1. Characteristics and Repair of Capacitor Failures in Industrial Control Circuit Boards
Failures caused by capacitor damage are the most common in electronic devices, especially with electrolytic capacitors being the most prevalent.
Capacitor damage manifests as:
Reduced capacity;
Complete loss of capacity;
Leakage;
Short circuit.
Since capacitors serve different functions in circuits, the characteristics of the resulting failures vary. In industrial control circuit boards, digital circuits dominate, and capacitors are mostly used for power filtering, with fewer used in signal coupling and oscillation circuits. If an electrolytic capacitor used in a switching power supply fails, the power supply may fail to oscillate, resulting in no voltage output; or the output voltage may not be well-filtered, causing voltage instability and logical confusion in the circuit, which can manifest as intermittent operation or failure to power on. If the capacitor is placed between the positive and negative terminals of the digital circuit’s power supply, the failure will exhibit the same symptoms.
This is particularly evident in computer motherboards, where many computers fail to power on after a few years, and upon opening the case, bulging electrolytic capacitors are often visible. If the capacitors are removed and measured, their capacity is often found to be significantly lower than the actual value.
The lifespan of a capacitor is directly related to the ambient temperature; the higher the temperature, the shorter the lifespan. This principle applies to electrolytic capacitors as well as other types. Therefore, when searching for faulty capacitors, one should focus on those located near heat sources, such as next to heat sinks and high-power components. The closer they are, the higher the likelihood of damage.
I once repaired a power supply for an X-ray flaw detector, where the user reported smoke coming from the power supply. Upon opening the case, I found a large 1000uF/350V electrolytic capacitor leaking a substance resembling oil. After removal and measurement, its capacity was only a few tens of uF, and it was the only capacitor located closest to the rectifier’s heat sink, while others further away were intact and had normal capacity. Additionally, I found ceramic capacitors that had shorted, which were also close to heat-generating components. Thus, during inspections, one should prioritize these areas.
Some capacitors exhibit severe leakage, becoming hot to the touch, and these must be replaced.
When troubleshooting intermittent faults, if poor contact has been ruled out, the majority of cases are often due to capacitor damage. Therefore, when encountering such faults, it is advisable to focus on inspecting the capacitors, as replacing them often yields surprising results (of course, care should be taken to choose quality capacitors from reputable brands like Rubycon or Nichicon).
2. Characteristics and Identification of Resistor Failures
It is common to see many beginners struggling with resistors during circuit repairs, often disassembling and soldering repeatedly. However, with experience, one can understand the characteristics of resistor failures and avoid unnecessary hassle.
Resistors are the most numerous components in electrical devices, but they are not the most failure-prone. The most common failure mode for resistors is open circuit, while resistance value increasing is less common, and decreasing resistance value is rare. Common types include carbon film resistors, metal film resistors, wire-wound resistors, and fusible resistors.
The first two types are widely used, with their failure characteristics being that low resistance values (below 100Ω) and high resistance values (above 100kΩ) have a higher failure rate, while mid-range values (from hundreds of ohms to tens of kilohms) are rarely damaged. Low resistance resistors often burn and turn black when damaged, making them easy to spot, while high resistance resistors typically show little to no visible signs of damage.
Wire-wound resistors are generally used for high current limiting and have low resistance values. When cylindrical wire-wound resistors fail, some may turn black or develop surface blisters or cracks, while others may show no signs. Cement resistors, a type of wire-wound resistor, may break upon failure, but otherwise have no visible signs. When fusible resistors fail, some may have a portion of the surface blown off, while others may show no signs, but they will never turn black.
Based on these characteristics, we can first observe whether there are any blackened low resistance resistors on the circuit board. Then, based on the most common failure modes of open circuit or increasing resistance value, as well as the tendency for high resistance resistors to fail, we can use a multimeter to directly measure the resistance across high resistance resistors on the circuit board. If the measured resistance is significantly higher than the rated value, that resistor is definitely damaged (note that one should wait for the resistance reading to stabilize before concluding, as capacitive components in the circuit may cause a charging and discharging effect). If the measured resistance is lower than the rated value, it can generally be disregarded. By measuring each resistor on the circuit board, even if a thousand are misjudged, none will be overlooked.
3. Methods for Determining the Quality of Operational Amplifiers
Determining the quality of operational amplifiers can be challenging for many electronic repair technicians, not only due to educational background (many of my students are undergraduates who, without instruction, would struggle to understand, even with teaching, it takes a long time for them to grasp it. I have a graduate student specializing in frequency control who is similarly challenged!). Here, I would like to discuss this with everyone in hopes of providing some help.
