RESISTOR : Fixed Value Resistor

Fixed - Value Resistor

  1. Definition and Function
    • A fixed - value resistor is a passive electrical component that has a constant resistance value. It is used to limit the flow of electric current in a circuit and to divide voltages.
    • According to Ohm's law , where  is the voltage across the resistor,  is the current flowing through it, and  is the resistance. The resistance of a fixed - value resistor does not change with the applied voltage or current (assuming the resistor is within its rated power and temperature limits).
  2. Construction and Types
    • Carbon Composition Resistors:
      • Made from a mixture of carbon powder and a binder. They are inexpensive and have a wide range of resistance values but have relatively low precision and high - temperature coefficients (meaning their resistance changes more with temperature compared to other types).
    • Metal Film Resistors:
      • Have a thin metal film (such as nichrome) deposited on a ceramic substrate. They offer better precision, lower - temperature coefficients, and lower noise compared to carbon composition resistors.
    • Metal Oxide Film Resistors:
      • Made with a metal oxide film on a ceramic core. They have good stability, high - power handling capabilities, and are suitable for high - voltage applications.
    • Wire - wound Resistors:
      • Constructed by winding a resistive wire (such as nichrome or manganin) around a ceramic or fiberglass core. They can handle high power but are generally more inductive due to the wire coil, which may limit their use in high - frequency applications.
  3. Resistance Value and Tolerance
    • The resistance value of a fixed - value resistor is indicated by colored bands (for through - hole resistors) or by printed values (for surface - mount resistors).
    • Color Coding:
      • For example, a resistor with the color bands brown, black, red, and gold has a resistance value of  with a tolerance of  (where brown = 1, black = 0, red = , and gold = ).
    • Tolerance:
      • It indicates the range within which the actual resistance value of the resistor can deviate from the nominal value. Common tolerances are , , and for more precise resistors,  or even lower.
  4. Power Rating
    • The power rating of a resistor indicates the maximum amount of power it can dissipate without being damaged. It is calculated using the formula  or 
    • Common power ratings for fixed - value resistors are , , , 1W, and higher for wire - wound resistors.
  5. Applications
    • Voltage Dividers:
      • Used to divide a voltage into smaller fractions. For example, in a simple voltage divider circuit with two resistors  and  connected in series across a voltage source , the output voltage 
    • Current Limiting:
      • In LED circuits, a resistor is used to limit the current flowing through the LED to prevent it from burning out. Given the forward voltage drop  of the LED and the supply voltage , the resistor value , where  is the desired current through the LED.
    • Biasing in Amplifier Circuits:
      • In transistor amplifier circuits, resistors are used to set the proper bias voltages and currents to ensure the transistor operates in the desired region (such as the active region for a bipolar transistor).
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