Explain in detail the components that are prone to causing circuit failures
Time:2024-02-25
Views:213
Capacitor fault
The failure caused by capacitor damage is the highest in electronic devices, especially in electrolytic capacitors where damage is the most common. Capacitor damage manifests as: reduced capacity, complete loss of capacity, leakage, and short circuit.
The role of capacitors in circuits varies, 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 filtering, with fewer used for signal coupling and oscillation circuits. If the electrolytic capacitor used in a switching power supply is damaged, the switching power supply may not vibrate and there may be no voltage output; Or if the output voltage filtering is not good, the circuit may experience logic confusion due to unstable voltage, manifested as the machine working in good or bad conditions or unable to start. If the capacitor is between the positive and negative poles of the digital circuit power supply, the fault behavior is the same as above.
This is particularly evident on computer motherboards, as many computers may experience issues after several years of use, such as sometimes not being able to turn on the computer and sometimes being able to turn it on again. When opening the computer case, you can often see a bulge in the electrolytic capacitor. If you remove the capacitor and measure its capacity, it will be much lower than the actual value.
The lifespan of a capacitor is directly related to the ambient temperature, and the higher the ambient temperature, the shorter the lifespan of the capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors. So when searching for faulty capacitors, it is important to focus on checking capacitors that are closer to the heat source, such as capacitors near heat sinks and high-power components. The closer they are to them, the greater the likelihood of damage. So when conducting maintenance and search, emphasis should be placed on it.
Some capacitors have severe leakage, and when touched with fingers, they may even burn your hands. These capacitors must be replaced. When troubleshooting, apart from ruling out the possibility of poor contact, most of the faults are caused by capacitor damage. So when encountering such faults, you can focus on checking the capacitor, and replacing the capacitor can often be surprising.
Resistance fault
It is common to see many beginners tinkering with resistors while repairing circuits, dismantling and soldering. In fact, with more repairs, as long as you understand the damage characteristics of resistors, you don‘t need to go to great lengths.
Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. Resistance damage is most common in open circuits, with increasing resistance values being rare and decreasing resistance values being rare. Common types include carbon film resistors, metal film resistors, wire wound resistors, and fuse resistors.
The first two types of resistors are the most widely used, and their damage characteristics are that the damage rate is higher for low resistance values (below 100 Ω) and high resistance values (above 100k Ω), with very little damage for intermediate resistance values (such as hundreds of ohms to tens of thousands of ohms); Secondly, when a low resistance resistor is damaged, it often burns and turns black, which is easy to detect, while when a high resistance resistor is damaged, there are few traces.
Wire wound resistors are generally used for high current limiting and have low resistance values; When cylindrical wire wound resistors are burned out, some may turn black or have surface peeling or cracks, while others may have no traces; Cement resistance is a type of wire wound resistance that may break when burned out, otherwise there are no visible traces; When the safety resistor is burned out, some surfaces may explode with a piece of skin, while others may not have any traces, but they will never burn or turn black. Based on the above characteristics, emphasis can be placed on checking the resistance to quickly identify the damaged resistance.
Based on the characteristics listed above, we can first observe whether there are any burnt marks on the low resistance resistance resistance on the circuit board. Then, according to the characteristics of most open circuits or increased resistance when the resistance is damaged, and the high resistance resistance resistance is easily damaged, we can use a multimeter to directly measure the resistance values at both ends of the high resistance resistance resistance on the circuit board.
If the measured resistance value is greater than the nominal resistance value, then the resistance must be damaged (note to wait for the resistance value to stabilize before making a conclusion, because there may be parallel capacitor components in the circuit, which involves a charging and discharging process). If the measured resistance value is smaller than the nominal resistance value, it is generally not necessary to pay attention to it. In this way, every resistor on the circuit board is measured once, and even if one thousand resistors are accidentally killed, one will not be missed.
Operational amplifier malfunction
The discrimination of the quality of operational amplifiers poses certain difficulties for many electronic repairers, not only due to their educational level. Here, we will discuss it with everyone, hoping to be helpful.
