Basic Guide To Resistor Selection

Step 5 - Environmental and Advanced Factors

For the majority of electronic designs, environmental factors are barely worth mentioning. Circuits operating in laboratories, homes, offices, or other controlled indoor spaces live very comfortable lives. Temperatures are stable, humidity is reasonable, vibration is negligible, and atmospheric pressure never changes enough to matter. In these situations, a correctly specified resistor will usually perform exactly as expected for its entire service life, and no further thought is required.

However, that assumption stops being valid the moment a design moves into harsher application spaces, including automotive, industrial, medical, and aerospace environments. In these domains, environmental factors are not edge cases, but are primary design constraints as they impose stresses that push the limits of passive components rapidly. Ignoring these environmental factors does not always cause instant failure, which itself makes the problem worse. Instead, it leads to gradual drift, intermittent faults, and reliability issues that only appear after deployment.

Operating Temperature

Operating temperature is almost always the first consideration, as all resistors change resistance with temperature, regardless of technology (this factor is described by the temperature coefficient). Furthermore, as ambient temperature increases, a resistor’s ability to dissipate heat also decreases. Power ratings for resistors (and all components in general) are defined at specific reference temperatures, typically around room temperature, and must be derated as temperatures rise. A resistor that is comfortably within limits on the bench can exceed its safe operating temperature in an engine bay or industrial enclosure without any change in electrical load. To aid engineers, datasheet derating curves are often provided to prevent this exact mistake and should be treated as mandatory reading rather than optional detail.

Moisture and Humidity

Moisture and humidity introduce another class of problems. Water ingress alters surface resistance, promotes corrosion of terminations, and degrades resistive films over time. Some resistor technologies tolerate moisture much better than others, but none are entirely immune. In wet or high-humidity environments, relying on component choice alone is rarely enough, and system-level mitigation such as conformal coating, potting, or sealed enclosures is usually far more effective.

Mechanical Stress

Mechanical stress also plays a major role in demanding environments, as vibration and repeated shock can induce fatigue in solder joints and create micro-cracks in substrates or resistive elements. These failures often manifest as intermittent resistance changes that are difficult to reproduce in testing, making them hard to predict and track. Small surface mount resistors are particularly sensitive to board flex and vibration, especially on large or poorly supported PCBs. Through-hole resistors, with their compliant leads and larger mechanical mass, tend to perform better under sustained vibration. This is one of the few areas where older packaging technologies retain a clear advantage, and can still be found in modern high-end systems.

High acceleration and sustained g-forces add further complications during resistor selection and integration. Resistors must be mechanically secured to prevent movement, and lead strength becomes a real concern. In such applications (including aircraft, missiles, and munitions), resistor selection cannot be separated from PCB layout, mounting strategy, and mechanical reinforcement.

Atmospheric Pressure

Low atmospheric pressure environments, often found in aerospace applications, introduce more specialised issues. Reduced pressure lowers dielectric strength, increasing the risk of arcing at higher voltages. Certain materials may also outgas over time, releasing contaminants that can damage sensitive assemblies or optics. These are niche concerns, but when they apply, they are non-negotiable.

Environmental conditions vary widely, and there is no universally correct resistor choice. The key requirement is awareness. Engineers must understand where and how their design will operate and treat environmental stress as a first-order input. Resistors are simple components, but the environments they survive in often are not.