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Type 2: Enhancement of Instrument Range

For Concept, refer to Measurement K-Notes, Electro-mechanical Instruments
Point to remember:
In Ammeter, external resistance is added in parallel to meter and in voltmeter additional
resistance is added in series.
Sample Problem 2:

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An ammeter has a current range of 0-5 A, and its internal resistance is 0.2 Ω. In order to
change the range to 0-25 A, we need to add a resistance of
(A) 0.8 Ω in series with the meter

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(B) 1.0 Ω in series with the meter
(C) 0.04 Ω in parallel with the meter

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(D) 0.05 Ω in parallel with the meter
Solution: (D) is correct option
Given that full scale current is 5A
Current in shunt I’=IR-Ifs En
20  R sh  5  0.2
= 25-5=20A
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R sh  0.05
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Unsolved Problems:
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Q.1 A 0-10mA DC Ammeter with internal resistance of 100 is used to design a DC voltmeter
with full scale voltage of 10 V. The full scale range of this voltmeter can be extended to 50V
by connecting an external resistance of
(A) 900 (B) 499.9k (C) 4000 (D) 4900
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Q.2 What is the value of series resistance to be used to extend (0-200)V range voltmeter
having 2000Ω/V sensitivity is to be extended to (0-2000)V range.
(A)44.44KΩ (B)55.55KΩ (C)34.56KΩ (D)45.25KΩ
Q.3 A DC ammeter has a resistance of 0.1Ω and its current range is 0-100 A. If the range is to
be extended to 0-500 A, then meter required the following shunt resistance?
(A)0.010Ω (B)0.011Ω (C)0.025Ω (D)1.0Ω

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Q.4 The coil of a measuring instrument has a resistance of 10Ω and the instrument reads up
to 250V, when a resistance of 4.999Ω is connected in series. Now the same instrument is used
as an ammeter by connecting a shunt resistance of 1/499Ω across it. What is the current range
of the ammeter?
(A)30A (B)26A (C)20A (D)24A
Q.5 A D’Arsonval movement with a full scale deflection current of 10 mA and internal
resistance of 500Ω is to be converted into the different range of voltmeters. If Ra,Rb and Rc are
the required series resistance for the ranges 0-20V, 0-50 V and 0-100 V respectively, then
Ra:Rb:Rc is?

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(A)2:5:10

Type 3:PMMC
(B)1:1:1 (C)3:9:19 (D)5:11:21

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For Concept, refer to Measurement K-Notes, Electro-mechanical Instruments
Point to remember:
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PMMC always measures the average value of the output and the pointer vibrates around the
zero position for pure AC input.
Sample Problem 3:
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The input impedance of the permanent magnet moving coil (PMMC) voltmeter is infinite.
Assuming that the diode shown in the figure below is ideal, the reading of the voltmeter in
Volts is?
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(A)4.46
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(B)3.15

(C)2.23
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(D)0

Solution: (A) is correct option

PMMC voltmeter reads average value.

For the +ve half cycle of I/p voltage, diode will be forward biased (Vg = 0, ideal diode)
Therefore, the voltmeter will be short circuited and reads
V1 = 0 volt (for +ve half cycle)
Now, for -ve half cycle, diode will be reverse biased and treated as open circuit. So, the
voltmeter reads the voltage across 100 kW. Which is given by

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14.1400
V2  100   14.14
(100  1)
14
So, V2,rms  V
2
Therefore, the average voltage for the whole time period is obtained as
14
0
V  V2,rms 2  14  4.94  4.46V
Vavg  1 
2 2 2 2

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Unsolved Problems:

Q.1 A PMMC has an internal resistance of 100  and requires 1 mA dc for full scale deflection.
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Shunting resistor Rsh placed across the movement has a value of 100. Diodes D1and D2 have
forward resistance of 400  and infinite reverse resistance. For 10 V ac range,

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The value of series multiplier is (RS) and voltmeter sensitivity for ac range is
(A) 9550 , 250 /V
(B) 4550 , 225 /V En
(C) 5000 , 500 /V gin
(D) 1800 , 250 V/
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Q.2 A Thermocouple produces a voltage of 50 mv. Its internal resistance is 50  . The
resistance of leads is 10. The output is read by a PMMC meter having an internal resistance
of 120  the output voltage indicated will be
(A) 50 mV (B) 40 mV (C) 33.3 mV .ne
(D) 25.0 mV
Q.3 An Electronic AC voltmeter is constructed using a full wave bridge Rectifier, with a scale
calibrated to read rms of a symmetrical square wave having zero mean. If this voltmeter is
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used to measure a voltage V(t)=10 sin 314t, then The reading of the voltmeter and magnitude
of percentage error in reading respectively are?
(A) 7.07V, 11% (B) 0.707V, 11.1%
(C) 6.36V, 9.9% (D) 11.1V, 0%
Q.4 The coil of moving coil voltmeter is 50 mm long and 40 mm wide and has 120 turns on it.
The control spring exerts a torque of 180 x 10-6 N.m. When the deflection is 120 divisions on
full scale, flux density of the magnetic field in the air gap is 1.2wb/m2. Neglect the resistance
of the coil. Resistance that must be put in the series with the coil to give one volt per division
is?
(A) 50 K (B) 63.7 K (C) 83 K (D) 91.7 K

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