Ideal operational amplifiers have the characteristics of "virtual short" and "virtual break", which are very useful for analyzing operational amplifier circuits in linear applications. To ensure linear application, the operational amplifier must operate in a closed loop (negative feedback). If there is no negative feedback, the operational amplifier under open-loop amplification becomes a comparator. If you want to judge the quality of a 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 short amplifier, if the operational amplifier is working properly, the voltage at the same input and reverse input terminals must be equal, even if there is a difference, it is still at the level of mv. Of course, in some high input impedance circuits, the internal resistance of the multimeter may have a slight impact on voltage testing, but it generally does not exceed 0.2V. If there is a difference of 0.5V or more, the amplifier will undoubtedly fail.
If the device is used as a comparator, it is allowed to have unequal input terminals in the same direction and reverse direction. If the same voltage is greater than the reverse voltage, the output voltage is close to the maximum positive value; If the same voltage<reverse voltage, the output voltage is close to the maximum value of 0V or negative (depending on whether it is a dual power supply or a single power supply). If the voltage detected does not comply with this rule, the device will undoubtedly fail! In this way, you don‘t need to use substitution method or remove the chip on the circuit board to determine the quality of the operational amplifier.
SMT component failure
Some SMT components are very small, making it inconvenient to test and repair them with ordinary multimeter probes. Firstly, it can cause short circuits, and secondly, it is inconvenient for circuit boards coated with insulation to come into contact with the metal parts of the component pins. Here is a simple method that will bring a lot of convenience to the detection.
Take two of the smallest sewing needles, place them tightly against the multimeter probe, then take a thin copper wire from a multi stranded cable, tie the probe and sewing needle together with the thin copper wire, and solder them firmly. This way, when using a probe with a small needle tip to measure SMT components, there is no longer a risk of short circuits, and the needle tip can puncture the insulation coating, directly hit key parts, and no longer have to worry about scraping those films.
Short circuit fault of public power supply
In circuit board maintenance, if there is a short circuit in the common power supply, it is often a big problem because many components share the same power supply, and every component using this power supply is suspected of a short circuit.
If there are not many components on the board, using the method of "hoeing the earth" can ultimately find the short circuit point; If there are too many components, it depends on luck whether the "hoe the earth" can hoe the situation. Here is a recommended method that works well. By using this method, you can achieve twice the result with half the effort and often quickly find the fault point.
There should be a power supply with adjustable voltage and current, with a voltage of 0-30V and a current of 0-3A. This type of power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the level of the device‘s power supply voltage. First, minimize the current and apply this voltage to the power supply voltage points of the circuit, such as the 5V and 0V terminals of the 74 series chip. Depending on the degree of short circuit, gradually increase the current. When touching a device with your hand, if you notice a significant heat generation, it is often a damaged component that can be removed for further measurement and confirmation. Of course, during operation, the voltage must not exceed the operating voltage of the device, and it must not be reversed, otherwise it will burn out other good devices.
Board failure
The number of boards used in industrial control is increasing, and many boards use the method of inserting gold fingers into slots. Due to the harsh industrial environment, which is dusty, humid, and contains corrosive gases, it is easy for the boards to have poor contact faults. Many friends may have solved the problem by replacing the boards, but the cost of purchasing boards is very considerable, especially for some imported equipment boards.
Actually, you can use an eraser to repeatedly wipe the dirt on the golden finger a few times, clean it up, and then try the machine again. The method may solve the problem, which is simple and practical.
Electrical faults
In terms of probability, various electrical faults with good and bad times can roughly include the following situations:
Poor contact: Poor contact between the board and the slot, inability to connect when the cable is broken internally, poor contact between the wire plug and wiring terminal, and virtual soldering of components all belong to this category;
Signal interference: For digital circuits, faults only appear under specific conditions. It is possible that the interference is too great and affects the control system to make errors. There are also individual component parameters or overall performance parameters on the circuit board that change, causing the anti-interference ability to approach the critical point and resulting in faults;
Poor thermal stability of components: From a large number of maintenance practices, it can be seen that electrolytic capacitors have poor thermal stability, followed by other capacitors, transistors, diodes, ICs, resistors, etc;
There is moisture, dust, etc. on the circuit board: Moisture and dust can conduct electricity and have a resistance effect, and the resistance value will also change during thermal expansion and contraction. This resistance value, together with other components, has a parallel effect. When this effect is strong, it will change the circuit parameters and cause faults to occur;
Software is also one of the factors to consider: many parameters in the circuit are adjusted using software, and the margin of certain parameters is set too low in the critical range. When the machine‘s operating conditions meet the reasons for software judgment of faults, an alarm will appear.
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