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Th
Direct Measurement of E Th and R Th For any physical network,
the value of E Th can be determined experimentally by measuring the
open-circuit voltage across the load terminals, as shown in Fig. 9.55;
E Th � V oc � V ab. The value of R Th can then be determined by completing
the network with a variable R L such as the potentiometer of Fig.
9.56(b). R L can then be varied until the voltage appearing across the
load is one-half the open-circuit value, or V L � E Th/2. For the series
circuit of Fig. 9.56(a), when the load voltage is reduced to one-half the
open-circuit level, the voltage across R Th and R L must be the same. If
we read the value of R L [as shown in Fig. 9.56(c)] that resulted in the
preceding calculations, we will also have the value of R Th, since R L �
R Th if V L equals the voltage across R Th.
E Th
E Th
+ V = 0 V –
RTh I = 0
(a)
R Th
E
+ Th –
2 ETh RL = RTh 2
(a)
a
+
E Th
–
b
a
b
Open circuit
+
–
Network
FIG. 9.55
Determining E Th experimentally.
Network
FIG. 9.56
Determining R Th experimentally.
Measuring V oc and I sc The Thévenin voltage is again determined
by measuring the open-circuit voltage across the terminals of interest;
�
+ –
(b)
(b)
(c)
a
b
R Th
THÉVENIN’S THEOREM ⏐⏐⏐ 337
a
+
E Th
–
b
V
+ –
V
+ –
+
E Th
2
–