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150 ⏐⏐⏐ SERIES CIRCUITS S
lead to point b. For single-subscript notation, hook up the red lead to the
point of interest and the black lead to ground, as shown in Fig. 5.60 for
V a and V b.
E
I
+ –
R 1
+ –
Red lead Black lead
Red lead
V ab
a b
+ –
R 2
+ –
V a
Black lead
+
R3 –
Red lead
+ –
V b
Black lead
FIG. 5.60
Measuring voltages with double- and single-subscript notation.
The internal resistance of a supply cannot be measured with an ohmmeter
due to the voltage present. However, the no-load voltage can be
measured by simply hooking up the voltmeter as shown in Fig. 5.61(a).
Do not be concerned about the apparent path for current that the meter
seems to provide by completing the circuit. The internal resistance of
the meter is usually sufficiently high to ensure that the resulting current
is so small that it can be ignored. (Voltmeter loading effects will be discussed
in detail in Section 6.9.) An ammeter could then be placed
directly across the supply, as shown in Fig. 5.61(b), to measure the
short-circuit current I SC and R int as determined by Ohm’s law: R int �
E NL /I SC. However, since the internal resistance of the supply may be
very low, performing the measurement could result in high current levels
that could damage the meter and supply and possibly cause dangerous
side effects. The setup of Fig. 5.61(b) is therefore not suggested. A
better approach would be to apply a resistive load that will result in a
supply current of about half the maximum rated value and measure the
terminal voltage. Then use Eq. (5.14).
+
E
–
+ 0 V –
R int
+
V NL = E
–
(a)
+ V –
E
R int
I sc
(b)
+ I –
Not recommended !!
FIG. 5.61
(a) Measuring the no-load voltage E; (b) measuring the short-circuit current.