See through the cause of circuit failure

Capacitor fault characteristics and maintenance

The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitors is the most common. Capacitor damage is manifested as: capacity becomes smaller; complete loss of capacity; leakage; short circuit.

Capacitors play different roles in the circuit, and the faults caused by them also 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 well filtered, and the circuit is logically confused due to unstable voltage, which is manifested as good or bad when the machine is working or open. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault is the same as above.

This is especially obvious on the computer motherboard. Many computers have been used for several years, sometimes they can't be turned on, and sometimes they can be turned on again. When you open the case, you can often see the phenomenon of bulging electrolytic capacitors. If you remove the capacitors, measure the capacity. , found to be much lower than the actual value.

The life of the 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 capacitors that are close to the heat source, such as capacitors next to heat sinks and high-power components. The closer you are to them, the greater the possibility of damage.

Once repaired the power supply of an X-ray flaw detector, the user reported that there was smoke coming out of the power supply. After disassembling the case, it was found that there was a 1000uF/350V large capacitor with oil-like things flowing out. Remove a large amount of capacity. There are only dozens of uF, and it is also found that only this capacitor is closest to the heat sink of the rectifier bridge, and the others far away are intact and the capacity is normal. In addition, there is a short circuit in the ceramic capacitor, and it is also found that the capacitor is relatively close to the heating component. Therefore, attention should be paid to the maintenance and search.

Some capacitors have serious leakage and even burn your hands when touched with your fingers. Such capacitors must be replaced.

In the case of good and bad faults during maintenance, except for the possibility of poor contact, most of the faults are caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising.

Characteristics and Discrimination of Resistor Damage

It is often seen that many beginners toss on the resistor when repairing the circuit, dismantling and soldering. In fact, it is repaired a lot. As long as you understand the damage characteristics of the resistor, you don't have to spend a lot of time.

Resistors are the most numerous components in electrical equipment, but not the ones with the highest damage rate. The open circuit is the most common damage to the resistance, and it is rare for the resistance value to increase, and it is very rare for the resistance value to decrease. Commonly used are carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.

The first two types of resistors are the most widely used, and the characteristics of their damage are that the damage rates of low resistance (below 100Ω) and high resistance (above 100kΩ) are relatively high, and the intermediate resistance (such as hundreds of ohms to tens of thousands of ohms) has a high damage rate. Very little damage; Second, when the low-resistance resistor is damaged, it is often scorched and blackened, which is easy to find, while the high-resistance resistor is rarely damaged.

Wirewound resistors are generally used for large current limiting, and the resistance value is not large. When the cylindrical wirewound resistor is burned out, some will turn black or the surface will explode, crack, and some will have no traces. Cement resistors are a type of wirewound resistors, which may break when burned out, otherwise there will be no visible traces. When the fuse resistor burns out, some surfaces will blow off a piece of skin, and some will have no traces, but they will never be scorched and 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 any traces of burning black, and then according to the characteristics of most open circuits or increased resistance when the resistance is damaged, and the high-resistance resistance is 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 (pay attention to wait for the resistance value display to stabilize before lowering In conclusion, because there may be parallel capacitive elements in the circuit, there is a charge and discharge process), if the measured resistance value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistor on the circuit board is measured again, even if one thousand is "killed by mistake", one will not be missed.

Operational amplifier is good or bad judgment

It is difficult for many electronic repairers to judge the quality of operational amplifiers. It is not only related to the level of education. I would like to discuss with you here, and I hope it will be helpful to everyone.

The ideal op amp has the characteristics of "virtual short" and "imaginary off", which are very useful for analyzing op amp circuits in linear applications. In order to guarantee linear operation, the op amp must work in 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 virtual short of the amplifier, that is to say, if the operational amplifier works normally, the voltages at the non-inverting input terminal and the inverting input terminal must be equal, and even if there is a difference, it is at the level of mv. The internal resistance of the amplifier will affect the voltage test a bit, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken! (I use the FLUKE179 multimeter)

If the device is used as a comparator, the non-inverting input and the inverting input are allowed to be unequal. If the same voltage > reverse voltage, the output voltage is close to the positive maximum value; if the same direction voltage < reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on dual power supply or single power supply). If the detected voltage does not meet this rule, the device must be broken! That way you don't have to use substitution, you don't have to remove the chip on the board to tell if the op amp is good or bad.

SMT component testing tips

Some SMD 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 it is inconvenient to contact the metal parts of the component pins on the circuit board coated with the insulating coating. Here is a simple method that will bring a lot of convenience to the detection.

Take two smallest sewing needles and place them close to the multimeter pen, then take a thin copper wire from a multi-strand cable, use the thin copper wire to tie the test pen and sewing needle together, and then use solder to solder them firmly. . In this way, there is no risk of short circuit when testing those SMT components with a test pen with a small needle tip, and the needle tip can pierce the insulating coating and directly hit the key parts, so there is no need to bother to scrape those membranes.

Public power short circuit maintenance

In circuit board maintenance, if there is a short circuit of the public power supply, it is often a big problem, because many devices share the same power supply, and each device using this power supply is suspected of short circuit. If there are not many components on the board, use "hoeing the earth" In the end, the short-circuit point can be found in the way of hoarding. If there are too many components, it will depend on luck whether the "hoeing the earth" can hoe the situation. A more effective method is recommended here. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly.

There must be a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. 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 power supply voltage point 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, use When you touch the device by hand, when you touch a device with obvious heat, 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 it cannot be reversed, otherwise other good devices will be burned.

Erasers solve big problems

More and more boards are used in industrial control, and many boards use gold fingers to be inserted into slots. Due to the harsh environment of the industrial site, the dusty, humid, and corrosive gas environment is easy to cause poor contact failure of the board. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some Boards for imported equipment.

In fact, you might as well use an eraser to wipe the gold finger several times, clean the dirt on the gold finger, and then try the machine again. Maybe the problem has been solved! The method is simple and practical.

Electrical Failure Analysis

Various good and bad electrical faults roughly include the following situations in terms of probability:

1. Poor contact

Poor contact between the board and the slot, failure to connect when the cable is internally broken, poor contact between wire plugs and terminals, and virtual welding of components, etc.

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 large, which affects the control system and makes it go wrong. There are also changes in the parameters of the individual components of the circuit board or the overall performance parameters, which makes the anti-interference. Capability tends to a tipping point and thus malfunctions;

3. The thermal stability of components is not good

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

4. There is moisture, dust, etc. on the circuit board

Moisture and dust will conduct electricity and 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, it will change the circuit parameters and cause failure. occur;

5. Software is also a factor to consider

Many parameters in the circuit are adjusted by software. The margin of some parameters is adjusted too low and is in the critical range. When the operating conditions of the machine conform to the reason for the software to determine the fault, then the alarm will appear.