Many beginners often struggle with resistors when troubleshooting circuits, taking them apart and re-soldering them. In fact, after gaining some experience, understanding the characteristics of resistor damage can save a lot of unnecessary effort.
Resistors are the most numerous components in electrical devices, but they are not the most failure-prone components. The most common type of resistor failure is an open circuit, while an increase in resistance is less common, and a decrease in resistance is very rare. Common types include carbon film resistors, metal film resistors, wire-wound resistors, and fusible resistors. The first two types are the most widely used, and their failure characteristics are: first, low resistance (below 100Ω) and high resistance (above 100kΩ) have a higher failure rate, while mid-range resistances (from several hundred ohms to several tens of kilohms) rarely fail; second, low resistance resistors often burn and turn black when damaged, making them easy to identify, while high resistance resistors usually show little evidence of damage. Wire-wound resistors are generally used for high current limiting and have low resistance values. When cylindrical wire-wound resistors burn out, some may turn black or show surface peeling or cracks, while others may show no signs. Cement resistors are a type of wire-wound resistor that may break when burned out, otherwise showing no visible signs. Fusible resistors may have a piece of their surface blown off when damaged, while others may show no signs, but they will never burn and turn black. Based on these characteristics, when checking resistors, one can focus on quickly identifying damaged resistors.
Based on the characteristics listed above, we can first observe whether there are any burn marks on the low resistance resistors on the circuit board. Then, according to the characteristic that most resistors fail as open circuits or increase in resistance, and that high resistance resistors are prone to failure, we can use a multimeter to directly measure the resistance across the terminals of high resistance resistors on the circuit board. If the measured resistance is greater than the nominal value, this resistor is definitely damaged (note that the resistance value should be stable before drawing conclusions, as there may be parallel capacitor components in the circuit that could affect the readings during charging and discharging). If the measured resistance is less than the nominal value, it is generally not worth worrying about. By measuring each resistor on the circuit board this way, even if we mistakenly identify a thousand, we won’t miss one.
Determining the quality of operational amplifiers can be quite challenging for many electronics repair technicians, not just due to differences in education levels (I have many undergraduates who won’t understand unless taught, and even then it takes a long time for them to grasp it. There’s even a graduate student specializing in frequency control who is similarly challenged!). Here, I would like to discuss this with everyone, hoping to help.
An ideal operational amplifier has the characteristics of “virtual short” and “virtual open”, which are very useful for analyzing linear applications of op-amp circuits. To ensure linear applications, the op-amp must operate under closed-loop (negative feedback) conditions. If there is no negative feedback, the open-loop amplified op-amp acts as a comparator. To determine the quality of the device, one must first clarify whether the device is being used as an amplifier or a comparator in the circuit.
From the diagram, we can see that regardless of the type of amplifier, there is a feedback resistor Rf. Therefore, during repairs, we can check this feedback resistor from the circuit, using a multimeter to measure the resistance between the output and inverting input terminals. If the resistance is excessively high, such as several MΩ, we can be fairly certain that the device is being used as a comparator. If this resistance is low (0Ω to several tens of kΩ), we should check if there is a resistor connected between the output and inverting input terminals; if so, it is definitely being used as an amplifier.
According to the principle of virtual short in amplifiers, if this operational amplifier is functioning normally, the voltages at the non-inverting and inverting input terminals must be equal, and even if there is a difference, it should only be in the millivolt range. Of course, in some high input impedance circuits, the internal resistance of the multimeter may slightly affect the voltage measurement, but it generally should not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier is undoubtedly faulty! (I am using a FLUKE179 multimeter)
If the device is being used as a comparator, then it is acceptable for the non-inverting and inverting input voltages to be unequal.
If the non-inverting voltage > inverting voltage, the output voltage approaches the positive maximum value;
If the non-inverting voltage < inverting voltage, the output voltage approaches 0V or the negative maximum value (depending on whether it is a dual or single supply).
If the detected voltages do not conform to this rule, the device is undoubtedly faulty!
This way, you do not need to use substitution methods or remove the chip from the circuit board to determine the quality of the operational amplifier.
Some surface mount components are very small, making it inconvenient to test them with ordinary multimeter probes. This can easily lead to short circuits, and it is difficult to contact the metal parts of component pins on circuit boards coated with insulating coatings. Here I will share a simple method that can make testing much easier.
Take two of the smallest sewing needles, and place them snugly against the multimeter probes. Then take a thin copper wire from a multi-strand cable, and use the thin copper wire to bind the probes and needles together, securing them with solder. This way, when using probes with small needle tips to measure those SMT components, there is no risk of short circuits, and the needle tips can pierce through the insulating coating to access critical areas without the hassle of scraping off the coating.
