Resistor Types and Characteristics Explained

Resistors are fundamental electronic components classified into two main groups: fixed resistors and variable resistors. Understanding their types and characteristics is crucial for circuit design and analysis.

Fixed Resistors

Fixed resistors possess a unique, constant resistance value between their terminals. They are designed to maintain a specific resistance under normal operating conditions.

Types of Fixed Resistors

• Wire-Wound Resistors

  Manufactured by winding a resistive wire (often an alloy) around a ceramic cylinder. These are typically used for power applications (e.g., >2W) due to their ability to dissipate significant heat.

• Carbon Film Resistors

  Produced by depositing a thin film of carbon pyrolytically onto a ceramic cylinder. A small spiral is then cut into the film to achieve the desired resistance. Terminals are attached using metal caps for insertion or assembly.

Characteristics of Fixed Resistors

• Nominal Value

  This is the stated ohmic value of the resistor, often indicated by screen printing directly on the component or by a series of four or five colored rings. For color-coded resistors, the value is read from left to right.

• Power Rating

  The power rating specifies the maximum amount of electrical power (work done by current per unit time) the resistor can safely dissipate without damage. This is manifested as heat emission and is expressed in watts.

• Tolerance

  Tolerance indicates the permissible range of deviation from the nominal resistance value. It represents the accuracy guaranteed by the manufacturer for the resistor in question.

Variable Resistors

Variable resistors are components whose resistive value can be intentionally changed by the action of an external agent. This agent could be light, mechanical movement, temperature, or voltage.

Types of Variable Resistors

• Potentiometers

  Potentiometers are variable resistors whose resistive value changes through the mechanical action of an arm or wiper moving across a resistive element. They typically have three terminals: two fixed terminals connected to the ends of the resistive track, and one movable terminal connected to the wiper.

  • Installation: Can be mounted on a chassis or a printed circuit board (PCB).
  • Materials: Common resistive materials include carbon or metallic wire.
  • Resistance Taper: Can have a linear or logarithmic change in resistance.

• Photoresistors (LDRs)

  Light-Dependent Resistors (LDRs) are components whose resistive value varies inversely with the intensity of light they are exposed to. As light intensity increases, their resistance decreases.

• Thermistors (Temperature-Variable Resistors)

  Thermistors are components specifically designed to change their electrical resistance in response to variations in temperature. They are widely used in temperature sensing and control applications.

  • NTC Thermistors

    Negative Temperature Coefficient (NTC) thermistors are variable resistors whose resistance decreases as the temperature increases.

  • PTC Thermistors

    Positive Temperature Coefficient (PTC) thermistors are variable resistors whose resistance increases as the temperature increases.

• Varistors (Voltage-Dependent Resistors)

  Varistors are nonlinear resistors whose resistivity (and thus resistance) decreases significantly as the voltage across them increases beyond a certain threshold. They are commonly used for surge protection.