I. Fault Characteristics and Repair of Damaged Capacitors on Industrial Control Circuit Boards
Capacitor failure is the most common cause of malfunctions in electronic devices, with electrolytic capacitor failure being the most frequent.
The symptoms of a damaged capacitor are:
The capacity has decreased;
Completely lost capacity;
Leakage current;
Short circuit.
Capacitors play different roles in circuits, and the faults they cause vary accordingly. In industrial control circuit boards, digital circuits make up the vast majority, and capacitors are mostly used for power supply filtering, with fewer used for signal coupling and oscillation circuits. If an electrolytic capacitor used in a switching power supply fails, the switching power supply may not oscillate and there may be no voltage output; or the output voltage filtering may be poor, and the circuit may experience logical errors due to voltage instability, manifesting as intermittent operation or failure to power on. If the capacitor is connected in parallel between the positive and negative terminals of the power supply in a digital circuit, the fault symptoms are the same.
This is especially noticeable on computer motherboards. Many computers, after a few years of use, sometimes fail to boot and sometimes boot up. When you open the case, you can often see that the electrolytic capacitors are bulging. If you remove the capacitors and measure their capacitance, you will find that the capacitance is much lower than the actual value.
The lifespan of a capacitor is directly related to the ambient temperature; the higher the ambient temperature, the shorter the capacitor's lifespan. This rule applies not only to electrolytic capacitors but also to other types of capacitors. Therefore, when searching for faulty capacitors, special attention should be paid to capacitors located close to heat sources, such as those near heat sinks and high-power components. The closer they are to these sources, the greater the likelihood of damage.
I once repaired the power supply of an X-ray flaw detector. The user reported smoke coming out of the power supply. After opening the chassis, I found an oily substance leaking from a large 1000uF/350V capacitor. Upon measurement, its capacitance was only a few tens of uF. I also discovered that this was the only capacitor closest to the rectifier bridge heatsink; the others, further away, were intact and had normal capacitance. Additionally, a ceramic capacitor was found to be short-circuited, also located close to heat-generating components. Therefore, during troubleshooting, one should focus on specific areas.
Some capacitors have serious leakage, and they can even get hot to the touch with your fingers. These capacitors must be replaced.
When troubleshooting intermittent faults, after ruling out poor contact, the most common cause is usually a damaged capacitor. Therefore, when encountering such faults, focus on checking the capacitors; replacing them often yields positive results (of course, pay attention to the quality of the capacitors; choose reputable brands such as Rubycon or Black Diamond).
II. Characteristics and Identification of Resistor Damage
I often see beginners messing around with resistors when troubleshooting circuits, taking them apart and soldering them. But once you've repaired a lot, you'll understand the characteristics of resistor failure and won't need to go through all that trouble.
Resistors are the most numerous components in electrical appliances, but they are not the components with the highest failure rate. Open circuit is the most common cause of resistor failure; increased resistance is less common, and decreased resistance is extremely rare. Common types include carbon film resistors, metal film resistors, wire-wound resistors, and fusible resistors.
The first two types of resistors are the most widely used. Their failure characteristics are: firstly, the failure rate of low resistance (below 100Ω) and high resistance (above 100kΩ) resistors is relatively high, while the failure rate of intermediate resistance (such as hundreds of ohms to tens of thousands of ohms) resistors is very low; secondly, when low resistance resistors fail, they are often charred and blackened, which is easy to find, while high resistance resistors rarely show any traces when they fail.
Wire-wound resistors are generally used for high-current limiting and have relatively low resistance values. When cylindrical wire-wound resistors burn out, some may turn black or have surface peeling and cracks, while others show no signs of damage. Cement resistors are a type of wire-wound resistor; when they burn out, they may break, otherwise, there are no visible traces. When fusible resistors burn out, some may have a piece of their outer layer chipped off, while others show no signs of damage, but they will never char or turn black. Based on these characteristics, you can focus on these aspects when inspecting resistors to quickly identify the damaged ones.
Based on the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board show signs of burning. Then, considering that most resistor failures involve open circuits or increased resistance, and that high-resistance resistors are prone to failure, we can use a multimeter to directly measure the resistance across the high-resistance resistors on the circuit board. If the measured resistance is higher than the nominal resistance, then this resistor is definitely damaged (note that we should wait for the resistance reading to stabilize before drawing a conclusion, as there may be parallel capacitors in the circuit, which undergo a charging and discharging process). If the measured resistance is lower than the nominal resistance, it can generally be ignored. By measuring every resistor on the circuit board in this way, even if a thousand are mistakenly identified, one will not be overlooked.
III. Methods for determining the quality of operational amplifiers
Determining whether an operational amplifier is good or bad can be quite difficult for many electronics repair technicians, and it's not just a matter of education level (I have many undergraduates who definitely won't know how to use it without being taught, and even after being taught it, it takes them a long time to grasp it; I even have a graduate student who specifically studied frequency converter control with his supervisor, and he still struggles with it!). Here, I'd like to discuss this with everyone, hoping it will be helpful.
