Characteristics and Maintenance of Common Circuit Board and Component Failures

1. The fault characteristics and maintenance of industrial control circuit board capacitor damage

The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of the electrolytic capacitor is the most common.

Capacitor damage manifests itself as: 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 also have their own characteristics. In industrial control circuit boards, digital circuits account for the vast majority. Capacitors are mostly used for power supply filtering and less for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not be able to vibrate and there is no voltage output; or the output voltage is not well filtered, and the circuit is chaotic due to the unstable voltage. Can not machine, if the capacitor and the digital circuit between the positive and negative power supply, the fault performance is the same as above.

This is especially obvious on computer motherboards. Many computers have been used for a few years and sometimes they cannot be turned on, and sometimes they can be turned on. When you open the case, you can often see electrolytic capacitors bulging. If you remove the capacitors, measure the capacity. It is 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, it is important to check the capacitors that are close to the heat source, such as the capacitors near the heat sink and high-power components. The closer to them, the greater the possibility of damage.

I once repaired the power 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 a large capacitor of 1000uF / 350V had an oily substance flowing out, and removed a certain amount of capacity. Only a few tens of uF, it is also found that only this capacitor is closest to the heat sink of the rectifier bridge, and the others that are far away are intact and the capacity is normal. In addition, there is a short-circuit situation in the ceramic capacitor, and it is also found that the capacitor is relatively close to the heating component. Therefore, there should be a focus on maintenance and search.

Some capacitors have serious leakage currents, which can even be hot when touched with a finger. Such capacitors must be replaced.

In the case of good and bad faults during maintenance, except for the possibility of poor contact, most of them are faults caused by capacitor damage. Therefore, when encountering such a fault, you can check the capacitor focus. It is often a surprise after replacing the capacitor (of course, pay attention to the quality of the capacitor, and choose a better brand, such as ruby, black diamond, etc.).

2. Characteristics and discrimination of resistance damage

I often see many beginners tossing on resistors while disassembling and repairing circuits. In fact, they have been repaired a lot. As long as you understand the damage characteristics of resistors, you don’t have to worry about it.

Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. An open circuit is the most common type of resistance damage. It is rare for resistance to become large, and it is very rare for resistance to become small. Common types are carbon film resistors, metal film resistors, wire wound resistors, and fuse resistors.

The first two types of resistors are the most widely used. Their damage characteristics are low resistance (below 100Ω) and high resistance (above 100kΩ). The damage rate is higher. It is rarely damaged; the second is that when the low resistance resistor is damaged, it is often burnt and blackened, which is easy to find, and when the high resistance resistor is damaged, there are few traces.

Wire-wound resistors are generally used for high current limiting, and the resistance is not large. When the cylindrical wire-wound resistor is burned out, some of it will become black or the surface will explode, crack, and some will not be traced. Cement resistance is a kind of wire wound resistance, which may break when burned out, otherwise, there will be no visible traces. When the fuse is burnt out, some surfaces will blast off a piece of skin, and some will have no trace, but they will never burn and become black. According to the above characteristics, there can be a focus when checking the resistance to quickly find the damaged resistance.

Based on the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have burnt traces, and then according to the characteristics of most of the open circuits or resistance values ​​becoming larger and the high-resistance resistors easily damaged when the resistance is 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 value is stable before it is lowered). In conclusion, because it is possible to connect capacitive elements in parallel in the circuit, there is a charge-discharge process.) If the measured resistance is smaller than the nominal resistance, it is generally ignored. In this way, each resistor on the circuit board is measured once, and even if one thousand are wrongly killed, one will not be missed.

3. The method of judging the quality of the operational amplifier

Judging the quality of operational amplifiers is quite difficult for quite a few electronic repairers, not only because of their educational level (there are many undergraduates who don’t teach if they don’t teach, and it takes a long time to understand after teaching. The postgraduate students who study frequency conversion control are actually the same!), I would like to discuss with you here, I hope to help everyone.

