The selection of resistors may seem simple, but it actually involves multiple key factors. Choosing the wrong one may lead to a decrease in circuit performance, heating, or even failure. The following are systematic rules and considerations for selecting resistors:
1、 Core electrical parameters
Nominal value: Select the specification closest to the calculated value from the standard series (E6, E12, E24, E48, E96, E192).
Accuracy (tolerance):
Ordinary circuit: ± 5% (E24), ± 1% (E96) is usually sufficient.
Precision circuits (reference source, ADC/DAC): require ± 0.5%, ± 0.1% or higher accuracy.
Calculation basis: Calculate theoretical values based on circuit topology (voltage division, current limiting, load, feedback, etc.) and Ohm's law, and then select according to the standard series.
rated power
Calculate power consumption: ` P=I ² R ` or ` P=V ²/R ` or ` P=V I `, calculate the actual power borne by the resistor.
Reduced usage: must be strictly followed! Select rated power>actual power consumption derating factor:
General occasions: Reduce the rating to 5070% (if the actual power consumption is 0.5W, choose a 1W resistor).
High temperature environment/high reliability requirement: derating to 3050%.
Heat dissipation considerations: High power resistors require consideration of heat dissipation fins and PCB copper foil heat dissipation design.
Maximum working voltage
Ensure that the maximum operating voltage of the resistor is greater than the actual peak voltage in the circuit.
Special attention should be paid to high-voltage applications (>250V) to avoid breakdown caused by insufficient creepage distance.
temperature coefficient
TCR: represents the degree of change in resistance with temperature (in ppm/° C).
Selection criteria:
Common application: ± 100ppm/° C~± 300ppm/° C (such as thick film surface mount resistors).
Precision applications (measurement, instrumentation): ± 5ppm/° C~± 25ppm/° C (such as metal foil resistors, precision film resistors).
Attention: Self heating caused by power consumption can also change the resistance value (temperature rise needs to be calculated in conjunction with TCR and power consumption).
2、 Environmental and reliability requirements
Working temperature range
The rated operating temperature range of the selected resistor should fully cover the application environment temperature (including self heating temperature rise).
Wide temperature resistors are required for harsh environments such as automotive electronics (40 ° C~+125 ° C/150 ° C) and industrial equipment (40 ° C~+85 ° C).
Environmental adaptability
Humidity/chemical environment: Choose moisture-proof and corrosion-resistant packaging (such as anti sulfurization thick film resistors, sealed resistors).
Mechanical stress: In high vibration situations, choose surface mount resistors or axial resistors with reinforced leads to avoid winding resistors.
Flame retardancy: Safety related circuits must comply with flame retardancy levels such as UL94 V0.
3、 Technology type and materials
Selection of Resistance Technology
Universal:
Surface mounted thick film resistor (low cost, small size, most widely used)
Carbon film/metal film axial resistance (via installation)
Precision/Low TCR:
Thin film resistor (high precision, low TCR)
Metal foil resistor (extremely high precision, extremely low TCR, best stability, high cost)
High power:
Winding resistance (high power, high inductance)
Metal oxide film resistor (high power, high temperature resistance, non inductive design optional)
High frequency/pulse:
Non inductive resistor (thin film, metal foil, special winding method)
Pay attention to the effects of parasitic inductance and capacitance.
4、 Frequency characteristics and parasitic parameters
High frequency applications
Parasitic inductance: The winding resistance has the highest inductance and the thin film resistance has the lowest. At high frequencies, the inductive impedance may exceed the resistance value.
Parasitic capacitance: Structural capacitance forms a bypass at high frequencies.
Countermeasure: Choose small-sized surface mount resistors (such as 0201, 0402), dedicated high-frequency resistors, or non inductive resistors.
Pulse load capacity
Instantaneous power may far exceed rated power (such as power start-up, surges, pulse circuits).
It is necessary to refer to the pulse power curve or single/repeated pulse endurance data of the resistor to ensure that it can withstand transient stress.
5、 Packaging and Process
Packaging form
Surface mount resistors: mainstream choices (0201, 0402, 0603, 0805, 1206, etc.), note that power and withstand voltage decrease with size reduction.
Axial lead resistance: through-hole plug, slightly better heat dissipation, suitable for high power or special requirements.
Power resistors: TO220, TO247, aluminum shell packaging, etc., requiring the installation of a heat sink.
procedure
Surface mount resistors need to adapt to the reflow soldering temperature curve (lead-free processes require higher heat resistance).
Axial resistance needs to adapt to wave soldering.
Avoid incompatibility between packaging materials (such as terminals) and solder.
6、 Cost and Supply Chain
cost optimization
Under the premise of meeting the requirements, priority should be given to selecting universal, standardized, and cost-effective models (such as thick film surface mount resistors with standard resistance values, common precision, and commonly packaged).
Avoid excessive design (such as using ± 5% for ordinary LED current limiting, no need for ± 0.1%).
Procurability and lifespan
Choose standard models from mainstream manufacturers to ensure long-term supply.
Pay attention to the device lifecycle and avoid using models that are about to be discontinued.
Summary of Selection Process
Clear requirements: circuit functionality, voltage, current, power consumption, accuracy, frequency, environmental conditions, reliability requirements, cost targets.
Calculate core parameters: theoretical resistance, actual power consumption, maximum voltage.
Preliminary screening:
Choose the technology type (thick film/thin film/winding, etc.).
Determine the packaging/installation method (patch/plug-in/power type).
Choose standard resistance (E series) and basic accuracy.
Key parameter review:
Is the power derating sufficient (most importantly!).
Is the maximum operating voltage met.
Does the working temperature range cover (including self heating).
Does TCR meet the temperature drift requirements.
Whether the high-frequency/pulse requirements are met (parasitic parameters, pulse capability).
Environmental and reliability confirmation: moisture resistance, corrosion resistance, sulfur resistance, flame retardancy, vibration resistance, etc.
Cost and supply chain confirmation: price, supply, lifecycle.
Application verification: Conduct actual circuit testing (temperature rise, stability, noise, etc.) when conditions permit.
Key reminder: Power derating is the first iron rule for selection! Even if other parameters are perfectly matched, insufficient power will inevitably lead to failure. It is necessary to conduct rigorous selection based on theoretical calculations, data manuals, and actual working conditions. For critical or harsh applications, it is recommended to consult the technical support of component manufacturers.