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S
S P P
The Ammeter
The maximum current that the d’Arsonval movement can read independently
is equal to the current sensitivity of the movement. However,
higher currents can be measured if additional circuitry is introduced.
This additional circuitry, as shown in Fig. 7.44, results in the basic construction
of an ammeter.
I max = 1 A
External terminal
I
a
Rotary switch
c
I m
I s
1 mA, 50 �
R shunt
FIG. 7.44
Basic ammeter.
1 mA, 50 �
I max = 1 A 0.05 � I
+ –
0.005 �
Imax = 10 A
Imax = 100 A 0.0005 �
FIG. 7.45
Multirange ammeter.
b
d
AMMETER, VOLTMETER, AND OHMMETER DESIGN ⏐⏐⏐ 231
Ammeter
The resistance Rshunt is chosen for the ammeter of Fig. 7.44 to allow
1 mA to flow through the movement when a maximum current of 1 A
enters the ammeter. If less than 1 A should flow through the ammeter,
the movement will have less than 1 mA flowing through it and will
indicate less than full-scale deflection.
Since the voltage across parallel elements must be the same, the
potential drop across a-b in Fig. 7.44 must equal that across c-d; that is,
(1 mA)(50 �) � RshuntIs Also, Is must equal 1 A � 1mA�999 mA if the current is to be limited
to 1 mA through the movement (Kirchhoff’s current law). Therefore,
(1 mA)(50 �) � Rshunt(999 mA)
(1 mA)(50 �)
Rshunt ���
999 mA
� 0.05 �
In general,
RmICS
Rshunt ��� Ima �
External terminal
(7.3)
One method of constructing a multirange ammeter is shown in Fig.
7.45, where the rotary switch determines the R shunt to be used for the
x
I CS