Ideal op amps have “virtual short” and “virtual break” characteristics. These two characteristics are very useful for analyzing linearly applied op amp circuits. To ensure linear operation, the op amp must work in a closed loop (negative feedback). Without 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 the amplifier’s virtual short, that is to say, if this op amp works normally, its voltage at the same input and reverse input must be equal, even if there is a difference, it is of mv level. Of course, in some high input impedance circuits, The internal resistance will have a little 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 bad!

If the device is used as a comparator, the non-inverting input and the inverting input are allowed to be different.

Same voltage> reverse voltage, the output voltage is close to the positive maximum;

If the voltage in the same direction is less than the reverse voltage, the output voltage is close to 0V or a negative maximum value (depending on the dual power supply or single power supply).

If it is detected that the voltage does not meet this rule, the device will definitely be bad!

This way you don’t have to use the substitution method, you can judge the quality of the operational amplifier without removing the chip on the circuit board.

4. A multimeter to test SMT components

Some patch components are very small, and it is very inconvenient to test and repair with ordinary multimeter test leads. One is that it is easy to cause a short circuit, and the other is that the circuit board coated with an insulating coating is inconvenient to contact the metal parts of the component pins. Here is a simple method that will bring a lot of convenience to the test.

Take the two smallest sewing needles, (Depth Industrial Control Maintenance Technology Column) and hold them tightly with the multimeter pen, then take a thin copper wire from a multi-strand cable, and tie the test pen and the sewing needle with the thin copper wire. Together, solder again with solder. In this way, when using a test pen with a small needle tip to test those SMT components, there is no risk of short circuit, and the needle tip can pierce the insulation coating and directly hit key parts, and no longer has to worry about scraping those films.

5. The maintenance method of the circuit board public power supply short circuit fault

Have a power supply with adjustable voltage and current, voltage 0–30V, current 0–3A, this power supply is not expensive, about 40$. Adjust the open-circuit voltage to the power supply voltage level of the device, first adjust the current to the minimum, and add this voltage to the circuit’s power supply voltage point such as the 5V and 0V terminals of the 74 series chips. Depending on the degree of the short circuit, slowly increase the current. Touch the device by hand. When you touch a device, the heat is obvious. This is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage during operation must not exceed the working voltage of the device, and cannot be reversed, otherwise, it will burn other good devices.

6. A small eraser to solve big problems

There are more and more boards used in industrial control, and many boards are inserted with gold fingers into the slots. Due to the harsh environment of the industrial site, the dusty, humid, and corrosive gas environment is prone to poor contact of the boards. A friend may solve the problem by replacing the card, but the cost of purchasing a card is considerable, especially for some imported equipment. In fact, everyone may wish to use the eraser to wipe the gold finger repeatedly, clean up the dirt on the gold finger, and then try the machine again. Maybe the problem is solved! The method is simple and practical.

7. Analysis of electrical faults

The various good and bad electrical faults include the following situations in terms of probability:

1)Poor contact

Poor contact between the board and the slot, failure of the cable when the cable is broken internally, poor contact of the wire plug and terminal, and false soldering of components are all such cases;

2). The signal is disturbed

For digital circuits, the fault will only appear under certain conditions. It may indeed be that the interference is too great to affect the control system and cause errors. There are also changes in the individual component parameters or the overall performance parameters of the circuit board, which makes it anti-interference. Capability approaches a tipping point, causing failure;

3). The thermal stability of the components is not good

From the perspective of a large number of maintenance practices, the first is the poor thermal stability of electrolytic capacitors, followed by other capacitors, transistors, diodes, ICs, resistors, etc .;

4). There are moisture and dust on the circuit board.

Moisture and dust will conduct electricity, have a resistance effect, and the resistance value will 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 to occur;

5). Software is also one of the considerations

Many parameters in the circuit are adjusted by software. The margins of some parameters are adjusted too low and are in the critical range. When the machine’s operating conditions meet the software’s reason for failure, then an alarm will appear.




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