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252 ⏐⏐⏐ SERIES-PARALLEL NETWORKS
40 mA
180 V
E
E
12 V
40 V
R 1
R 2
R 3
R 4
R 5
20 mA
R L2
R L3
FIG. 7.93
Problem 30.
+120 V
4 mA
–60 V
FIG. 7.94
Problem 31.
20 �
10 mA
1 k� Pot.
+
40 V
–
+
36 V
–
R L1
R1 R2 +
RL 10 k� 3 V
–
100 � Pot.
FIG. 7.95
Problem 32.
a
+
+
1 k� Vab –
b
+
10 k� Vbc –
c
100 V
–
S
S P P
*30. Determine the voltage divider supply resistors for the
configuration of Fig. 7.93. Also determine the required
wattage rating for each resistor, and compare their levels.
SECTION 7.5 Potentiometer Loading
*31. For the system of Fig. 7.94:
a. At first exposure, does the design appear to be a good
one?
b. In the absence of the 10-k� load, what are the values
of R1 and R2 to establish 3 V across R2? c. Determine the values of R1 and R2 when the load is
applied, and compare them to the results of part (b).
*32. For the potentiometer of Fig. 7.95:
a. What are the voltages V ab and V bc with no load
applied?
b. What are the voltages V ab and V bc with the indicated
loads applied?
c. What is the power dissipated by the potentiometer
under the loaded conditions of Fig. 7.95?
d. What is the power dissipated by the potentiometer with
no loads applied? Compare it to the results of part (c).
SECTION 7.6 Ammeter, Voltmeter, and
Ohmmeter Design
33. A d’Arsonval movement is rated 1 mA, 100 �.
a. What is the current sensitivity?
b. Design a 20-A ammeter using the above movement.
Show the circuit and component values.