Failures caused by capacitor damage are the highest in electronic equipment, and damage to electrolytic capacitors is the most common. The performance of capacitor damage is as follows:
1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.
Capacitors play different roles in the circuit, and the faults they cause 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, 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 filtered well, and the circuit is logically chaotic due to voltage instability, which shows that the machine is working well or broken No matter the machine, if the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault will be the same 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 faulty capacitors, 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.
I have repaired the power supply of an X-ray flaw detector. The user reported that smoke came out of the power supply. After disassembling the case, it was found that there was a 1000uF/350V large capacitor with oily things flowing out. Remove a certain amount of capacity It is only tens of uF, and it is found that only this capacitor is the closest to the heat sink of the rectifier bridge, and the others far away are intact with normal capacity. In addition, the ceramic capacitors were short-circuited, and the capacitors were also found to be relatively close to the heating components. 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. After replacing the capacitors, it is often surprising (of course, you must also pay attention to the quality of the capacitors, and choose a better brand, such as Ruby, Black Diamond, etc.).
1. Characteristics and judgment of resistance damage
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 types 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 high, 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.
Wirewound resistors are generally used for high current limiting, and the resistance is not large. When cylindrical wire wound resistors burn out, some will turn black or the surface will burst or crack, and some will have no traces. Cement resistors are a type of wire wound resistors, which may break when burned out, otherwise there will be no visible traces. When the fuse resistor burns out, a piece of skin will be blown off on some surfaces, and some have no traces, but they will never burn or turn black. 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 is greater than the nominal resistance, the resistance must be damaged (note that the resistance is stable before the display In conclusion, because there may be parallel capacitive elements in the circuit, there is a charge and discharge process), if the measured resistance is smaller than the nominal resistance, 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.
Second, the judgment method of operational amplifier
It is difficult to judge the quality of operational amplifiers for many electronic repairers, not only the education level (there are many undergraduates undergraduates, if you don’t teach, they will definitely not, it will take a long time to understand, there is a special The same is true for graduate students whose tutors are studying inverter control!), I would like to discuss with you here, and 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.