Capacitors play different roles in circuits, and the faults caused by them also have their own characteristics: in industrial control circuit boards, digital circuits account for the vast majority, capacitors are mostly used as power filter, and less capacitors are used as 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 logic is confused due to unstable voltage, which means that the machine is good or bad or cannot be started when it is working. 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 particularly evident on the computer motherboard. Many computers can not be started or can be started after several years of use. When the chassis is opened, the phenomenon of electrolytic capacitor bulging can often be seen. If the capacitor is removed to measure the capacity, it is found that it is much lower than the actual value. The life of capacitor is directly related to the ambient temperature. The higher the ambient temperature is, the shorter the life of capacitor is. This rule applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for fault capacitors, we should focus on the capacitors close to the heat source, such as those near the radiator and high-power components. The closer they are, the more likely they are to be damaged. Therefore, emphasis should be placed on maintenance and search. Some capacitors have serious electric leakage, which can even be hot when touched by fingers. Such capacitors must be replaced. In the maintenance of good and bad faults, except for the possibility of poor contact, most faults are caused by capacitor damage. Therefore, when encountering such faults, you can focus on checking the capacitor, which is often surprising after replacing it.
Resistance fault
It is often seen that many beginners toss and turn on the resistance when repairing the circuit, which is both disassembled and welded. In fact, it is much more repaired. As long as you understand the damage characteristics of the resistance, you don't need to spend much time. 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 resistance damage. It is rare for the resistance value to become larger, and it is rare for the resistance value to become smaller. Common types include carbon film resistor, metal film resistor, wire wound resistor and fuse resistor. The first two types of resistors are the most widely used, and their damage characteristics are: first, the damage rate of low resistance (below 100 Ω) and high resistance (above 100k Ω) is high, and the damage rate of intermediate resistance (such as hundreds of ohms to tens of kiloohms) is rare; Second, when the low resistance resistance is damaged, it is often burnt and blackened, which is easy to find, while when the high resistance resistance is damaged, there is little trace. Wire wound resistance is generally used for large current limiting, with small resistance value; When the cylindrical wire wound resistance is burnt out, some will be blackened or the surface will be cracked, and some will have no trace; Cement resistance is a kind of wire wound resistance, which may break when burned, otherwise there is no visible trace; When the fuse resistor is burnt out, some surfaces will blow off a piece of skin, and some have no traces, but they will never be burnt and blackened. According to the above characteristics, we can focus on checking the resistance and quickly find out the damaged resistance. According to the characteristics listed above, we can first observe whether the low resistance resistance on the circuit board has burnt black traces. Then, according to the characteristics of most open circuits or large resistance values when the resistance is damaged, and the high resistance resistance is easy to be damaged, we can use a multimeter to directly measure the resistance at both ends of the high resistance resistance on the circuit board. If the measured resistance value is greater than the nominal resistance value, the resistance must be damaged (it is important to make a conclusion after the resistance value is stable, because there may be a charging and discharging process in the circuit due to parallel capacitor elements). If the measured resistance value is smaller than the nominal resistance value, it is generally ignored. In this way, each resistance on the circuit board is measured once. Even if you "kill" 1000 by mistake, you will not miss one.
Operational amplifier fault
It is difficult for quite a number of electronic maintainers to judge whether the operational amplifier is good or bad, not only because of the relationship between education, but also to discuss with you here, hoping to help you. The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", which are very useful for analyzing the operational amplifier circuit of linear application. In order to ensure the linear application, the operational amplifier must work in a closed loop (negative feedback). If there is no negative feedback, the op amp under open-loop amplification becomes a comparator. If you want to judge whether the device is good or bad, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit. According to the principle of virtual short of the amplifier, that is, if the operational amplifier works normally, the voltages at the same direction input end and the reverse input end must be equal, even if there is a difference, it is mv level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have some 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 will definitely be damaged. If the device is used as a comparator, the same direction input end and reverse direction input end are allowed to be unequal. If the same direction voltage is greater than the reverse voltage, the output voltage is close to the maximum positive value; If the voltage in the same direction is less than the reverse voltage, the output voltage is close to the maximum value of 0V or negative (depending on dual power supply or single power supply). If the detected voltage does not conform to this rule, the device will be damaged! In this way, you do not need to use the substitution method, and you do not need to remove the chip on the circuit board to judge the quality of the operational amplifier.
SMT component failure
Some SMD components are very small, which makes it inconvenient to test and repair with an ordinary multimeter probe. First, it is easy to cause short circuit. Second, it is inconvenient for the circuit board coated with insulating coating to contact the metal part of the component pin. Here is a simple method, which will bring a lot of convenience to the detection. Take two sewing needles of the smallest size, close them to the multimeter probe, and then take a thin copper wire in a multi strand cable, bind the probe and the needle together with the thin copper wire, and then solder them firmly. In this way, when measuring SMT components with a stylus with a small tip, there is no risk of short circuit. In addition, the tip can puncture the insulation coating, directly hit key parts, and no longer bother to scratch the film.
Public power short circuit fault
During circuit board maintenance, the fault of short circuit of public power supply is often serious, because many components share the same power supply, and each component using this power supply is suspected of short circuit. If there are not many components on the board, the short circuit point can be found by "hoeing the ground"; If there are too many components, it will depend on luck to hoe the earth. A more effective method is recommended here. With this method, you can get twice the result with half the effort and often find the fault point quickly. There should be a power supply with adjustable voltage and current. The voltage is 0-30V, and the current is 0-3A. This power supply is inexpensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level. First, adjust the current to the minimum. Apply this voltage to the power supply voltage points of the circuit, such as the 5V and 0V terminals of 74 series chips. Slowly increase the current depending on the degree of short circuit. Touch the device with your hand. When you feel that a device is obviously hot, it is often a damaged component, and you can take it down for further measurement and confirmation. Of course, during operation, the voltage must not exceed the working voltage of the device, and the connection must not be reversed, otherwise other good devices will be burned.
Board failure
There are more and more boards used in industrial control, and many of them are inserted into the slots with golden fingers. Due to the harsh environment of the industrial site and the dusty, humid and corrosive gas environment, it is easy to cause the board to have a bad contact fault. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you might as well use an eraser to repeatedly wipe the golden finger, clean up the dirt on the golden finger, and then try the machine again, which may solve the problem. The method is simple and practical.
Electrical fault
In terms of probability, various kinds of good and bad electrical faults generally include the following situations: poor contact: poor contact between the board and the slot, no connection when the cable is broken, poor contact between the wire plug and the terminal block, faulty soldering of components, etc; Signal interference: for digital circuits, the fault will appear only under specific conditions. It may be that the interference really affects the control system so much that it makes an error. It may also be that the parameters of individual components or the overall performance parameters of the circuit board have changed, making the anti-interference ability tend to the critical point, thus causing the fault; Poor thermal stability of components: from a large number of maintenance practices, the thermal stability of electrolytic capacitors is poor, followed by other capacitors, triodes, diodes, ICs, resistors, etc; There are moisture, dust, etc. on the circuit board: moisture and dust will conduct electricity with resistance effect, and the resistance value will change in the process of thermal expansion and cold contraction. This resistance value has a parallel effect with other components. If this effect is strong, circuit parameters will be changed to cause faults; Software is also one of the factors to be considered: many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low to be in the critical range. When the operating conditions of the machine meet the reason for the software to determine the failure, the alarm will appear.
