Resistors, also referred to as resistances, regulate the current flow in electronic circuits by limiting the amount of current that passes through a particular branch.
Resistors are commonly referred to simply as “resistances.” They are components designed to limit current flow; when integrated into a circuit, these resistors have a fixed resistance value, typically with two leads, and they restrict the amount of current that can pass through the branch to which they are connected. A resistor with an unchangeable resistance value is known as a fixed resistor. Those with variable resistance values are called potentiometers or variable resistors. An ideal resistor behaves linearly, meaning the instantaneous current passing through it is directly proportional to the applied instantaneous voltage.
Variable resistors used for voltage division have one or two movable metal contacts that press tightly against an exposed resistive element. distributor of electronic components of the contact determines the resistance between one end of the resistive element and the contact itself.
The relationship between terminal voltage and current in a resistor is defined by a specific function, reflecting the device’s ability to convert electrical energy into other forms of energy. This relationship is represented by the letter R, with the unit of measurement being ohms (Ω). Practical devices such as light bulbs, heating filaments, and resistors can all be represented as resistive elements.
The resistance value of a resistive component generally depends on factors such as temperature, material, length, and cross-sectional area. The physical quantity that measures the effect of temperature on resistance is the temperature coefficient, defined as the percentage change in resistance value for each 1°C increase in temperature. The primary physical characteristic of a resistor is its ability to convert electrical energy into heat energy, which is why it can also be considered a power-dissipating element; as current flows through it, internal energy is generated. In circuits, resistors commonly serve the roles of dividing voltage and current. As for signals, both alternating current (AC) and direct current (DC) signals can pass through a resistor.
Material composition
Composed of resistive material and structured in a specific form, resistors are two-terminal electronic components that serve to limit the flow of current within an electrical circuit. Those with unalterable resistance values are known as fixed resistors, while those whose resistance can be adjusted are referred to as potentiometers or variable resistors. In an ideal scenario, resistors are linear, meaning the instantaneous current passing through them is proportional to the instantaneous voltage applied across them. However, certain specialized resistors, such as thermistors, varistors, and other sensitive elements, exhibit a non-linear relationship between voltage and current.
Resistors are among the most frequently used components in electronic circuits, and they are typically organized into different series based on power rating and resistance value for circuit designers to choose from. Their primary role within circuits is to regulate and stabilize currents and voltages, functioning as current dividers and voltage dividers, as well as matching loads for circuit compatibility. Depending on the requirements of the circuit, resistors can also be employed for negative or positive feedback in amplification circuits, conversion between voltage and current, and as protective components against voltage or current overload during input surges. Additionally, when combined with capacitors, they form RC circuits, which can be used for oscillation, filtering, bypassing, differentiation, integration, and as timing elements.
To expand the range of resistance values and provide fine adjustments, cylindrical resistive elements use grooving methods, while planar resistive elements use etched serpentine patterns.
The voltage-current relationship at the terminals of a resistor is represented by its current-voltage characteristic curve. When the voltage and current are proportional (resulting in a linear characteristic), it is called a linear resistor; otherwise, it is referred to as a non-linear resistor.
The main parameters characterizing the resistor’s properties and characteristics include nominal resistance value and allowable deviation, rated power, load characteristic, and resistance temperature coefficient.
Nominal resistance value: It is the designed resistance value marked on the resistor using numbers or color codes. The units for resistance values are ohms (Ω), kilo-ohms (kΩ), mega-ohms (MΩ), and tera-ohms (TΩ).
Resistance values are manufactured according to standardized preferred number series, corresponding to allowable deviations. The marking methods for resistance values and allowable deviations include direct marking, color code marking, and alphanumeric symbol marking.
Allowable deviation: The maximum permissible deviation between the actual resistance value and the nominal resistance value, expressed as a percentage. Common tolerances include ±5%, ±10%, and ±20%. Precision resistors can have tolerances below ±1%, and high-precision resistors can reach 0.001%. Accuracy is determined by both the allowable deviation and irreversible resistance value changes.
Rated power: The maximum power dissipation allowed for the resistor during continuous operation at the rated temperature (highest ambient temperature). For each type of resistor, there is also a specified maximum working voltage. Even if the rated power is not reached, exceeding the maximum working voltage is not permissible, especially for higher resistance values. The symbol used to represent resistor power in circuit diagrams is shown in the provided figure.
Load characteristic: Resistors should not exceed their rated power when operating at temperatures below the rated temperature (tR). When the temperature exceeds tR, the load power must be reduced accordingly. Each type of resistor has specified load characteristics. Additionally, under low air pressure, the load is correspondingly reduced. For pulse loads, the average power is much lower than the rated power, and separate specifications exist.
Resistance temperature coefficient: It represents the average relative change in resistance value per 1℃ temperature change within the specified temperature range. It is expressed in parts per million per degree Celsius (ppm/°C). In addition to the mentioned parameters, other technical specifications include non-linearity (degree of deviation from linear relationship between current and applied voltage), voltage coefficient (relative change in resistance value per unit voltage change), current noise (effective value of noise potential generated by current flow in the resistive element, expressed using current noise index), high-frequency characteristics (relationship between resistance value and operating frequency due to distributed capacitance and inductance effects within the resistive element), and long-term stability (resistor’s irreversible changes in resistance value due to environmental conditions during prolonged usage or storage).