A series-parallel circuit combines characteristics of both types of circuits. Some of the resistors are in series, and some are in parallel. This type of circuit is more common in electronic equipment than in the circuits used to supply electrical power for agricultural equipment.
Study Figure 26.3. In circuit A, all of the electricity must pass through the 2.5-Q resistor, but then it has two alternative paths. Part of the current will pass through the 4.0 Q resistor and the rest will pass through the 3.2 Q resistor. Circuit B is the same circuit, just drawn differently so that it is easier to see the relationship between the three resistors.
To solve for the total resistance of a series-parallel circuit, start by reducing the parallel resistors. The circuit then acts as a series circuit, and the resistances can be added. It is helpful to redraw the circuit after each pair of parallel resistors is condensed. To determine the total resistance for Figure 26.3, the first step is to find the equivalent resistance of the parallel branch of the circuit. Either parallel circuit equation can be used. The equation using pairs of resistances is considered easier to use.
The resistance of the parallel branch of the circuit has an equivalent resistance of 1.8 Q. The next step is to combine this equivalent resistance with the remaining resistors in the circuit.
Figure 26.4 shows that the series-parallel circuit can be reduced to a series circuit. The total resistance of this circuit is:
Rt = 2.5 Q + 1.777. ..Q = 4.3 Q The series-parallel circuit has a total resistance of 4.3 Q.
Determining Voltage and Amperage in Circuits 335
FIGURE 26.4. Equivalent series resistance of Figure 26.3.
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