Series-Parallel Circuits

When electrical components are wired together one after another so that there is only one path for current flow, they are said to be wired “in series.” When they are wired together side by side, so there is more than one path for current to flow, they are said to be wired “in parallel.” A series-parallel circuit is made of both a series circuit and a parallel circuit. The series circuit can be before or after the parallel portion of the circuit. Series-parallel circuits can be analyzed using the same electrical laws that apply to separate series or parallel circuits; you just have to apply the series laws to the series portion and the parallel laws to the parallel portion.

FIGURE 36-33
Typical dash light circuit using a variable resistor in series with bulbs connected in parallel to each other.

A circuit for dash lights is an example of a series-parallel circuit in an automotive application Figure 36-33. In this situation, a variable resistor is connected in series with a number of dash lights, which are connected in parallel to each other. When the variable resistor is turned, the resistance changes. As the resistance increases, the voltage drop across the potentiometer increases and the current flow decreases. This lowers the voltage and current flow to the dash lights, making the dash lights dimmer. When the variable resistor is turned such that its resistance is lowered, the voltage drop across it is reduced and the current flow in the circuit increases, making the dash lights brighter. Since the dash lights are in parallel, they each receive the same amount of voltage, and the total circuit current flow is divided equally between them, making them equally bright as one another.