Core principle: Measure the resistance value to ensure it is within its nominal value and allowable tolerance range, while observing its physical state and functional performance in the circuit.
Testing steps and methods:
Appearance inspection (preliminary screening):
Visual observation: Carefully inspect the resistor body for any obvious physical damage, such as:
Burnt/discolored: The surface or near the solder pads at both ends may appear burnt black, yellow, or discolored (usually due to overheating caused by overcurrent or overvoltage).
Crack/fracture: Cracks or complete fracture of the resistor or end cap (due to excessive mechanical or thermal stress).
Abnormal solder joints: virtual soldering, cold soldering, excessive soldering causing bridging, and pad detachment (poor soldering or external damage).
Packaging bulging/deformation: Very rare, but if present, it indicates severe internal failure.
Conclusion: If any obvious physical damage (especially burning or cracking) is found, it can be basically determined that the resistor has been damaged and needs to be replaced. If the appearance is normal, proceed to the next step of electrical measurement.
Static resistance measurement during power outage (core step):
Key requirement: At least one end of the resistor must be disconnected (soldered off) from the circuit board, or the entire circuit board must be completely powered off during measurement, and the circuit where the resistor is located must be isolated from other components (especially parallel components). Otherwise, the parallel path will seriously interfere with the measurement results, resulting in readings far below the actual resistance value.
Choose the appropriate measuring tool:
Digital multimeter: It is a commonly used tool.
Select the lowest ohm range (such as 200 Ω range or lower).
Attention to accuracy: Alloy resistors typically have low resistance values (ranging from milliohms to a few ohms). The low resistance range accuracy and resolution of a regular multimeter may be insufficient, especially for milliohm resistors. Pay attention to the significant digits and decimal point when reading.
Millimeter/Micro ohmmeter: Highly recommended for precise measurement of low resistance alloy resistance. These instruments are designed for high-precision measurement of low resistance and typically use a four wire system (Kelvin connection) to eliminate the influence of test line resistance and contact resistance.
Measurement method:
Connect the instrument probe or Kelvin clip tightly and firmly to both ends of the resistor (ensuring good contact).
Read a stable resistance value.
Judgment criteria:
Compare the measured value (R_measured) with the nominal value (R_nominal) and allowable tolerance (tolerance, such as ± 1%, ± 5%) of the resistance.
Qualified: R_measured is within the range of [R_nominal (1 Tolerance), R_nominal (1+Tolerance)].
Example: If the nominal value is 0.1 Ω± 1%, the qualified range is approximately 0.099 Ω and 0.101 Ω.
Exception:
Open circuit: The instrument displays "OL" (overload) or a maximum resistance value (several orders of magnitude higher than the nominal value) indicating that the resistor has burned out.
Short circuit: Resistance close to 0 Ω (far below the nominal value and far below the lower tolerance limit), severe internal or external short circuit (rare, but may be accompanied by physical damage).
Resistance drift: The measured value exceeds the tolerance range but the non open/short circuit resistance performance deteriorates (such as overloading causing damage to the alloy layer).
Conclusion: Static resistance is the most direct basis for judging good or bad. Exceeding the tolerance range can be considered as a defect.
Functional verification under powered on state (auxiliary judgment):
Prerequisite: The static resistance measurement is normal, but there are still issues with the circuit function (such as inaccurate current detection, protection circuit misoperation, etc.).
Method:
Solder the resistor back to the circuit board as it is.
When powered on, use an oscilloscope or high-precision voltmeter to:
Measure the voltage drop (V_drop) across the resistor.
Simultaneously (or as known) measure the current (I) flowing through the resistor.
Calculate actual resistance value: R_calculated=V_drop/I
Comparison: Compare R_calculated with nominal and static measurement values.
Judgment criteria:
Normal: The calculated value is close to the static measurement value and nominal value (within tolerance), and the voltage waveform is clean (without abnormal oscillation).
Exception:
Abnormal resistance calculation: If there is a significant deviation from the static or nominal value (especially when the current is high), it may indicate that the resistance is experiencing excessive temperature drift under load (although the temperature drift of alloy resistors is usually small) or that there is poor contact (such as solder joint cracking) causing unstable resistance.
Abnormal heating: After being powered on for a period of time, if the resistance becomes abnormally hot (exceeding the expected temperature rise), it may indicate that it has been subjected to excessive power (due to insufficient design margin or circuit failure causing overcurrent), which can accelerate deterioration or directly burn out over time.
Abnormal voltage waveform: The oscilloscope displays spikes, oscillations, or distortions in the voltage waveform, which may indicate that the resistor itself (or its degradation) has introduced a problem at a specific frequency, but it is more likely to be a problem with other parts of the circuit.
Conclusion: Power on verification can identify dynamic performance issues that are difficult to capture through static measurements (such as changes in resistance under high current, poor contact) or overload risks. Abnormal heating or significant deviation in calculated resistance values are signs of poor performance. The abnormal voltage waveform requires further analysis.
Temperature coefficient considerations (advanced/specific requirements):
An important advantage of alloy resistors is their extremely low temperature coefficient (TCR).
If the application requires extremely high stability of resistance with temperature changes (such as precision measurement), static resistance measurements can be repeated at different ambient temperatures (or local heating/cooling of the resistor) to observe whether the changes exceed the TCR range calibrated in the specification book.
Conclusion: TCR exceeding the standard indicates insufficient stability of the resistance during temperature changes, which is considered poor in precision applications.
Substitution method (ultimate verification):
When the above methods still cannot determine the quality of the resistance, or suspect that it is the only/main cause of circuit failure, you can try replacing it with a known good, new alloy resistor of the same specification.
Conclusion: If the circuit function returns to normal after replacement, it can be basically confirmed that the original resistor is faulty.
Summarize and judge the logical chain:
Appearance: Is there any physical damage? → Yes=bad; No → Next step.
Power off resistance measurement: (must be isolated for measurement!)
Open circuit or short circuit? → Yes=bad;
Is the resistance within the tolerance range? → Yes → Preliminary judgment is good; No=bad.
Power on verification (if static is good but the circuit is still abnormal):
Abnormal/unstable resistance calculation? → Yes=bad;
Abnormal fever? → Yes=bad or about to break;
Normal? The resistor itself may be fine, check other parts of the circuit.
(Optional) Checking temperature drift: TCR exceeding the standard? → Yes=bad (for high-precision applications).
(Difficult and complicated symptoms) Replacement method: Is the circuit better after replacing with a new one? → Yes=The original resistor is broken.
Key precautions:
Safety first: power-off measurement! Be careful when conducting electrical measurements to avoid short circuits and electric shock.
Accuracy matching: Choose measurement tools with appropriate accuracy, especially for milliohm level resistors.
Four wire system priority: For precise measurement of low resistance values, try to use the four wire system function of a milliohmmeter or multimeter as much as possible.
Isolation measurement: Static measurement must ensure that the resistance is disconnected from other parts of the circuit, which is the most common mistake.
Specification reference: Always refer to the official datasheet of the resistor to check its nominal value, tolerance, rated power, TCR and other parameters.
Understanding application: Judging good or bad ultimately serves the functionality of the circuit. A resistor with a resistance value within the tolerance, if its power, voltage, TCR and other parameters do not meet the actual circuit requirements, is also considered "bad" (not suitable).
By following the above logical and step-by-step inspection method, combined with necessary tools and careful operation, the quality of alloy resistance can be effectively judged.