Repair of a switching power supply: 12 volt UPS malfunctions, their diagnosis and elimination

In modern consumer electronics, pulse-type power supplies (UPS) are actively used. They are necessary to rectify and lower the input voltage to a predetermined value. Despite the relatively high reliability, the UPS can fail. If the user has certain knowledge in the field of electronics, then he will be able to repair the 12 volt switching power supply on his own.

UPS operating principle

Most power supplies are based on typical circuits and have similar faults. If a person has at least basic knowledge in the field of electronics, then he can try to restore the UPS with his own hands. Since some parts of the power supply are live, care must be taken even during the initial inspection.

High voltage UPSs use diode bridges to convert AC to DC voltage. Also, a smoothing capacitor is provided in the design of the power supply. Since the high voltage is converted into a pulsed voltage with a frequency of 10 to 100 kHz, it became possible to abandon the use of large-sized step-down low-frequency transformers. Instead of them, impulse devices are now used that are small in size.

In low-voltage UPSs, the voltage is first reduced to the required value, and then it is rectified, stabilized and smoothed. As a result, it is possible to obtain the voltage indicator that is necessary for the operation of the equipment. To increase the reliability of power devices and obtain stable output parameters, their design contains various control circuitry solutions.

Troubleshooting

Please be aware that not every power supply can be repaired. Today, many manufacturers produce electronic devices in which the blocks are subject to complete replacement. In them, printed circuit boards are often filled with a compound solution. In such a situation, even professionals do not undertake the restoration of the UPS.

The most common faults of switching power supplies most often caused by:

• problems with the operation of the high-voltage part, arising from the breakdown of diodes or failure of the capacitor;
• breakdown of the transistor located in the high-voltage part of the UPS and designed to generate high-frequency pulses;
• failure of diodes installed in the low-voltage section;
• burnout of the filter choke coil installed at the outlet.

There are other possible reasons for the failure of this device, but they can only be detected when using special instruments, for example, an oscilloscope. In such a situation, high requirements are imposed on the master who repairs the device. If the cause of the UPS failure is not related to the four most common faults, then you should seek professional help.

It is quite easy to detect problems with the operation of the high-voltage section. To diagnose them, it is enough to check the voltage after the fuse. If there is an input voltage on the low-voltage section, but there is no output voltage, then the cause of the malfunction must be looked for here.

If the fuse fails, you need to inspect the board. A burnt-out capacitor can be identified by the bulging of its body. To check the diode bridge installed in the high-voltage section, it is necessary to evaporate each component, and then examine the device with a multimeter.

To exclude the possibility of a repeated malfunction after repair, you need to check all the details. After completing these works, you can proceed to check the UPS. To identify a burnt out choke, it is necessary to check the coils of all elements with a tester. If you cannot find the required replacement part, then you can rewind the burnt one yourself. However, this is a rather complicated process, so it is sometimes easier to buy a new PSU.

Restore standard devices

Most often, at home, attempts are made to restore the power supplies of TVs and computers. It is advisable to first find a diagram of a specific device. First of all, this applies to TVs with picture tubes, since their UPSs provide a wide range of voltages. It is easier with desktop PCs, because their power supply units are made according to a typical scheme.

TV repair

Problems with the power supply are indicated by an inoperative "sleep" LED. First, you should check the functionality of the power cord. If no problem was found, then you can start preliminary repair work:

• disassembling TV and freeing electronic printed circuit boards;
• visual inspection of the UPS for external faults such as swollen capacitors;
• checking the soldering points (special attention should be paid here to the contacts of the pulse transformer).

If visual inspection does not give positive results, then the fuse, diodes, capacitors and transistors are sequentially checked. It is quite difficult to establish the operability of microcircuits.

Among the main malfunctions of TV power supply units you can note:

• breakage of ballast resistors;
• failure of the filtering high-voltage capacitor;
• breakdown of the diode bridge;
• failure of the secondary voltage filter capacitors.

All these parts, except for the diodes, can be checked directly on the board. After replacing defective elements, a conventional incandescent lamp is connected instead of a fuse, and the TV is connected to the network. Here are possible the following behaviors of the restored unit:

1. The sleep LED turns on and the lamp turns on and begins to dim. At the same time, a raster appears on the screen. In this case, it is necessary to check the line scan voltage indicator. If its value turned out to be increased, then the reason may lie in faulty capacitors or optocouplers.
2. When the LED does not light up, there is no raster on the screen, and the lamp flashes and goes out, then the pulse generator is inoperative. In such a situation, you need to check the voltage across the capacitor. If its value turned out to be less than 280 V, then one of the bridge diodes may be broken or the capacitor has failed.
3. When the lamp is on brightly, you need to check all the elements of the UPS again.

This algorithm of actions will allow you to identify the main problems with the power supply unit of the TV.

Desktop computer

It should be remembered that the repair of switching power supplies with a PWM controller is difficult, so in some situations it is worth just replacing the UPS. It is these power supplies that are installed in modern desktop PCs. If there is a problem the following signs indicate:

• the computer does not start;
• the UPS cooler does not rotate;
• there is a repeated start of the power supply device.

To carry out repair work, it is necessary to remove the UPS from the system unit and remove the casing from it. Then you need to remove dust from the boards and parts with a brush. After that, a visual inspection of the elements of the block is carried out, then the load is connected to it. The algorithm for further actions is similar to repairing a TV.

If the transistors of the pulse generator or the PWM controller are out of order, then you just need to buy a new UPS. This is a rather complicated device and it is difficult to repair this type of switching power supply yourself.

When carrying out repair work, you must observe safety rules and exercise caution. It is also worthwhile to correctly assess your capabilities, because sometimes it is better to turn to professionals.