capacitors, diodes, transistors, etc. with a multimeter.
- Inspect the back of the board. Check for cracks, gaps between tracks, and reliability of soldering of parts.
Resistors can:
- darken
- crack,
- The quality of pin soldering deteriorates.
If all this is noticeable visually, it makes sense to replace the resistors with new ones with a deviation from the original of no more than plus or minus 5%.
If nothing is noticeable externally, you should check the resistors with a multimeter. The resistor is faulty if resistance = 0 or?.
Faulty electrolytic capacitors appear swollen. Their capacity is also checked. Permissible deviations are plus or minus 5%.
A working silicon diode has a resistance in the forward direction of 3-6 kOhm, and in the reverse direction - ?.
To measure resistance, you need to unsolder the diode. To check, set the multimeter to resistance measurement mode with a limit of 20 kOhm.
The second option is to check with a multimeter without soldering the diode. In this case, the multimeter must be set to the voltage drop measurement mode (should be up to 0.7 V). If the multimeter shows 0 or near zero, the diode will still have to be unsoldered and checked again. If the readings do not change, a breakthrough has probably occurred. A part needs to be replaced.
Bipolar transistors are tested in both directions (forward and reverse) at the transitions:
- collector base,
- base-emitter.
The test involves measuring the voltage drop in the transistors. It is also important to check that there is no breakdown in the collector-emitter junction.
Serviceable transistors behave like diodes, faulty ones need to be rechecked completely - the entire “piping”:
- diodes,
- resistors,
- capacitors.
To check the supply voltage of a switching power supply, you will need:
- his diagram
- two incandescent lamps? 100W.
Algorithm of actions:
- Using the diagram, find the output to the horizontal scanning stage.
- Disable output.
- Connect the incandescent lamp.
- Connect the power supply through the second lamp.
If the lamp comes on and is bright, the power supply is faulty. If the light comes on and goes out or shines dimly, the input circuits of the power supply are working properly.
To determine which element is broken (which is why the light comes on), you need to refer to the diagram.
The test voltage measurement is carried out with a connected light bulb on the B+ load. The diagram shows what the voltage should be. Usually it is 110-150V. If it is correct, the power supply is working.
If the voltage is increased (200V), check the elements of the primary circuit of the power supply. If it is lowered - secondary circuits.
All faulty parts are soldered off and new ones are soldered in their place.
Remember! It is impossible to repair the TV power supply yourself without knowledge and experience. Even more important is that handicraft and amateur repairs are a direct threat to the health and even lives of people!
Malfunctions of modern switching power supplies
Often the causes of failures of pulsed voltage sources lie in poor-quality mains voltage. Lowering and increasing network voltage, power surges, and network shutdowns negatively affect the reliability of electronic components of power circuits.
Impulse power block
Such surges and network outages are especially painful for power diodes, powerful transistors, PWM controllers, and capacitors. It’s good when your voltage converter is made without filling with compound. You can do the repair of such switching power supplies yourself.
Increasingly, voltage sources filled with compound are appearing. They are not accepted for repair even in specialized workshops. For them, the only repair option is to replace it with a new one. Improper operation of these sources or connection of more powerful loads can also cause their failure.
There is no need to immediately send these converters for repair; the reasons for their failure can be quite simple, and you can easily deal with them. For more complex faults, some knowledge of electronics is required. Experience in repair comes with time, the more you do it, the more knowledge you will gain.
Diagnosis of faults in switching power supplies
The most important thing in repair is to find the fault, and fixing it is a matter of technique. The circuitry of switching power supplies can be divided into input and output parts. The input part includes a high-voltage circuit, and the output part has a low-voltage circuit.
In the high-voltage part of the board, all elements operate under high voltage, so they fail more often than elements in the low-voltage part. The high-voltage circuit has a network filter, diode bridges for rectifying the alternating voltage of the network, transistor switches and a pulse transformer.
Small isolating transformers are also used, which are controlled by PWM controllers and supply pulses to the gates of field-effect transistors. Thus, galvanic isolation of network and secondary voltages occurs. For such decoupling, optocouplers are often used in modern circuits.
Switching power supply circuit using transistors
The output voltages are also galvanically isolated from the network through a power transformer. In simple conversion circuits, instead of PWM controllers, self-generating transistors are used. These cheap voltage sources are used to power halogen lamps, LED lamps, etc.
The peculiarity of such schemes is simplicity and a minimum of elements. However, simple and cheap voltage sources do not start without a load, the output voltage is unstable and has increased ripple. Although these parameters do not affect the lighting of halogen lamps.
