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.
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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.