When repairing circuit boards, encountering a common power supply short circuit can be quite frustrating, as many components share the same power supply, and each component using this power supply may be suspected of being shorted. If there are not many components on the board, a “broad search” approach can eventually find the short circuit point. However, if there are too many components, whether the broad search will yield results relies on luck. Here, I recommend a relatively effective method that can often quickly locate the fault point.
You need a voltage and current adjustable power supply, with a voltage range of 0-30V and a current range of 0-3A. This power supply is inexpensive, around 300 yuan. Set the open-circuit voltage to the level of the device’s power supply voltage, first adjusting the current to the minimum, and apply this voltage to the power supply points of the circuit, such as the 5V and 0V terminals of the 74 series chips. Depending on the severity of the short circuit, gradually increase the current, and use your hand to feel the components. When you feel that a certain component is noticeably warm, this is often the damaged component, which can be removed for further measurement and confirmation. Of course, during operation, the voltage must not exceed the working voltage of the component, and it must not be reversed; otherwise, other good components may be damaged.
As industrial control boards become more prevalent, many boards use gold fingers inserted into slots. Due to harsh industrial environments that are dusty, humid, and contain corrosive gases, boards can develop contact failure issues. Many friends may have solved problems by replacing the boards, but the cost of purchasing boards can be significant, especially for certain imported equipment. In fact, you might try using an eraser to clean the gold fingers a few times, and after clearing away the dirt, try the machine again; it might just solve the problem! This method is simple and practical.
Intermittent electrical faults can generally be categorized into the following situations based on probability:
1. Poor contact
Contact issues between the board and slot, intermittent breaks in internal cables, poor contact at plugs and terminals, and cold solder joints all fall into this category;
2. Signal interference
For digital circuits, faults may only appear under specific conditions, possibly due to excessive interference affecting the control system, or changes in the parameters of individual components or overall performance pushing the anti-interference capability to a critical point, resulting in faults;
3. Poor thermal stability of components
From extensive repair practice, the most common issue is poor thermal stability of electrolytic capacitors, followed by other capacitors, transistors, diodes, ICs, and resistors;
4. Moisture and dust on the circuit board.
Moisture and dust can conduct electricity, creating resistance effects, and during thermal expansion and contraction, the resistance value may change. This resistance can have a parallel effect with other components, and when this effect is strong, it can alter circuit parameters, leading to faults;
5. Software is also a factor to consider.
Many parameters in circuits are adjusted using software. If certain parameters are set too close to their limits, they may fall within a critical range. When the machine operates under conditions that meet the software’s fault criteria, an alarm will trigger.
Modern electronic products come in a wide variety, with an ever-increasing number of components. In circuit repair, especially in the industrial circuit board repair field, many components may be unfamiliar or even unheard of. Additionally, even if the data for certain components on a board is complete, sifting through this information on a computer without a quick search method can significantly reduce repair efficiency. In the field of industrial electronics repair, efficiency is money; neglecting efficiency means neglecting cash in your pocket.
We should be grateful to live in this great era. The internet’s vast reach makes every corner of the world seem within reach, and free information is available everywhere for everyone.
First, I recommend a website http://www.alldatasheet.com, where you can search for any component, whether it’s a chip, transistor, relay, or even an LED. It covers everything. To facilitate future searches, each time you download a datasheet, you can save it in a designated folder. Important Note: Pay attention to the dropdown selection box in the upper left corner (included/start with/end/match). Choosing wisely will help you find what you need faster. After pressing search, also pay attention to the manufacturer’s logo on the right side of the chip. Choose the manufacturer that matches the labeling on the chip, as some chips, especially Japanese ones, may have incomplete markings and many similar keywords. In this case, you need to combine the chip’s actual packaging and manufacturer to search. For example, the keyword for 7805 may yield results for both a three-terminal voltage regulator and an ADC chip. A simple keyword search may return a large number of components but may not lead you to what you want. This website is excellent, with a fairly complete range of components, and you don’t need to register to search; however, the server is abroad, so the speed may not always be satisfactory, and some obscure component data may also be unavailable.
For domestic websites, Shengming Parts Network http://www.icminer.com seems to provide faster downloads, with a more comprehensive range of components, even some data not publicly available from manufacturers scanned into PDF documents. When you’ve searched other websites without success, trying this site might yield pleasant surprises. The only inconvenience is that you need to register and log in to download.
