Resistors

• Types of Resistors


  • Wire-wound resistors
  • Carbon-composition resistors
  • Film-type resistors
  • Surface-mount resistors (chip resistors)
  • Fusible resistors
  • Thermistors
• Wire-wound resistors


  • Typically used in high current applications with low resistance and appreciable power
• Carbon-composition resistors


  • Made of carbon or graphite mixed with a powdered insulating material
  • Metal caps with tinned copper wire (called axial leads) are joined to the ends of the carbon resistance element
• Carbon film resistors


  • Compared to carbon composition resistors, have tighter tolerances, are less sensitive to temperature changes and aging, and generate less noise
• Metal film resistors


  • Have very tight tolerances, are less sensitive to temperature changes and aging, and generate less noise
• Surface-mount resistors (chip resistors)


  • Temperature-stable and rugged
  • Their end electrodes are soldered directly to a circuit board
  • Much smaller than conventional resistors with axial leads
  • Power dissipation rating is usually 1/8 to 1/4 W
• Fusible resistors


  • Wire-wound resistors made to burn open easily when the power rating is exceeded
  • Serve a dual function as both a fuse and a resistor
• Thermistors


  • Temperature-sensitive resistors whose resistance value changes with changes in operating temperature
  • Used in electronic circuits where temperature measurement, control, and compensation are desired
• Resistance, R


  Provides the required reduction in current or the desired drop in voltage
• Power rating, P


  Indicates the amount of power the resistor can safely dissipate as heat
• The wattage rating is always more than the actual amount of power dissipated by the resistor, as a safety factor
• Resistor color coding


  • Colors represent numerical values
  • Standardized by the Electronic Industries Alliance (EIA)
• Resistor color code


  • Band A: first digit
  • Band B: second digit
  • Band C: decimal multiplier
  • Band D: tolerance
• Tolerance


  The amount by which the actual R can differ from the color-coded value
• Tolerance is usually stated in percentages
• Rheostats


  Two-terminal variable resistances used to vary the amount of current in a circuit
• Potentiometers


  Three-terminal variable resistances used to vary the amount of voltage in a circuit
• A potentiometer may be used as a rheostat by simply using the wiper terminal and one of the other terminals, the third terminal is left unconnected and unused
• Resistors can become open or they can drift out of tolerance
• Some controls (especially volume and tone controls) may become noisy or scratchy-sounding, indicating a dirty or worn-out resistance element
• Resistors can short out internally, or become short-circuited by another component in the circuit
• Resistance measurements are made with an ohmmeter
• The ohmmeter has its own voltage source, so voltage must be off in the circuit being tested. Otherwise the ohmmeter may become damaged