Theory- 6 :- Resister - Definition, Types And Classification

 What is a Resistor?

A resistor is an electrical component that opposes the flow of electric current in a circuit. Its opposition to current flow is called electrical resistance, and it's measured in ohms ($\Omega$). Resistors are fundamental in electronics, used to control current, divide voltage, and protect other components.

Resistor Symbols

Resistor symbols vary depending on the standard used in a circuit diagram. The two most common are the American and IEC (International Electrotechnical Commission) standards. A fixed resistor is shown as a zigzag line in the American standard and a rectangle in the IEC standard. Variable resistors, which can have their resistance adjusted, are typically represented with an arrow or a diagonal line through the symbol.

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Classification of Resistors

Resistors are broadly categorized into two types based on their behavior: Linear Resistors and Non-Linear Resistors.

1. Linear Resistors

These resistors have a resistance value that remains constant regardless of the voltage across them or the current flowing through them. This means they perfectly follow Ohm's Law ($V=I\times R$). They are further classified as fixed or variable. 

• Fixed Resistors: As the name suggests, these have a fixed resistance value that cannot be changed. They are used in circuits where a precise, unchanging resistance is needed. 

  
  
  

  • Carbon Film Resistors: Made by depositing a thin carbon film on a ceramic rod. They are inexpensive and widely used for general purposes. 

    
    
    

  • Metal Film Resistors: Created by depositing a metal film. They offer better tolerance and temperature stability than carbon film resistors. 

    
    
    

  • Wire-Wound Resistors: Made by winding a resistive wire around an insulating core. They are suitable for high-power applications. 

                                                             

    
    
    

• Variable Resistors: The resistance of these components can be adjusted manually. They have a resistive element and a sliding contact (wiper). 

  
  
  

  • Potentiometers: A three-terminal device used to vary resistance and act as a voltage divider. A common example is the volume control knob on a radio.

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  • Rheostats: A two-terminal device used to vary resistance to control the current flowing in acircuit.  

  • Trimmer Resistors: Small potentiometers used for fine-tuning circuits. They are often set once during assembly. 

    
    
    

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2. Non-Linear Resistors

These resistors do not follow Ohm's Law because their resistance changes based on external factors like voltage, temperature, or light. They are used for specialized sensing and protection applications.

• Thermistors: These are temperature-sensitive resistors. Their resistance changes significantly with temperature. 

  
  
  

  • NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases.

  • PTC (Positive Temperature Coefficient): Resistance increases as temperature increases.

• Varistors (Voltage-Dependent Resistors): 

  
  
  The resistance of a varistor is non-linearly proportional to the voltage applied across it. They are used for surge protection.

• Photoresistors (Light-Dependent Resistors or LDRs): 

  
  
  The resistance of an LDR decreases as the intensity of light falling on it increases. They are used as light sensors in applications like automatic street lights.