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172 ⏐⏐⏐ PARALLEL CIRCUITS<br />

EXAMPLE 6.3 Determine the total resistance for the network of<br />

Fig. 6.8.<br />

RT RT R1 = 2 Ω R2 4 Ω<br />

R3 = 5 Ω<br />

R1 2 Ω R2 4 Ω<br />

Solution:<br />

1 1 1 1<br />

�� ������ RT<br />

R1 R2 R3<br />

1 1 1<br />

�������0.5 S � 0.25 S � 0.2 S<br />

2 � 4 � 5 �<br />

� 0.95 S<br />

1<br />

and RT ���1.053 �<br />

0.95 S<br />

The above examples demonstrate an interesting and useful (for<br />

checking purposes) characteristic of parallel resistors:<br />

The total resistance of parallel resistors is always less than the value<br />

of the smallest resistor.<br />

In addition, the wider the spread in numerical value between two parallel<br />

resistors, the closer the total resistance will be to the smaller resistor.<br />

For instance, the total resistance of 3 � in parallel with 6 � is 2 �,<br />

as demonstrated in Example 6.1. However, the total resistance of 3 � in<br />

parallel with 60 � is 2.85 �, which is much closer to the value of the<br />

smaller resistor.<br />

For equal resistors in parallel, the equation becomes significantly<br />

easier to apply. For N equal resistors in parallel, Equation (6.2) becomes<br />

1 1 1 1<br />

�� � �� � �R � � �R � � . . . 1<br />

� �� R R<br />

R<br />

T<br />

1<br />

� N� �<br />

� R<br />

=<br />

FIG. 6.8<br />

Example 6.3.<br />

⎬<br />

⎪<br />

⎪<br />

⎪<br />

⎭<br />

R<br />

and RT � �� (6.3)<br />

N<br />

In other words, the total resistance of N parallel resistors of equal value<br />

is the resistance of one resistor divided by the number (N) of parallel<br />

elements.<br />

For conductance levels, we have<br />

N<br />

G T � NG<br />

⎫<br />

⎪<br />

⎪<br />

⎪<br />

R 3<br />

P<br />

5 Ω<br />

(6.4)

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