Failures Caused by Capacitor Damage Are the Most Common in Electronic Devices
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Capacitor damage manifests as: reduced capacitance; complete loss of capacitance; leakage; short circuit.
The role of capacitors in circuits varies, and the characteristics of the failures they cause are also distinct. In industrial control circuit boards, digital circuits account for the vast majority, with capacitors primarily used for power filtering, and fewer used for signal coupling and oscillation circuits. If the electrolytic capacitor used in a switching power supply is damaged, the switching power supply may fail to oscillate and have no voltage output; or the output voltage may not be well-filtered, leading to unstable voltage and logical confusion in the circuit, which manifests as the machine working intermittently or failing to start. If the capacitor is between the positive and negative terminals of the power supply in a digital circuit, the failure symptoms are the same.
This is particularly evident on computer motherboards, where many computers exhibit the phenomenon of sometimes failing to start after a few years of use. Opening the case often reveals bulging electrolytic capacitors. If you remove the capacitor and measure its capacitance, you will find it significantly 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 looking for faulty capacitors, priority should be given to those located near heat sources, such as near heat sinks and high-power components; the closer they are, the greater the likelihood of damage.
Characteristics and Identification of Resistor Damage
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Many beginners often struggle with resistors during circuit repairs, taking them off and soldering them repeatedly. However, with enough experience, if you understand the characteristics of resistor damage, you won’t need to go through such trouble.
Resistors are the most numerous components in electrical devices, but they are not the components with the highest failure rate. The most common failure mode for resistors is open circuit, while an increase in resistance value is less common, and a decrease in resistance value is very 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, and their failure characteristics are that low resistance values (below 100Ω) and high resistance values (above 100kΩ) have a higher failure rate, while intermediate resistance values (such as a few hundred ohms to tens of kilohms) rarely fail. Low-value resistors often show signs of burning and blackening when they fail, making them easy to identify, while high-value resistors usually show little to no signs of damage.
Wire-wound resistors are generally used for large current limiting and have low resistance values. When cylindrical wire-wound resistors fail, some may turn black or show surface peeling and cracks, while others may show no signs. Cement resistors are a type of wire-wound resistor, which may break when burned, otherwise showing no visible signs. Fusible resistors may have a piece of their surface blown off when they fail, but they will never be blackened. Based on the above characteristics, we can focus our checks on resistors that are likely to be damaged.
Based on the characteristics listed above, we can first check if there are any signs of burning on low-value resistors on the circuit board. Then, according to the fact that most resistors fail as open circuits or increase in resistance value, and that high-value resistors are prone to failure, we can use a multimeter to measure the resistance across high-value resistors on the circuit board. If the measured resistance is significantly higher than the nominal value, this resistor is definitely faulty (note that the resistance reading should stabilize before drawing a conclusion, as there may be parallel capacitive components in the circuit, resulting in a charging and discharging process). If the measured resistance is lower than the nominal value, it can generally be ignored. By measuring every resistor on the circuit board, even if we mistakenly identify a thousand, we will not miss a single one.
Methods for Judging the Quality of Operational Amplifiers
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Judging the quality of operational amplifiers can be quite challenging for many electronic repair technicians, not only due to educational background (many undergraduate students struggle to grasp the concepts, and even those who study frequency control with their mentors take a long time to understand!) Here, I would like to discuss this with everyone in the hope of helping you.
An ideal operational amplifier has the characteristics of “virtual short” and “virtual open,” which are very useful for analyzing linear applications of operational amplifier circuits. To ensure linear operation, the operational amplifier must work in closed-loop (negative feedback). Without negative feedback, the operational amplifier in open-loop becomes a comparator. To determine the quality of the device, one must first clarify whether the device is used as an amplifier or as a comparator in the circuit.
Regardless of the type of amplifier, there is a feedback resistor Rf. Therefore, during repairs, we can check this feedback resistor on the circuit. Use a multimeter to check the resistance between the output and inverting input terminals. If the resistance is absurdly high, such as several MΩ, we can be fairly certain that the device is used as a comparator. If this resistance is small (0Ω to several tens of kΩ), we should check if there is a resistor connected between the output terminal and the inverting input terminal; if so, it is definitely used as an amplifier.
According to the principle of virtual short in amplifiers, if this operational amplifier is functioning normally, the voltage at the non-inverting input and the inverting input must be equal, with any difference being in the millivolt range. Of course, in certain high input impedance circuits, the internal resistance of the multimeter may slightly affect the voltage measurement, but generally, it should not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be faulty!
If the device is used as a comparator, the non-inverting input and inverting input are allowed to differ. If the detected voltage does not comply with this rule, the device is definitely 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.
A Tip for Testing SMT Components with a Multimeter
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Some surface-mounted components are very small, making it inconvenient to test them with a standard multimeter probe. This is due to the risk of short circuits and the difficulty of accessing the metal parts of the component pins on insulated circuit boards. Here, I would like to share a simple method that will greatly facilitate testing.
Take two of the smallest sewing needles, place them against the multimeter probes, and then use a thin copper wire from a multi-stranded cable to tie the probes and sewing needles together, securing them with solder. This way, when testing SMT components with the fine-tipped probes, there will be no risk of short circuits, and the needle tips can pierce the insulation, allowing access to critical areas without the need to scrape off the coating.
Repair Methods for Short Circuit Faults in Common Power Supplies on Circuit Boards
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In circuit board repairs, encountering short circuit faults in common power supplies can be daunting because many components share the same power supply, and each component using this power supply is suspect of having a short circuit. If there are not many components on the board, using a “broad search” approach can eventually locate the short circuit point. However, if there are too many components, relying on luck may not yield results. Here, I recommend a relatively effective method that can help you quickly find the fault point.
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 power supply is not expensive, 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. Apply this voltage to the power supply points of the circuit, such as the 5V and 0V terminals of a 74 series chip. Depending on the severity of the short circuit, gradually increase the current. When you touch a component that becomes noticeably hot, that is often the damaged component, which can then be removed for further measurement and confirmation. Of course, during operation, the voltage must not exceed the operating voltage of the components, and it must not be connected in reverse, or other good components may be damaged.
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