An ideal operational amplifier possesses the characteristics of “virtual short” and “virtual break,” which are very useful for analyzing linear applications of op-amp circuits. To ensure linear applications, the op-amp must operate in a closed loop (negative feedback). Without negative feedback, the open-loop op-amp functions as a comparator. To determine the quality of a device, one must first clarify whether it is used as an amplifier or as 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, measure the resistance between the output and the inverting input. If the resistance is outrageously high, such as several MΩ, we can be fairly certain that the device is functioning as a comparator. If this resistance is relatively low, from 0Ω to several tens of kΩ, we should check for resistors connected between the output and the inverting input; if present, it is functioning 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 inputs must be equal, and even if there is a difference, it should be in the millivolt range. Of course, in some high input impedance circuits, the internal resistance of the multimeter may slightly affect voltage measurements, but generally, it shouldn’t exceed 0.2V. If the difference exceeds 0.5V, the amplifier is undoubtedly faulty! (I used a FLUKE 179 multimeter).
If the device is functioning as a comparator, then the voltages at the non-inverting and inverting inputs may not be equal:
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 supply or single supply).
If the detected voltages do not conform to this rule, the device is undoubtedly faulty!
Thus, you can determine the quality of the operational amplifier without needing to use substitution methods or remove the chip from the circuit board.
4. A Small Trick for Testing SMT Components with a Multimeter
Some surface-mounted components are very small, making it inconvenient to test them with standard multimeter probes. This can easily lead to short circuits, and it is difficult to contact the metal parts of the component leads on circuit boards coated with insulating layers. Here is a simple method that will greatly facilitate testing.
Take two of the smallest sewing needles, and attach them tightly to the multimeter probes. Then take a thin copper wire from a multi-stranded cable, and use it to bind the probes and needles together, securing them with solder. This way, using the fine-tipped probes to measure SMT components eliminates the risk of short circuits, and the tips can pierce through the insulating coating to reach critical areas without the need to scrape off the coating.
5. Troubleshooting Short Circuit Failures in Common Power Supply on Circuit Boards
When repairing circuit boards, encountering short circuit failures in common power supplies can be quite daunting, as many components share the same power supply, and each component using this power supply may be suspected of shorting. If the board has few components, using a “digging method” can ultimately locate the short circuit point. However, if there are too many components, whether the digging method can find the fault depends on luck. Here, I recommend a relatively effective method that can significantly expedite the process of locating faults.
You need a power supply that can adjust both voltage and current, with a voltage range of 0-30V and a current range of 0-3A. This type of power supply is not expensive, around 300 yuan. Set the open circuit voltage to the level of the device’s power supply voltage, and initially set the current to the minimum. Apply this voltage to the circuit’s power supply points, such as the 5V and 0V terminals of 74 series chips, and gradually increase the current based on the severity of the short circuit. By touching the components, when you feel a noticeable heat from a particular component, that is likely the damaged component, which can then be removed for further measurement and confirmation. Of course, during operation, the voltage must not exceed the working voltage of the components, and it must not be reversed, or it may damage other functioning components.
6. A Small Eraser Solves Big Problems
With the increasing use of boards in industrial control, many boards utilize gold fingers inserted into slots. Due to harsh industrial environments—dusty, humid, and corrosive gases—boards can develop contact failure issues. Many may have resolved the problem by replacing the boards, but the cost of purchasing new boards can be significant, especially for certain imported equipment. Instead, try using an eraser to repeatedly clean the gold fingers; after cleaning off any dirt, try powering the device again, and it might just solve the problem! This method is simple and practical.
7. Analysis of Intermittent Electrical Faults
Various intermittent electrical faults can generally be categorized into the following situations based on probability:
1. Poor contact
Poor contact between the board and slot, intermittent internal breaks in cables, poor contact at connectors and terminals, and cold solder joints all fall into this category;
2. Signal interference
For digital circuits, faults may only present under specific conditions, possibly due to excessive interference affecting the control system, or due to changes in individual component parameters or overall performance parameters on the circuit board, leading to a critical point in anti-interference capability, thus causing faults;
3. Poor thermal stability of components
Based on extensive repair practice, electrolytic capacitors are the most notable for poor thermal stability, followed by other capacitors, transistors, diodes, ICs, resistors, etc.;
4. Moisture and dust on the circuit board
Moisture and dust can conduct electricity, exhibiting resistance effects, and during thermal expansion and contraction, the resistance value may change. This resistance can parallel other components, and if this effect is strong enough, it can alter circuit parameters, causing faults;
5. Software is also a consideration
Many parameters in circuits are adjusted using software, and if certain parameter margins are set too low, they may fall within critical ranges. When the machine operates under conditions that meet the software’s criteria for fault detection, alarms may trigger.
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