Failures caused by capacitor damage are the highest among electronic devices, and damage to electrolytic capacitors is the most common.
Capacitor damage is manifested as: the capacity becomes smaller, the capacity is completely lost, the leakage, and the short circuit.
Capacitors play different roles in circuits, and the faults caused by them have their own characteristics: in industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power filtering, and less capacitors 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 there is no voltage output.
Or the output voltage is not well filtered, and the circuit is logically chaotic due to voltage instability, which is manifested as the machine works badly or cannot be turned on. If the capacitor is connected between the positive and negative poles of the digital circuit, the fault will behave as above.
This is especially obvious on computer motherboards. Many computers sometimes fail to turn on after a few years, and sometimes they can be turned on. Open the case, you can often see the phenomenon of electrolytic capacitors bulging, if you remove the capacitors to measure the capacity , 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 a faulty capacitor, you 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 you are, the greater the possibility of damage. Therefore, there should be some emphasis when checking and repairing.
Some capacitors have serious leakage current, and even burn your hands when touched with your fingers. This type of capacitor must be replaced. In the case of ups and downs during maintenance, except for the possibility of poor contact, most of the failures are generally caused by capacitor damage.
Therefore, when encountering such failures, you can focus on checking the capacitors, and it is often surprising after replacing the capacitors.
It is often seen that many beginners are tossing on the resistance while repairing the circuit, and it is dismantled and welded. In fact, it has been repaired a lot. As long as you understand the damage characteristics of the resistance, you don’t need to spend a lot of 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 type of resistance damage. It is rare that the resistance value becomes larger, and the resistance value becomes smaller. Common ones include carbon film resistors, metal film resistors, wire wound resistors and insurance resistors.
The first two kinds of resistors are the most widely used. One of the characteristics of their damage is that the damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ) is higher, and the middle resistance value (such as hundreds of ohms to tens of kiloohms) Very little damage; second, when low-resistance resistors are damaged, they are often burnt and blackened, which is easy to find, while high-resistance resistors are rarely damaged.
Wire wound resistors are generally used for high current limiting, and the resistance is not large; some cylindrical wire wound resistors will turn black or the surface will burst or crack when burned out, and some have no traces; cement resistors are a kind of wire wound resistors , It may break when burned out, otherwise there is no visible trace; when the fuse resistor burns out, a piece of skin will be exploded on the surface, and some have no trace, but it will never be burnt or 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 burnt black marks, and then according to the characteristics that most of the resistors are open or the resistance becomes larger and the high-resistance resistors are 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 greater than the nominal resistance value, the resistance must be damaged (note that the resistance value is stable before drawing a conclusion, because there may be parallel capacitive elements in the circuit, there is a charging and discharging process), if the resistance is measured If the value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistance on the circuit board is measured again, even if one thousand is “wrongly killed”, one will not be missed.
Operational amplifier failure
It is difficult to judge the quality of operational amplifiers for quite a lot of electronic repairers. It is not only related to education level, but I would like to discuss with you here. I hope it will be helpful to everyone.
The ideal operational amplifier has the characteristics of “virtual short” and “virtual break”, these two characteristics are very useful for analyzing the operational amplifier circuit of linear application. In order to ensure linear application, the op amp 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 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 virtual shortness of the amplifier, that is to say, if the operational amplifier works normally, the voltage of the same direction input terminal and the reverse input terminal must be equal, even if there is a difference, it is mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will have some influence 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 undoubtedly be broken.
If the device is used as a comparator, the same direction input terminal and reverse input terminal are allowed to be different. In the same direction voltage>reverse voltage, the output voltage is close to the positive maximum value; in the same direction voltage
SMT component failure
Some SMD components are very small and inconvenient to test and repair with ordinary multimeter pens. One is that it is easy to cause a short circuit, and the other is that it is inconvenient for the circuit board coated with an insulating coating to touch the metal part of the component pin. Here is an easy way to tell everyone, it will bring a lot of convenience to the detection.
Take two sewing needles of the smallest size and close them to the multimeter pen, then take a thin copper wire from a multi-strand cable, tie the test pen and the sewing needle together with a thin copper wire, and then solder firmly . In this way, there is no risk of short circuit when measuring those SMT components with a test pen with a small needle tip, and the needle tip can pierce the insulating coating and ram the key parts directly, without having to bother to scrape the film.
Short circuit failure of public power
In circuit board maintenance, if you encounter a short-circuit of the public power supply, the fault is often serious, because many devices share the same power supply, and every device using this power supply is suspected of being short-circuited.
If there are not many components on the board, the method of “hoeing the earth” can eventually find the short-circuit point; if there are too many components, whether the “hoeing the earth” can hoe the situation depends on luck. A more effective method is recommended here. Using this method will get twice the result with half the effort, and the fault point can often be found 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 device power supply voltage level, 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, and slowly increase the current depending on the degree of short circuit.
Touch the device with your hands. When you touch a device that heats up significantly, 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 the connection cannot be reversed, otherwise it will burn out other good devices.
More and more boards are used in industrial control, and many boards use golden fingers to insert into slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environment, it is easy to cause poor contact failure of the board. Many friends may solve the problem by replacing the board, but the cost of buying the board is very considerable, especially some Boards of imported equipment.
In fact, you might as well use an eraser to rub the gold finger several times, clean up the dirt on the gold finger, and try the machine again. The problem may be solved. The method is simple and practical.
In terms of probability, various electrical faults with good and bad times include the following situations:
Poor contact between the board and the slot, failure of the connection when the cable is broken, poor contact between the plug and terminal, and false soldering of components are all this type;
Signal is interfered:
For digital circuits, faults will only appear under certain conditions. It is possible that too much interference has affected the control system and caused errors. There are also changes in individual component parameters or overall performance parameters of the circuit board, which makes anti-interference ability. Towards a critical point and thus fail;
Poor thermal stability of components:
From a large number of maintenance practices, the thermal stability of electrolytic capacitors is the first to be poor, followed by other capacitors, triodes, diodes, ICs, resistors, etc.;
Moisture, dust, etc. on the circuit board:
Moisture and dust will conduct electricity and have a resistance effect, and the resistance value will also 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;
Software is also one of the considerations:
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 conform to the reason for the software to determine the failure, then an alarm will appear.