Troubleshooting circuit failures

The continuous development of technology has made circuits more and more complex. When there is a problem with the circuit, everyone needs to be able to troubleshoot it. Failures caused by capacitor damage are the highest in electronic equipment, and damage to electrolytic capacitors is especially common. Capacitor damage manifests as: reduced capacity; complete loss of capacity; leakage; short circuit. Capacitors play different roles in circuits, and the faults they cause have different characteristics. In industrial control circuit boards, digital circuits account for the vast majority. Capacitors are mostly used for power supply filtering, and fewer 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 have no voltage output; or the output voltage may not be filtered well, and the circuit may be logically confused due to voltage instability, which may manifest as the machine working intermittently or turning on and off. If the capacitor is not 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. After a few years of use, many computers sometimes fail to turn on and sometimes can turn on. When you open the case, you can often see bulging electrolytic capacitors. If you remove the capacitor, 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 relatively close to the heat source, such as capacitors next to heat sinks and high-power components. The closer they are to them, the greater the possibility of damage.

I 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, I found a large 1000uF/350V capacitor with oil-like substance flowing out. I removed a certain amount of capacity. It was only a few tens of uF. It was also found that only this capacitor was closest to the heat sink of the rectifier bridge. The other capacitors that were far away were intact and had normal capacity. In addition, there are cases of short circuit in ceramic capacitors, and it is also found that the capacitor is relatively close to the heating component. Therefore, some emphasis should be placed on maintenance and search.

Some capacitors have serious leakage and may even be hot when touched with fingers. Such capacitors must be replaced. When repairing faults that come and go, except for the possibility of poor contact, most faults are generally caused by damaged capacitors. Therefore, when encountering such a fault, you can focus on checking the capacitor. After replacing the capacitor, you will often be pleasantly surprised.

Characteristics and identification of resistor damage

It is often seen that many beginners are tossing around with resistors when repairing circuits, disassembling and welding them. In fact, they can repair a lot more. As long as you understand the damage characteristics of resistors, you don't have to spend a lot of time. Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. The most common type of resistor damage is an open circuit. It is less common for the resistance value to increase, and it is very rare for the resistance value to decrease. Common ones include carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.

The first two types of resistors are the most widely used. One of their damage characteristics is that the damage rate is higher for low resistance values ​​(below 100Ω) and high resistance values ​​(above 100kΩ), and for intermediate resistance values ​​(such as several hundred ohms to tens of kiloohms). Rarely damaged; secondly, when low-resistance resistors are damaged, they are often burnt and blackened, which is easy to find, while when high-resistance resistors are damaged, there are few traces.

Wirewound resistors are generally used for large current limiting, and their resistance is not large. When cylindrical wire-wound resistors are burned out, some will turn black or have skin or cracks on the surface, while others will have no traces. Cement resistors are a type of wirewound resistor and may break when burned out, otherwise there will be no visible traces. When the fuse burns out, some parts of the surface will explode, and some may leave no trace, but they will never be burnt or blackened. Based on the above characteristics, you can focus on checking the resistor and quickly find the damaged resistor.

Based on the characteristics listed above, we can first observe whether there are any traces of burnt black on the low-resistance resistors on the circuit board. Then, based on the characteristics that most of the resistors are open-circuited or the resistance increases when the resistors are damaged, 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 resistor must be damaged (be careful not to wait until the resistance value is stable before measuring the resistance). Conclusion, because there may be capacitive elements connected in parallel in the circuit and there is a charging and discharging process), if the measured resistance is smaller than the nominal resistance, generally ignore it. In this way, every resistor on the circuit board is measured once. Even if a thousand are "killed by mistake", one will not be missed.

Judging the quality of operational amplifiers

It is difficult for many electronic repairmen to judge the quality of operational amplifiers, not only because of their education level. I would like to discuss it with you, hoping it will be helpful to everyone.

The ideal operational amplifier has the characteristics of "virtual short" and "virtual open". These two characteristics are very useful for analyzing operational amplifier circuits for linear applications. In order to ensure linear operation, 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 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 circuit of the amplifier, that is to say, if the operational amplifier is working normally, the voltage of its non-inverting input terminal and the inverting input terminal must be equal. Even if there is a difference, it will be mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will have some influence 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 broken! (I used a FLUKE179 multimeter)

If the device is used as a comparator, the non-inverting input terminal and the inverting input terminal are allowed to be different. If the same direction voltage > the reverse voltage, the output voltage is close to the positive maximum value; if the same direction voltage < the reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on the dual power supply or single power supply). If the detected voltage does not comply with this rule, the device must be broken! In this way, you can judge the quality of the operational amplifier without using the substitution method or removing the chip on the circuit board.

SMT component testing tips

Some chip components are very small, and it is very inconvenient to use ordinary multimeter test leads to test and repair them. First, it is easy to cause short circuits. Second, it is inconvenient for circuit boards coated with insulating coating to touch the metal parts of component pins. Here I will tell you a simple method, which will bring a lot of convenience to the detection. Take two of the smallest sewing needles and place them close to the multimeter pens. Then take a thin copper wire from a multi-strand cable. Use the thin copper wire to tie the test pens and sewing needles together, and then solder them firmly. . 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 insulating coating and go straight to the key parts, so you no longer have to bother to scrape those membranes.

Public power supply short circuit maintenance

During circuit board maintenance, if you encounter a public power supply short-circuit fault, you will often have a big problem, because many devices share the same power supply, and every device using this power supply is suspected of being short-circuited. If there are not many components on the board, use "hoeing the ground" The short circuit point can be found after all. If there are too many components, it depends on luck whether the problem can be found by "hoeing the ground". Here we recommend a more effective method. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly.

You need 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 device power supply voltage level. First, adjust the current to the minimum. Then 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 the short circuit, slowly increase the current. Touch the device with your hands. When you feel that a certain device is obviously hot, this is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be connected reversely, otherwise other good devices will be burned out.

Rubber solves big problems

More and more boards are used in industrial control, and many boards use gold fingers to insert into the slots. Due to the harsh environment of the industrial site, dusty, humid, and corrosive gas environments can easily cause poor contact failures in the boards. Many boards My friend may have solved the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you might as well use an eraser to rub the gold finger repeatedly, clean the dirt on the gold finger, and then try the machine again. The problem may be solved! The method is simple and practical.

Electrical failure analysis

Various electrical faults that come and go may include the following situations in terms of probability:

1. Poor contact

Poor contact between the board and the slot, poor communication when the cable is broken internally, poor contact between the wire plug and terminal block, and weak soldering of components all fall into this category;

2. Signal interference

For digital circuits, faults will occur under specific circumstances. It may be that the interference is too great and affects the control system, causing it to go wrong. There may also be changes in individual component parameters or overall performance parameters of the circuit board, making the anti-interference Capabilities tend to reach critical points and thus malfunctions occur;

3. The thermal stability of components is not good

Judging from a large number of maintenance practices, the thermal stability of electrolytic capacitors is first of all, 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 relatively strong, it will change the circuit parameters and cause malfunctions. occur;

5. Software is also a factor to consider.

Many parameters in the circuit are adjusted using software. The margin of some parameters is adjusted too low and is in a critical range. When the machine operating conditions meet the reasons for the fault determined by the software, an alarm will appear. The above are some methods for checking some circuit problems. I hope it will be helpful to everyone.