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Yangzhou Kaihong Power Technology Co., Ltd. (Yangzhou Headquarters)
Sales phone: 0514-87979963
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Company address: Xi'an Jiaotong University Pioneer Park, East Wuzhou Road, Yangzhou

Analysis and repair of linear DC power circuit board technology
Edit: Yangzhou Kaihong Power Technology Co., Ltd. Release time: 2019-05-30

First, the fault characteristics and maintenance of the capacitor damage of the linear DC power circuit board. The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitor is the most common. Capacitor damage manifests itself as: 1. capacity becomes smaller; 2. complete loss of capacity; 3. leakage; 4. short circuit.

Analysis and repair of linear DC power circuit board technology

Capacitors play different roles in the circuit, and the faults they cause also have their own characteristics. In linear DC power circuit boards, digital circuits account for the vast majority of capacitors. Capacitors are mostly used for power supply filtering, and capacitors used for signal coupling and oscillation circuits are less. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not be able to vibrate and there is no voltage output; or the output voltage is not well filtered, and the circuit is chaotic due to the unstable voltage. Can't stop the machine, if the capacitor is between the positive and negative poles of the power supply of the digital circuit, the fault performance is the same as above.

This is especially obvious on computer motherboards. Many computers have been used for a few years and sometimes they cannot be turned on, and sometimes they can be turned on. When you open the chassis, you can often see the phenomenon of electrolytic capacitors bulging. It is found to be much lower than the actual value.

The life of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life of the capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for faulty capacitors, it is important to check the capacitors that are close to the heat source, such as the capacitors near the heat sink and high-power components. The closer to them, the greater the possibility of damage.

The user reported that there was smoke coming out of the linear DC power supply. After disassembling the case, it was found that a 1000uF / 350V large capacitor had an oily substance flowing out. The amount of the removed capacitor was only tens of uF, and it was only found The capacitor and the heat sink of the rectifier bridge are closest to each other, and the others that are far away are intact and the capacity is normal. In addition, there are short-circuits in ceramic capacitors. It is also found that the capacitor is relatively close to the heating component. Therefore, there should be a focus on maintenance and search.

Some capacitors have serious leakage currents, which can even be hot when touched with fingers. Such capacitors must be replaced.

In the case of good and bad faults during maintenance, except for the possibility of poor contact, most of them are faults caused by capacitor damage. Therefore, when encountering such a fault, you can check the capacitor. It is often a surprise after replacing the capacitor (of course, you must pay attention to the quality of the capacitor, and choose a better brand, such as ruby, black diamond, etc.).

Second, the characteristics and discrimination of resistance damage

I often see many beginners tossing on resistors when they are repairing circuits, and they are disassembled and soldered. In fact, they are repaired a lot. As long as you understand the damage characteristics of resistors, you don't have to worry about it.

Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Open circuit is the most common type of resistance damage. It is rare to see a large resistance value, and it is very rare to have a small resistance value. Common types are carbon film resistors, metal film resistors, wire wound resistors and fuse resistors.

The first two types of resistors are the most widely used. Their damage characteristics are: low resistance (below 100Ω) and high resistance (above 100kΩ). The damage rate is high. It is rarely damaged; secondly, when the low-resistance resistor is damaged, it is often burnt and blackened, which is easy to find, and when the high-resistance resistor is damaged, there are few traces.

Wire-wound resistors are generally used for high current limiting, and the resistance is not large. When the cylindrical wire-wound resistor is burned out, some will be black or the surface will be cracked or cracked, and some will not be traced. Cement resistance is a type of wire wound resistance, which may break when burned out, otherwise there will be no visible traces. When the fuse is burnt out, some surfaces will blast off a piece of skin, and some will have no trace, but they will never burn and become black. According to the above characteristics, you can focus on the resistance check and quickly find the damaged resistance.

Based on the characteristics listed above, we can first observe whether the low-resistance resistor on the linear DC power supply circuit board has burnt traces, and then according to the fact that most resistors are open-circuited or the resistance value becomes larger and the high-resistance resistor is easily damaged when the resistance is damaged. Characteristics, we can use a multimeter on the linear DC power circuit board to directly measure the resistance of the two ends of the high-resistance resistor. If the measured resistance is greater than the nominal resistance, the resistor must be damaged (pay attention to the equal resistance The conclusion can only be reached after the value display is stable, because the circuit may have capacitors connected in parallel, and there is a charge and discharge process). If the measured resistance is smaller than the nominal resistance, it is generally ignored. In this way, each resistor on the linear DC power supply circuit board is measured once, even if a thousand are "missed" by mistake, one will not be missed.

Third, the method of judging the quality of the operational amplifier

Judging the quality of operational amplifiers is quite difficult for quite a few electronic repairers, not only because of their education level (many undergraduates are under the control, they will definitely not if they do n’t teach, it also takes a long time to understand, and there is a special The postgraduate who studies frequency control is actually the same!), I will discuss with you here, I hope it will be helpful to everyone.

