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What is the most prone to failure component on a circuit board?

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Capacitor failure


The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitors is the most common. Capacitor damage is manifested as: the capacity becomes smaller, the capacity is completely lost, leakage, and short circuit.

The functions of capacitors in the circuit are different, and the faults caused by them also have their own characteristics: 


In industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power supply filtering, less capacitor are used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate and have no voltage output;Or the output voltage filtering is not good, and the circuit is logically confused due to unstable voltage, which is manifested as good or bad when the machine is working or cannot be turned on. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault will behave as above.

This is especially obvious on the computer motherboard. Many computers have been used for several years, sometimes they can't be turned on, and sometimes they can be turned on again. When you open the case, you can often see the phenomenon of bulging electrolytic capacitors. If you remove the capacitors, measure the capacity, found to be much lower than the actual value.

The influence of ambient temperature on capacitors:


The life of the 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, we should focus on checking the capacitors that are close to the heat source, such as the capacitors next to the heat sink and high-power components. The closer it is, the greater the possibility of damage.

Some capacitors have serious leakage and even burn your hands when touched with your fingers. Such capacitors must be replaced. When there is a fault that the maintenance is sometimes good and sometimes bad, except for the possibility of poor contact, most of the faults are caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising.


PCBA debug


Resistor failure


It is often seen that many beginners toss on the resistor when repairing the circuit, dismantling and soldering. In fact, it is repaired a lot. As long as you understand the damage characteristics of the resistor, you don't have to spend a lot of time.

Resistors are the most numerous components in electrical equipment, but not the ones with the highest damage rate. The open circuit is the most common damage to the resistance, and it is more common for the resistance value to become larger, and it is very rare for the resistance value to become smaller. Commonly used are carbon film resistors, metal film resistors, wire wound resistors and fusible resistors.

The first two types of resistors are the most widely used, and their damage characteristics are that the damage rates of low resistance (below 100Ω) and high resistance (above 100kΩ) are relatively high, and the intermediate resistance (such as hundreds of ohms to tens of thousands of ohms) very little damage; Second, when the low-resistance resistor is damaged, it is often scorched and blackened, which is easy to find, but there are few traces when high-value resistors are damaged.

Wire wound resistors


Wire wound resistors are generally used for high current limiting, and the resistance value is not large; when cylindrical wire wound resistors burn out, some will turn black or the surface will explode, crack, and some have no traces; cement resistors are a kind of wire wound resistors, it may break when it is burned out, may there will be no visible traces; when the fusible resistor is burned out, some surfaces will blow up a piece of skin, and some will have no traces, but they will never be scorched and blackened. According to the above characteristics, you can focus on checking the resistance and quickly find the damaged resistance.

According to the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have any traces of burning black, and then according to the characteristics of most open circuits or increased resistance when the resistance is damaged, and the high-resistance resistance is easily damaged, we can use a multimeter to directly measure the resistance at both ends of the high-resistance resistor on the circuit board. If the measured resistance value is larger than the nominal resistance value, the resistance must be damaged (pay attention to wait for the resistance value to display stable before drawing a conclusion, because there may be parallel capacitive elements in the circuit, and there is a charging and discharging process), if the measured resistance value is smaller than the nominal resistance value, it is generally ignored.

Operational Amplifier Failure


It is difficult for many electronic repairers to judge the quality of operational amplifiers. I will discuss it with you here, and hope it will be helpful to everyone.

The ideal operational amplifier has the characteristics of "virtual short" and "imaginary off", which are very useful for analyzing operational amplifier circuits in linear applications. In order to guarantee linear operation, the operational amplifier must work in closed loop (negative feedback). Without negative feedback, the operational amplifier 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.

Principle of the virtual short of the amplifier


That is to say, if the operational amplifier works normally, the voltages at the non-inverting input terminal and the inverting input terminal must be equal, and even if there is a difference, it is at the level of mV. The internal resistance will have a little impact on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken.

If the device is used as a comparator, the non-inverting input and the inverting input are allowed to be unequal. If the same voltage > reverse voltage, the output voltage is close to the positive maximum value; if the same direction voltage < reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on dual power supply or single power supply). If the detected voltage does not meet this rule, the device must be broken! That way you don't have to use substitution, you don't have to remove the chip on the board to tell if the op amp is good or bad.

SMT component failure


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 it is inconvenient to contact the metal parts of the component pins on the circuit board coated with the insulating coating. Here is a simple method that will bring a lot of convenience to the detection.

Take the two smallest sewing needles, close them with the multimeter test leads, and then take a thin copper wire in a multi-strand cable, use the thin copper wire to tie the test leads and the sewing needle together, and then use solder to fasten them. In this way, there is no risk of short circuit when testing those SMT components with a test lead with a small needle tip, and the needle tip can pierce the insulating coating and directly hit the key parts, so there is no need to bother to scrape those membranes.

Short circuit fault of public power


In circuit board maintenance, if there is a short circuit of the public power supply, it is often a big problem, 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, you can eliminate them one by one to find the problem; if there are too many components, when you find the problem depends on luck. A more effective method is recommended here. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly.

It is necessary to have a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this kind of 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, add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip, it seems to be short-circuited, and then slowly increase the current.

Short circuit fault of public power

Touch the device with your hand. When you touch a device with obvious heat, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be reversed, otherwise other good devices will be burned.

Board failure


More and more boards are used in industrial control, and many boards use gold fingers to be inserted into slots. Due to the harsh environment of the industrial site, the dusty, humid and corrosive gas environment is easy to cause poor contact failure of the board. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some boards for imported equipment.

In fact, you might as well use an eraser to wipe the gold finger several times, clean the dirt on the gold finger, and then try the machine again. Maybe the problem will be solved. The method is simple and practical.

Electrical failure


Various some times good and sometimes bad electrical faults roughly include the following situations in terms of probability:

Poor contact: Poor contact between the board and the slot, no power supply when the cable is broken inside, poor contact between the wire plug and the terminal, and virtual welding of components, etc.;

Signal interference: For digital circuits, faults will only appear under certain conditions. It may be indeed that the interference is too large, which affects the control system and causes errors. There are also changes in the parameters of individual components of the circuit board or the overall performance parameters, make the anti-interference ability tend to the critical point and cause failure;

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

There is moisture, dust, etc. on the circuit board: moisture and dust will conduct electricity and 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, and this effect is relatively strong When the circuit parameters are changed, the fault occurs;

Software is also one of the factors to consider: many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low and is in the critical range. When the operating conditions of the machine meet the reason for the software to determine the fault, then an alarm will appear.


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