Parallel Circuits

Automotive brake light systems use parallel circuits. A parallel circuit is different than a serial circuit because in a parallel circuit if one of the bulbs, or anything else in the circuit fails the other bulbs will function normally. This is not true in a series circuit. In a series circuit if one of the bulbs burned out or any of the components of the circuit fails the entire circuit is rendered inoperable.

So if one bulb goes out all the others still work. This is because in a parallel circuit current can flow through more than one branch at a time. If one of the branches fails and creates an open, the energy will travel to the next branch. In a parallel circuit voltage applied to each branch is the same. So each bulb in the brake light circuit receives 12 volts regardless of how many bulbs are added.

Each item added to a parallel circuit will decrease the resistance in turn increasing the current. Current flows through all of the bulbs regardless of their resistance. Remember Ohm's Law (I=E/R) whenever working with electrical circuits. Always be careful and check specifications before adding additional items to a factory system. Adding items ends up lowering resistance. This increases the current (amperage) and can damage components or the entire circuit.