Resistor Tutorial

Resistors Tutorial and Summary

• 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.

I_total = I1 = I2 = I3 .... etc

• The total circuit resistance of series resistors is equal to:
• R_total = R₁ + R₂ + R₃ + ..... R_n etc.

• Total circuit voltage is equal to the sum of all the individual voltage drops.

V_total = 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.

V_S = V1 = V2 = V3 .... etc

• Total resistance of a parallel circuit is equal to:

• Total circuit current flow is equal to the sum of all the individual branch currents added together.
• I_total = 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.
• 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:

• In AC Circuits the voltage and current flowing in a pure resistor are always "in-phase" producing 0^o 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.