How to efficiently diagnose electronic component failures?

Although electrical equipment contains numerous electronic components internally, its failure phenomena exhibit certain regularities.

As the most numerous component in electrical equipment, resistors are not necessarily the parts with the highest failure rate. Among the various failure modes of resistors, open-circuit failures are the most common, while increases in resistance value are relatively rare, and decreases in resistance value are extremely uncommon. Among the many types of resistors, carbon-film resistors and metal-film resistors are widely used. The failure characteristics of these two types of resistors are as follows: First, resistors with low resistance values (below 100Ω) and high resistance values (above 100kΩ) are more likely to fail, whereas resistors with intermediate resistance values (such as several hundred ohms to tens of kiloohms) rarely fail. Second, when low-resistance resistors fail, they typically show signs of burning and discoloration, making the failure easily detectable; in contrast, failures in high-resistance resistors tend to be less noticeable. Wirewound resistors, commonly used for current-limiting in high-current applications, have relatively low resistance values. When cylindrical wirewound resistors burn out, some may exhibit blackening, surface blistering, or cracking, while others may show no obvious signs of damage. Cement resistors, a type of wirewound resistor, may break upon burnout, but otherwise usually leave no visible traces of damage. As for fuse resistors, some may have a piece of their surface blown off upon burnout, while others show no obvious signs of damage—but under no circumstances will they exhibit signs of burning and blackening. In summary, when inspecting resistors, one can use these characteristic features to perform targeted screening, thereby quickly identifying damaged resistors.

Electrolytic capacitors are widely used in various electrical and electronic devices, yet they have a relatively high failure rate. The types of damage to electrolytic capacitors include: first, complete loss or significant reduction in capacitance; second, mild or severe leakage current; and third, loss or reduction in capacitance accompanied by leakage current. When inspecting damaged electrolytic capacitors, the following points should be carefully noted:

(1) Observation: When some capacitors are damaged, they may leak electrolyte, leaving an oily film on the surface of the circuit board at the bottom of the capacitor as well as on the exterior of the capacitor. Such capacitors must never be used again. Additionally, some capacitors will swell after being damaged; these capacitors also should no longer be put into service.

(2) Touching: After the device is powered on, some electrolytic capacitors with severe leakage may become hot—so hot that they can even burn your hand. In such cases, you must replace the affected capacitors.

(3) Electrolytic capacitors contain electrolyte internally. Prolonged exposure to heat can cause the electrolyte to evaporate, thereby reducing the capacitance. Therefore, during inspection, special attention should be paid to capacitors located near heat sinks and high-power components— the closer they are, the greater the likelihood of damage.

II. The primary characteristics of damage to semiconductor devices such as transistors are PN junction breakdown or open circuits; among these, short-circuit breakdown is relatively common. In addition to this, there are two other types of failure modes: one is a decline in thermal stability, which manifests as normal operation at startup but results in soft breakdown after the device has been running for a certain period; the other is a deterioration in PN junction characteristics. When measured with a multimeter on the R×1k range, each PN junction appears normal, yet the device fails to function properly once installed and powered on. However, if the R×10 or R×1 range is used for measurement, it will be found that the forward resistance of the PN junctions is higher than normal.

For the measurement of diodes and transistors, a pointer-type multimeter can be used for in-circuit testing. A more accurate method involves setting the multimeter to the R×10 or R×1 range (typically R×10 is preferred; if it’s difficult to observe, you can switch to the R×1 range) and measuring the forward and reverse resistance of the diode or transistor’s PN junction. If the forward resistance is relatively low (compared to its normal value) and the reverse resistance is sufficiently high (relative to the forward value), this indicates that the PN junction is functioning properly. Conversely, if the measurements do not meet these criteria, there is reason to suspect damage, and the component should be removed from the circuit and measured again. This is because, in most circuits, the external resistors surrounding diodes and transistors typically have resistances ranging from several hundred to several thousand ohms or higher. When using the multimeter’s low-resistance range for in-circuit measurements, the influence of these external resistors on the PN junction’s resistance can generally be disregarded.

The characteristic of integrated circuit damage lies in its complex internal structure and multifunctionality: any damage to even a single component can render the entire circuit unable to function properly. Integrated circuit damage can be broadly categorized into two types: complete failure and poor thermal stability. In cases of complete failure, the integrated circuit can be removed and compared with a normally functioning circuit of the same model. By measuring the forward and reverse resistance of each pin relative to ground, one can often detect pins with abnormal resistance values. As for integrated circuits with poor thermal stability, during equipment operation, use anhydrous alcohol to cool down the suspected circuit. If the fault is delayed or no longer occurs after cooling, it can be concluded that the issue stems from thermal stability problems. Typically, the only way to resolve such issues is by replacing the integrated circuit with a new one.