Resistor Tutorial Summary

Table of Contents

  • The job of a Resistor is to limit the current flowing through an electrical circuit.
  • Resistance is measured in Ohm’s and is given the symbol Ω
  • Carbon, Film and Wirewound are all types of resistors.
  • Resistor colour codes are used to identify the resistance and tolerance rating of small resistors.
  • The BS1852 Standard uses letters and is used to identify large size resistors.
  • Tolerance is the percentage measure of the accuracy of a resistor from its preferred value with the E6 (20%), E12 (10%), E24 (5%) and E96 (1%) series of tolerance values available.

Series Resistor Tutorial

  • Resistors that are daisy chained together in a single line are said to be connected in SERIES.
  • Series connected resistors have a common Current flowing through them.
  • Itotal = I1 = I2 = I3 …. etc

  • The total circuit resistance of series resistors is equal to:
  • Rtotal = R1 + R2 + R3 + ….. Rn etc.
  • Total circuit voltage is equal to the sum of all the individual voltage drops.
  • Vtotal = V1 + V2 + V3 …. etc

  • The total resistance of a series connected circuit will always be greater than the highest value resistor.

Parallel Resistor Tutorial

  • Resistors that have both of their respective terminals connected to each terminal of another resistor or resistors are said to be connected in PARALLEL.
  • Parallel resistors have a common Voltage across them.
  • VS = V1 = V2 = V3 …. etc

  • Total resistance of a parallel circuit is equal to:
  • parallel resistance equation
  • Total circuit current flow is equal to the sum of all the individual branch currents added together.
  • Itotal = I1 + I2 + I3 …. etc

  • The total resistance of a parallel circuit will always be less than the value of the smallest resistor.

Resistor Power Rating

  • The larger the power rating, the greater the physical size of the resistor to dissipate the heat.
  • All resistors have a maximum power rating and if exceeded will result in the resistor overheating and becoming damaged.
  • Standard resistor power rating sizes are 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W.
  • Low ohmic value power resistors are generally used for current sensing or power supply applications.
  • The power rating of resistors can be calculated using the formula:
  • resistor power equation
  • In AC Circuits the voltage and current flowing in a pure resistor are always “in-phase” producing 0o phase shift.
  • When used in AC Circuits the AC impedance of a resistor is equal to its DC Resistance.
  • The AC circuit impedance for resistors is given the symbol Z.

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