Diode bridge of ATX switching power supply
Repairing such a device is very simple due to the small number of elements. Most often, malfunctions occur in the high-voltage part of the circuit, when one or more diodes break down, electrolytic capacitors swell, and power transistors fail. The diodes of the low-voltage circuit also fail, the output filter inductors and the fuse blow out.
The malfunction of these elements can be detected with a multimeter. Other malfunctions of pulse units require the use of an oscilloscope or digital multimeter. In this case, it is better to send the unit to a workshop for repairs. The fuse can be easily tested with a multimeter for the presence of voltage after the fuse.
If a fuse has blown, you need to carefully visually check the entire circuit board circuit, tracks, soldering problems, darkening of circuit elements and sections of tracks, swelling of capacitors. If the diodes do not read well with a multimeter on the board, they are unsoldered and each one is checked separately.
All elements of the board are checked, the faulty one is replaced, and only then is the unit connected to the network for testing. During diagnostics, the capacitors are also unsoldered and checked with a tester. A burnt inductor can be rewound by determining the number of turns and wire cross-section. Finding the required throttle on sale will not be easy; it is better to restore it yourself.
Repair of UPS units of computers and televisions
To repair a pulse voltage source, you will need tools such as a soldering iron with temperature control, a set of screwdrivers, wire cutters, tweezers, a mounting knife, and a regular 100 W lamp. From the material you will need solder, flux, alcohol to remove rosin with a brush from the board solder. The equipment you will need is a multimeter.
Since switching power supplies (UPS) for televisions and computers have standard circuits, the method for detecting faults in them will be the same. A malfunction of the TV's voltage converter can be determined by the absence of LED backlighting.
The repair begins by checking the power cord, removing the power supply from the TV, and carefully inspecting the elements and traces of the board. They look for swollen capacitors, darkening of the tracks, cracked casing of alimony, charring of resistances, violation of the integrity of soldering joints, especially at the terminals of the pulse transformer.
If external damage is not found with a multimeter, check the fuse, diodes, power transistors of the keys, and the functionality of the capacitors. When you are sure that all elements are in good working order, but the device does not work, you need to change the pulse generator chip.
In a TV converter, the main faults occur in ballast resistors, low voltage electrolytic capacitors, and diodes. You can ring them without removing them from the boards (except for diodes). After troubleshooting, solder a 100 W lamp instead of the fuse and turn it on.
- The lamp lights up and goes out, and the sleep mode LED lights up. The TV screen lights up. Then check the horizontal voltage; if it is higher than normal, change the capacitors.
- The lamp lights up and goes out, but the LED does not light up, there is no raster. The reason is most likely in the pulse generator. Measure the voltage on the capacitor, which should be in the range of 280 - 300V. If the voltage is lower, the fault is sought in the diodes or in a capacitor leak. If there is no voltage on the capacitor, all circuits of high-voltage power supplies are checked again.
- The lamp burns brightly when some elements are faulty. The voltage source is checked again.
Using an incandescent lamp, you can find possible faults in the source. To repair a computer's ATX source, you need to assemble a load circuit as in the figure below or connect it to a computer. However, if the malfunction of the ATX unit is not corrected, you can burn the motherboard.
An external manifestation of an ATX unit failure may be when the motherboard does not turn on, the fans do not work, or the unit tries to turn on repeatedly. Before troubleshooting the device, you need to clean it from dust with a vacuum cleaner and a brush. A visual inspection of the elements and board tracks is also carried out, and only after that the load is turned on.
If the fuse blows, then connect a 100 W incandescent lamp, as when checking the voltage source on a TV. When the lamp lights up but does not go out, the fault is looked for in the capacitor, transformer and bridge diodes. If the fuse is intact, a malfunction could occur in the PWM controller, then it is necessary to replace the device. Also, repeated starting of the source indicates a malfunction of the reference voltage stabilizer.
Safety precautions when repairing a switching power supply
The high side of the device does not have galvanic isolation from the network, so you should not touch the elements of this part with both hands. If touched with one hand, you will receive a noticeable electric shock, but it is not fatal. You cannot check live elements with a screwdriver or tweezers.
The high-voltage circuit of the device is indicated by a wide stripe, and the inside by small strokes of paint. The device has a high-voltage capacitor, which, after turning off the unit, holds dangerous voltage for up to 3 minutes. Therefore, after turning off, you need to wait until the capacitors are discharged or discharge them through a 3 - 5 Kom resistor. You can increase safety when repairing a device by using a safety transformer.
This transformer has two 220 V windings with a power of up to 200 W (depending on the power of the UPS). Such a transformer is galvanically isolated from the network. The primary winding of the transformer is connected to the network, and the secondary winding with the lamp is connected to the UPS. In this case, you can touch the elements of the high part of the device with one hand, you will not receive an electric shock.
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