Ideal op amps have "virtual short" and "virtual break" characteristics. These two characteristics are very useful for analyzing linearly applied op amp circuits. To ensure linear operation, the op amp must work in a closed loop (negative feedback). Without negative feedback, the op amp under open-loop amplification becomes a comparator. If you want to judge the quality of the device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit.

According to the principle of the amplifier's virtual short, that is to say, if this operational amplifier works normally, the voltages of its non-inverting input and inverting input must be equal, even if there is a difference, it is of mv level. Of course, in some high input impedance circuits, the multimeter The internal resistance will have a little effect on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier will definitely be bad! (I use a FLUKE179 multimeter)

If the device is used as a comparator, the non-inverting input and the inverting input are allowed to be different.

Same voltage> reverse voltage, the output voltage is close to the positive maximum;

If the voltage in the same direction is less than the reverse voltage, the output voltage is close to 0V or a negative maximum value (depending on the dual power supply or single power supply).

If it is detected that the voltage does not meet this rule, the device will definitely be bad!

In this way, you don't have to use the replacement method, and you don't have to remove the chip on the linear DC power circuit board to judge the quality of the operational amplifier.

Fourth, a multimeter to test SMT components

Some SMD components are very small, and it is very inconvenient to test and repair with ordinary multimeter test leads. One is that it is easy to cause a short circuit, and the other is that the linear DC power circuit board coated with an insulation coating is inconvenient to contact the metal parts of the component pins. Here is a simple method that will bring a lot of convenience to the test.

Take the two smallest sewing needles, (Deep Maintenance Technical Column) and hold them tightly with the multimeter pen, then take a thin copper wire in a multi-strand cable, and tie the test pen and the sewing needle with the thin copper wire. , And then solder firmly. In this way, when using a test lead with a small tip to test those SMT components, there is no risk of short circuit, and the tip can pierce the insulation coating and directly hit key parts, and no longer has to worry about scraping those membranes.

Fifth, the maintenance method of the short circuit fault of the public power of the linear DC power circuit board

In the repair of linear DC power circuit boards, if there is a short circuit of the common power supply, the fault is often large, because many devices share the same power supply, and every device using this power supply is suspected of short circuit. If there are not many components on the board, use " After all, the short-circuit point can be found in the way of "Big Earth". If there are too many components, it will depend on luck if the "Big Earth" can reach the situation. A more effective method is recommended here. Using this method, you can do more with less and often find the fault point quickly.

Have a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the power supply voltage level of the device, first adjust the current to the minimum, and add this voltage to the circuit's power supply voltage point, such as the 5V and 0V terminals of 74 series chips. Depending on the degree of short circuit, slowly increase the current. Touch the device by hand. When you touch a device, the heat is obvious. This is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage during operation must not exceed the working voltage of the device, and cannot be reversed, otherwise it will burn other good devices.

6. A small eraser to solve big problems

There are more and more boards used in industrial control, and many boards use golden fingers to insert into the slot. Due to the harsh environment of the industrial site, the dusty, humid, and corrosive gas environment is prone to cause poor contact failure of the board, many A friend may solve the problem by replacing the board card, but the cost of buying a board card is very considerable, especially for some imported equipment. In fact, you may wish to use the eraser to wipe the gold finger repeatedly, clean up the dirt on the gold finger, and then try the machine again. Maybe the problem is solved! The method is simple and practical.

Seven, analysis of good and bad electrical faults

The various good and bad electrical faults include the following situations in terms of probability:

Poor contact

Poor contact between the board and the slot, failure to connect when the cable is broken internally, poor contact between the wire plug and the terminal, and false soldering of components, etc .;

2. The signal is disturbed

For digital circuits, the fault will only appear under certain conditions. It may indeed be that the interference is too great to affect the control system and cause errors. There are also changes in individual component parameters or overall performance parameters of the linear DC power circuit board. Make the anti-jamming capability approach a critical point, thereby causing failure;

3. The thermal stability of the components is not good

From the perspective of a large number of maintenance practices, the first is the poor thermal stability of electrolytic capacitors, followed by other capacitors, transistors, diodes, ICs, resistors, etc .;

4. There is moisture and dust on the linear DC power circuit board.

Moisture and dust will conduct electricity, have a resistance effect, and the resistance value will change during the process of thermal expansion and contraction. This resistance value will have a parallel effect with other components. When this effect is strong, the circuit parameters will be changed to cause failure. occur;

5. Software is also one of the considerations

Many parameters in the circuit are adjusted by software. The margins of some parameters are adjusted too low and are in the critical range. When the machine's operating conditions meet the software's reason for failure, then an alarm will appear.


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