Parallel Circuits

Parallel circuits are commonly used in automotive lighting circuits.  A parallel circuit has several unique characteristics:

• There is more than one path for electric current.
• The total amount of electric current is equal to the sum of the individual branch currents.
• The source voltage will be dropped across all of the electrical loads in each branch of the circuit.  There can be more than one load in each branch.
• An open anywhere in a branch will stop the current in that branch, but leave the other branch unaffected.
• The total circuit resistance is less than the resistance of the resistive loads of any single branch. 

  Example One: The resistance in the circuit shown below circuit must be less than 6 ohms.  It is actually 3 ohms.  There are several ways of calculating the resistance in a parallel circuit:

  • The easiest way is to divide the voltage by the total current, 12V / 4A = 3 ohms.
  • If there are only two parallel branches, divide the product (multiply) of the resistances by the sum (add) of the resistances.  For example (6 x 6) / (6 + 6) = 36 / 12 = 3 ohms.
  • If there are more than two branches,  take the inverse of the sum of the inverses of each branch resistance.  For example 1 / ((1 / 6) + (1 / 6)) = 3 ohms.

Example Two: The resistance in the circuit shown below circuit must be less than 6 ohms.  It is actually 3.43 ohms.  There are several ways of calculating the resistance in a parallel circuit:

• The easiest way is to divide the voltage by the total current, 12V / 3.5A = 3.43 ohms.
• If there are only two parallel branches, divide the product (multiply) of the resistances by the sum (add) of the resistances.  For example (8 x 6) / (8 + 6) = 48 / 14 = 3.43 ohms.
• If there are more than two branches,  take the inverse of the sum of the inverses of each branch resistance.  For example 1 / ((1 / 8) + (1 / 6)) = 3.43 ohms.