Seriesparallel Circuits

In a series-parallel circuit, current flows through the series portion of the circuit and then splits to flow through the parallel branches of the circuit. Some components are wired in series, others in parallel. Most automotive circuits are seriesparallel, and the same relationship between voltage, current, and resistance exists.

Use of Ohm's Law

Applying Ohm's Law to series-parallel circuits is a matter of simply combining the rules seen for series circuits and parallel circuits. First, calculate the equivalent resistance of the parallel loads and add it to the resistances of the loads in series.

The total resistance is then divided into the source voltage to find current. Voltage drop across series loads is current times resistance. Current in branches is voltage divided by resistance. For calculation examples, see page 6.

When troubleshooting a series-parallel circuit, problems in the series portion can shut down the entire circuit while a problem in one leg of the parallel portion may or may not affect the entire circuit, depending on the problem. Very high resistance in one leg would reduce total circuit current, but increase current in other legs. Very low resistance in one leg would increase total circuit current and possibly have the effect of bypassing other legs.

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