Ideal operational amplifiers possess the characteristics of "virtual short" and "virtual open," which are highly useful for analyzing op-amp circuits used in linear applications. To ensure linear operation, the op-amp must operate in a closed-loop (negative feedback) configuration. Without negative feedback, an op-amp in open-loop amplification becomes a comparator. To determine the quality of a component, one must first determine whether the component is used as an amplifier or a comparator in the circuit.
Regardless of the type of amplifier, there is a feedback resistor Rf. When repairing, we can check this feedback resistor in the circuit. Use a multimeter to check the resistance between the output terminal and the inverting input terminal. If it is unusually large, such as several MΩ or more, we can be fairly certain that the device is used as a comparator. If the resistance is small, from 0Ω to tens of kΩ, then check if there is a resistor connected between the output terminal and the inverting input terminal. If there is, it is definitely used as an amplifier.
According to the principle of virtual short in amplifiers, if the operational amplifier is working properly, the voltage at its non-inverting input terminal and the inverting input terminal must be equal. Even if there is a difference, it will be in the mV range. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have a slight impact on the voltage test, but it will generally not exceed 0.2V. If there is a difference of more than 0.5V, then the amplifier is definitely broken!
If the device is used as a comparator, then the non-inverting input and the inverting input can be different.
If the voltage in the same direction is greater than the voltage in the opposite direction, then the output voltage will be close to the maximum positive value.
If the voltage in the same direction is less than the voltage in the opposite direction, the output voltage will be close to 0V or the negative maximum value (depending on whether it is a dual power supply or a single power supply).
If the detected voltage does not conform to this rule, the device will undoubtedly fail!
This way, you can determine whether the operational amplifier is good or bad without using the substitution method or removing the chips from the circuit board.
IV. A little trick for testing SMT components with a multimeter
Some surface-mount components are very small, making testing and troubleshooting with ordinary multimeter probes inconvenient. Firstly, it easily causes short circuits, and secondly, it's difficult to access the metal parts of the component leads on circuit boards with insulating coatings. Here's a simple method that will greatly simplify the testing process.
Take two of the smallest sewing needles (from the Deep Industrial Control Repair Technology Column) and hold them close to the multimeter probes. Then, take a thin copper wire from a multi-strand cable and use it to tie the probes and sewing needles together. Finally, solder them securely. This way, when using the probes with their fine needle tips to test SMT components, there's no risk of short circuits. Furthermore, the needle tip can pierce the insulating coating and reach the critical parts, eliminating the need to painstakingly scrape off the protective films.
V. Troubleshooting methods for short circuit faults in the common power supply of circuit boards
In circuit board repair, encountering a short circuit in a common power supply is often a major headache, because many components share the same power supply, and every component using this power supply is a potential short circuit culprit. If there aren't many components on the board, a "digging deep" approach can eventually find the short circuit point. However, if there are too many components, whether this "digging deep" approach will uncover the problem depends on luck. Here, we recommend a more effective method that is twice as efficient and often allows you to quickly find the fault point.
You'll need a power supply with adjustable voltage and current (0-30V, 0-3A). This type of power supply is inexpensive, around 300 yuan. Adjust the open-circuit voltage to the device's power supply voltage level. First, set the current to its minimum. Apply this voltage to the circuit's power supply voltage points, such as the 5V and 0V terminals of a 74 series chip. Depending on the degree of short circuit, slowly increase the current. Touch the devices; when a device becomes noticeably hot, this is often the damaged component. Remove it for further measurement and confirmation. Of course, the voltage must never exceed the device's operating voltage, and the connections must not be reversed, otherwise other working components will be damaged.
VI. A small eraser solves a big problem
Industrial control systems utilize an increasing number of circuit boards, many of which employ a gold-finger insertion slot design. Due to the harsh industrial environment—dusty, humid, and prone to corrosive gases—contact problems can easily occur. Many people may solve the issue by replacing the circuit board, but the cost of new boards, especially those in imported equipment, can be substantial. A simple and practical solution is to gently rub the gold fingers with an eraser to clean them thoroughly before attempting to run the machine again.
VII. Analysis of Intermittent Electrical Faults
In terms of probability, various intermittent electrical faults can be roughly categorized into the following types:
Poor contact: Poor contact between the board and the slot, intermittent continuity due to internal breakage of the cable, poor contact of the plug and terminal, and poor soldering of components all fall into this category;
Signal interference: For digital circuits, faults will only appear under specific conditions. It is possible that the interference is too great and affects the control system, causing it to malfunction. Alternatively, the parameters of individual components or the overall performance parameters of the circuit board may change, causing the anti-interference capability to approach the critical point, thus resulting in a fault.
Poor thermal stability of components: Based on extensive repair experience, electrolytic capacitors are the most problematic due to their poor thermal stability, followed by other capacitors, transistors, diodes, ICs, resistors, etc.
Moisture and dust on the circuit board: Moisture and dust can conduct electricity and have a resistive effect. Moreover, 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 a fault.
Software is also a factor to consider: many parameters in the circuit are adjusted using software. If the margin of some parameters is set too low and they are in the critical range, an alarm will occur when the machine's operating conditions meet the reasons for the software's fault determination.
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