Useful information | Teach you to see the cause of circuit failure at a glance

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Capacitors play different roles in circuits, and the faults they cause 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 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 oscillate and there will be no voltage output; or the output voltage filtering is not good, and the circuit has logical confusion due to unstable voltage, which manifests as the machine working sometimes well and sometimes badly or failing to start. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault manifestation is the same as above.

This is particularly evident on computer motherboards. Many computers have the problem of sometimes not turning on and sometimes being able to turn on after a few years of use. When you open the computer case, you can often see bulging electrolytic capacitors. If you remove the capacitors and measure the capacity, you will find that it is 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 heat sources, such as capacitors next to heat sinks and high-power components. The closer they are to the heat source, the greater the possibility of damage.

I once repaired the power supply of an X-ray flaw detector. The user reported that smoke was coming out of the power supply. After disassembling the chassis, I found that there was something like oil flowing out of a large 1000uF/350V capacitor. After disassembling it, I measured the capacity and found that it was only a few dozen uF. I also found that only this capacitor was closest to the heat sink of the rectifier bridge, and the others that were far away were intact and had normal capacity. In addition, there was a short circuit in the ceramic capacitor, and it was also found that the capacitor was relatively close to the heating component. Therefore, we should focus on it when repairing and searching.

Some capacitors have serious leakage and may even burn your hands when you touch them. Such capacitors must be replaced.

When repairing a fault that works sometimes but sometimes not, excluding the possibility of poor contact, most of the faults are generally caused by capacitor damage. So when encountering such a fault, you can focus on checking the capacitor. After replacing the capacitor, you will often be pleasantly surprised.

It is common to see many beginners struggling with resistors when repairing circuits, disassembling and soldering them. In fact, if you repair them more often, you don't have to go through so much trouble as long as you understand the damage characteristics of resistors.

Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. The most common damage to resistors is open circuit, while resistance increase is less common and resistance decrease is very rare. Common types include carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.

The first two types of resistors are the most widely used. The characteristics of their damage are: first, the damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ) is higher, and the intermediate resistance (such as hundreds of ohms to tens of kiloohms) is rarely damaged; second, 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 have low resistance. When a cylindrical wirewound resistor burns out, some will turn black or have peeling and cracking on the surface, while others will show no trace. Cement resistors are a type of wirewound resistor, which may break when burned out, but otherwise there will be no visible trace. When a fuse resistor burns out, some will have a piece of skin blown off on the surface, while others will show no trace, but it will never be burnt and blackened. Based on the above characteristics, you can focus on checking the resistor and quickly find the damaged resistor.

According to the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have burnt marks. Then, based on the characteristics that most resistors are open circuit or the resistance value increases when they are damaged, and high-resistance resistors are easily damaged, we can use a multimeter to directly measure the resistance of the high-resistance resistors on the circuit board. If the measured resistance value is larger than the nominal resistance value, the resistor must be damaged (pay attention to wait until the resistance value display is stable before drawing a conclusion, because there may be parallel capacitor components in the circuit, and there is a charging and discharging process). If the measured resistance value is smaller than the nominal resistance value, generally ignore it. In this way, every resistor on the circuit board is measured, and even if a thousand are "killed by mistake", one will not be missed.

The ideal operational amplifier has the characteristics of "virtual short" and "virtual open", which are very useful for analyzing the operational amplifier circuit for linear application. In order to ensure linear application, the operational amplifier must work 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 of amplifier, that is to say, if the operational amplifier works normally, the voltage of its same direction input terminal and reverse input terminal must be equal, even if there is a difference, it is mV level. Of course, in some high input impedance circuits, the internal resistance of the multimeter 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 use FLUKE179 multimeter)

If the device is used as a comparator, the same-direction input terminal and the reverse input terminal are allowed to be unequal. If the same-direction voltage > reverse voltage, the output voltage is close to the maximum positive value; if the same-direction voltage < reverse voltage, the output voltage is close to 0V or the maximum negative value (depending on dual power supply or single power supply). If the voltage detected does not meet this rule, the device must be broken! In this way, you don't have to use the substitution method, and you don't have to remove the chip on the circuit board to judge the quality of the operational amplifier.

Take two smallest sewing needles, place them close to the multimeter probe, then take a thin copper wire from a multi-strand cable, use the thin copper wire to tie the probe and the sewing needle together, and then solder them firmly. In this way, when using the probe with a fine needle tip to test those SMT components, there is no need to worry about short circuits, and the needle tip can pierce the insulating coating and directly hit the key parts, so there is no need to bother scraping the membrane.

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, add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V ends of the 74 series chip, depending on the degree of short circuit, slowly increase the current, touch the device with your hand, when you touch a device that is obviously hot, this is often a damaged component, you can remove it for further measurement and confirmation. Of course, during operation, the voltage must not exceed the working voltage of the device, and it cannot be connected in reverse, otherwise other good devices will be burned.

Source: Internet compilation. If copyright is involved, please contact us to delete.

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