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228 ⏐⏐⏐ SERIES-PARALLEL NETWORKS
S
S P P
As noted above in the highlighted statement, the more the current
drain, the greater the change in terminal voltage with the application of
the load. This is certainly verified by the fact that I L 2 is about 33.5 times
larger with the 20-� loads.
The next example is a design exercise. The voltage and current ratings
of each load are provided, along with the terminal ratings of the
supply. The required voltage divider resistors must be found.
EXAMPLE 7.11 Determine R 1, R 2, and R 3 for the voltage divider supply
of Fig. 7.36. Can 2-W resistors be used in the design?
I s = 50 mA
E 72 V
R 1
R 2
R 3
a
b
R L2
10 mA
+
–12 V
20 V
–
R L1
20 mA
+
60 V
FIG. 7.36
Example 7.11.
Solution: R3: R3 � � VR
3
� � �
IR3 VR
12
V
3
� � �� � 240 �
Is
50
mA
PR3 � (IR3 ) 2 R3 � (50 mA) 2 240 ��0.6 W < 2 W
R1: Applying Kirchhoff’s current law to node a:
Is � IR1 � IL1 � 0
and IR1 � Is � IL1 � 50 mA � 20 mA
� 30 mA
R1 � � VR
1
� ��
IR1 VL � V
1
L2 60 V � 20 V
� ��� �
IR
30 mA
1
� 1.33 k�
PR1 � (IR1 ) 2 R1 � (30 mA) 2 1.33 k��1.197 W < 2 W
R2: Applying Kirchhoff’s current law at node b:
IR1 � IR2 � IL2 � 0
and IR2 � IR1 � IL2 � 30 mA � 10 mA
� 20 mA
R2 � � �1 k�
PR2 � (IR2 ) 2 R2 � (20 mA) 2 40 V
�
30 mA
VR2 20 V
� �
IR2 20 mA
1 k��0.4 W < 2 W
Since PR1 , PR2 , and PR3 are less than 2 W, 2-W resistors can be used for
the design.
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