When selecting a resistors for a specific application, engineers need to consider several key characteristics to ensure proper functionality and reliability. Here are the key factors to consider when selecting a resistor type,
Resistance Value
Resistor value should match circuit requirements. The most accurate value can be chosen using available resistor charts (such as E12, E24, E48, E96, or E192), or a combination of multiple values may be used to achieve the desired accuracy ( need to take tolerance into consideration)
Tolerance
Accuracy of the resistor. Commonly available with tolerances of 1% (E96), 5% (E24), and 10% (E12). Tolerance can be determined by the final color band of a through-hole resistor. A lower tolerance will result in a higher unit price.
Power Rating
Ensure it can handle the maximum power required. Commonly available power ratings include 0.25W, 0.5W, 1W, 2W, 5W, and 25W.
Temperature Coefficient of Resistance (TCR)
Needs to be low for stable performance. It is measured in ppm/°C (parts per million per degree Celsius) (1 ppm = 0.0001%) and is defined as TCR = (R2– R1)/ R1 (T2– T1).
Voltage Rating
Must exceed the maximum circuit voltage. The voltage rating is not equal to the calculated voltage from the resistor’s wattage rating (W = VxI).
Package Type and Size
Commonly available in three types (Through-Hole Axial, Through-Hole Radial, and SMD), through-hole resistor sizes vary based on wattage, while SMD resistor sizes are determined by their value code format.
Stability and Aging
Stability and aging of resistors are influenced by various factors, primarily dependent on working temperature, environmental conditions (such as temperature, exposure to humidity, chemicals, etc.), resistor material properties, voltage stress, mechanical stress, and more.
Noise Performance
The noise of a resistor is characterized by its “noise figure” or “noise voltage“, measured in microvolts or nanovolts per square root of hertz (uV/√Hz or nV/√Hz). Manufacturers provide noise figures in resistor datasheets, typically in terms of noise voltage density (uV/√Hz) or noise power density (nW/Hz).
Frequency Response
Resistor frequency response matters for high-frequency applications. It’s indicated by the “capacitance” parameter, usually in picofarads (pF). Capacitance can limit signal quality and introduce phase shifts. Datasheets provide capacitance values at specified frequencies, aiding in choosing suitable resistors for precise signal integrity.
Cost and Availability
Factoring in material cost and its accessibility.
