Introduction
Resistors, like capacitors and inductors, are fundamental electronic components used extensively in electrical and electronic equipment. They may appear ordinary to people working with electrical systems, but without resistors electrical or electronic circuits cannot be established. Resistors are therefore essential components.
Functions of Resistors
Resistors are passive components that provide a specific resistance. Their operation is based on Ohm's law: Voltage (V) = Current (I) × Resistance (R). The main functions are current control, voltage division, current sensing, and biasing.
Current control
Resistors limit the current in a circuit to below a rated value. For example, in an LED circuit, placing a resistor in series with the LED limits the current to prevent damage.
Voltage division
When two or more resistors are connected in series, the applied voltage is divided among them in proportion to their resistance values.
Current sensing
When current flows through a resistor, a voltage proportional to that current appears across its terminals. Measuring this voltage allows the current flowing in the circuit to be determined.
Biasing
Applying voltages to semiconductor devices such as transistors to establish operating points is called biasing. Bias voltages provided to the emitter, collector, and base of a transistor are often set with resistors.
In addition to the functions above, resistors are used as damping resistors, termination resistors, pull-up and pull-down resistors, and more.
Principle of Resistance
The resistance value of a resistor depends on the resistivity of the material, its cross-sectional area, and its length. As shown by the relation R = rho × L / S, where rho is resistivity, L is length, and S is cross-sectional area.
Reference values for the resistivity of common metals are provided for design considerations.
Resistor Terminology
Resistors are described by parameters that indicate specifications and ratings. These include various parameters related to value, tolerance, power rating, temperature coefficient, and more. Four fundamental parameters are highlighted in the referenced illustration.
Value and Tolerance Marking Standards
The resistor value and tolerance markings follow international standards such as IEC 60062: Marking codes for resistors and capacitors, and IEC 60063: Preferred number series for resistors and capacitors.
Resistor values are standardized into preferred number series (E-series). Values are not simple integers like 1 Ω, 2 Ω, 3 Ω, but commonly use values such as 2.2 Ω or 4.7 Ω according to the E-series. The E designation refers to exponent, and the series divides the range from 1 to 10 into equal-ratio steps. These standardized values make selection and proportional design easier in practical use.
Resistor Construction
The typical basic structures of resistors vary by application. In recent years, surface-mount resistors have become the mainstream choice in small devices.
Selecting Resistors
When selecting resistors for a design, choose parts that meet the PCB size and mounting constraints, required performance, and other specifications.
Selection by mounting method
Choose between surface mount or through-hole/terminal mounting. Surface mounting is increasingly common, but through-hole and screw-fixed resistors may still be required depending on circuit scale and specifications. Selection should be made with the overall PCB assembly method in mind; ideally the board should use a consistent mounting approach where possible.
Selection by performance, characteristics, and size
Consider rated power, accuracy, temperature characteristics, functional requirements, environmental resistance, heat dissipation, dimensions, and height. Beyond mounting method, resistor types differ in performance and dimensional constraints. For some demanding environmental conditions, options may be limited, requiring tradeoffs. Selection must consider performance, characteristics, size, and mounting method from a global design perspective.
Selecting SMD resistors
When choosing surface-mount resistors, satisfy the required performance and characteristics. The following general steps are recommended:
- Choose between a single SMD resistor or a compound SMD network.
- If single SMD, select between thick-film or thin-film SMD resistors.
- If compound SMD, choose a multi-element SMD network (independent circuits) or a resistor array (common connections).
- Choose the package shape based on operating voltage and power dissipation.
- When multiple candidate types meet basic requirements, select based on other performance criteria.
