Overview of fault factors in circuit design
Circuit failure is a headache for every engineer, so how to avoid circuit failure in circuit design?
1. Capacitor failure.
Faults caused by capacitor damage are common in electronic equipment, especially electrolytic capacitor damage. Capacitor damage is manifested as: capacity reduction, complete loss of capacity, leakage and short circuit.
Capacitors play different roles in circuits, and faults also have their own characteristics: in industrial control circuit boards, digital circuits account for the vast majority, capacitors are mostly used as power filters, and there are few capacitors in signal coupling and oscillation circuits.
If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate or there may be no voltage output. Or the output voltage filter is not good, and the logic of the circuit is confused due to unstable voltage, which shows that the machine works well or cannot be started. If the capacitor is between the positive and negative poles of the power supply of the digital circuit, the fault performance is the same.
This is particularly evident on the computer motherboard. Many computers sometimes fail to turn on the machine after several years of use, and sometimes they can turn on the machine. When opening the chassis, you can often see the bulge of electrolytic capacitor. If the capacitors are removed and measured, they are found to be much lower than the actual value.
The service life of the capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the service life of the capacitor. This Law applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for a faulty capacitor, we should focus on checking the capacitor close to the heat source, such as the capacitor next to the heat sink and the capacitor next to the high-power element. The closer we are to it, the greater the possibility of damage. Therefore, attention should be paid to maintenance and search.
Some capacitors have serious leakage and may even be hot when touched with your fingers. Eda365 electronic forum reminds you that such capacitors must be replaced. In addition to the possibility of poor contact, faults during maintenance are generally caused by capacitor damage. Therefore, in case of such a fault, the capacitor can be checked, which is often unexpected after replacement.
2. Resistance failure.
It is often seen that many beginners toss about the resistance, dismantle and weld when repairing the circuit. In fact, they repair a lot. As long as you understand the damage characteristics of the resistance, you don't have to bother and work hard.
Resistance is a large number of components in electrical equipment, but it is not a component with damage rate. Resistance damage is common in an open circuit. It is rare for the resistance to increase and it is rare for the resistance to decrease. Common are carbon film resistance, metal film resistance, winding resistance and safety resistance.
The first two resistors are widely used, and their damage is characterized by low resistance value (100 ω Below) and high resistance value (100 ω The above damage rate is high, and the damage of intermediate resistance value (e.g. hundreds of euros to tens of thousands of euros) is very few; Second, the low resistance is burnt and blackened when the resistance is damaged, which is easy to find, and there are few traces when the high resistance is damaged.
Wire winding resistance is usually used for high current current limiting, and the resistance value is small; When the resistance of cylindrical wire winding is burned out, some will be blackened or the surface will burst and crack, while others will have no trace; Cement resistance is a kind of wire winding resistance, which may break when burned out, otherwise there is no obvious trace; When the safety resistor is burnt out, some surfaces will blow off a piece of skin, while others will have no trace, but they will never be charred and blackened. In view of the above characteristics, we can focus on checking the resistance and quickly find out the damaged resistance.
According to the above characteristics, we can first observe whether the low resistance on the circuit board has the trace of burning black, and then according to the characteristics that most of the resistance is open circuit or the resistance becomes larger and the high resistance is easy to be damaged when the resistance is damaged, we can directly measure the resistance at both ends of the high resistance on the circuit board with a multimeter.
If the measured resistance value is greater than the nominal resistance value, the resistance will be damaged (pay attention to draw a conclusion after the resistance value is stable, because there may be parallel capacitor elements in the circuit and there is a charge and discharge process). If the measured resistance value is less than the nominal resistance value, it is generally unnecessary to care. In this way, if you measure each resistance on the circuit board, even if you kill a thousand by mistake, you won't miss one.
3. Operational amplifier failure.
For quite a number of electronic maintenance personnel, it is very difficult for electronic maintenance personnel, not just the relationship between education level. I'm here to discuss with you, hoping to be helpful to you.
The ideal operational amplifier has the characteristics of virtual short and virtual break, which is very useful for analyzing the operational amplifier circuit for linear applications. In order to ensure linear application, the operation must be carried out in a closed loop (negative feedback). If there is no negative feedback, the operation after open-loop amplification will become a comparator. To judge the quality of the equipment, we must first distinguish whether the equipment is used for amplifier or comparator in the circuit.
According to the principle of virtual short of the amplifier, that is, if the operating amplifier works normally, the voltage at the same input end and the reverse input end must be equal. Even if there is a difference, it is still mv level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have a certain 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 damaged.
If the device is used for comparator, the same direction input and reverse input are allowed to be different. If the voltage in the same direction is negative, the output voltage is close to positive; If the voltage in the same direction is the reverse voltage, the output voltage is close to 0V or negative value (depending on dual power supply or single power supply). If the detected voltage does not comply with this rule, the equipment will be damaged! Therefore, you do not need to use the replacement method or remove the chip on the circuit board to judge the quality of the operational amplifier.
4. SMT component failure.
Some SMD components are extremely small, so it is very inconvenient to test and maintain them with ordinary multimeter probes. (1) It is easy to cause short circuit, (2) it is inconvenient to contact the metal part of the component pin of the circuit board coated with insulating coating
Take two small sewing needles close to the multimeter probe, then take the thin copper wire in one or more cables, bind the probe with the sewing needle with the thin copper wire, and then solder it firmly. In this way, when measuring those SMT components with a probe with a small needle tip, there is no risk of short circuit. The needle tip can pierce the insulating coating, directly crush the key parts, and don't bother to scrape those films anymore.
5. Short circuit fault of common power supply.
During the maintenance of the circuit board, if the fault of short circuit of common power supply is encountered, the head is often large, because many devices share the same power supply, and each device using the power supply is suspected of short circuit.
If there are few plate elements, the short-circuit point can be found by hoeing; If there are too many plate elements, hoeing depends on luck.
It has a power supply with adjustable voltage and current. The voltage is 0-30V and the current is 0-3A. This power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the power supply voltage level of the equipment, first adjust the current in place, add this voltage to the 5V and 0V terminals of 74 series chips, and slowly increase the current according to the short circuit degree of 74 series chips and the short circuit degree. Touch the equipment by hand. When touching a equipment with obvious heating, it is usually damaged equipment, which can be removed for further measurement and confirmation. During operation, the voltage must not exceed the working voltage of the equipment or be connected reversely, otherwise other good equipment will be burned.
6. The board is broken.
More and more boards and cards are used in industrial control. Many boards and cards adopt the way of golden finger slot. However, the industrial site environment is bad, and the environment with much dust, humidity and corrosive gas is easy to lead to poor board contact. Many people will think of replacing the board to solve the problem, but the cost of buying the board is very considerable, especially for some imported equipment.
In fact, you might as well wipe the golden finger with an eraser several times, clean up the dirt on the golden finger, and try again. Maybe the problem will be solved. The method is simple and practical.
7. Electrical failure.
From the perspective of probability, various good and bad electrical faults probably include the following situations:
1) Poor contact:
Poor contact between the board and the slot, no connection when the cable is broken, poor contact between the wire plug and wiring terminal, false welding of parts, etc.
2) Signal disturbed:
For digital circuits, faults only occur under specific conditions, which may really be too much interference, affect the control system and lead to errors. In addition, the individual component parameters or overall performance parameters of some circuit boards have changed, so that the anti-interference ability tends to the critical point, resulting in failure.
3) Poor thermal stability of components:
From a large number of maintenance practices, the thermal stability of the first electrolytic capacitor is poor, followed by other capacitors, triodes, diodes, integrated circuits, resistors, etc.
