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2.1 Basic Meter Movement
2.2 Permanent Magnetic Moving
Coil (PMMC)
Principle Operation of PMMC
Meter
Advantages and Disadvantages of
PMMC Meter
2.3 DC Ammeter
Basic DC Ammeter
Multirange Ammeter
The Aryton Shunt / Universal
Shunt
Ammeter Insertion Effects
2.4 DC Voltmeter
Basic DC Voltmeter
Multirange Voltmeter
Loading Effects
2.5 AC Voltmeter
Basic AC Voltmeter
AC Voltmeter Using Half Wave
Rectifier
AC Voltmeter Using Full Wave
Rectifier
2.6 Ohmmeter (series Type
Ohmmeter)
Basic Ohmmeter
Calibration of Series Type
Ohmmeter ESE
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2.1 Basic Meter Movement
• The action of most commonly DC meter is based on the
fundamental principle of motor.
• The motor action is produced by the flow of a small current
through a moving coil which is positioned in the field of a
permanent magnet .
• The basic moving coil system is called “D‟ Arsonval meter”.
• The coil (rotor) moves in a rotary fashion. The amount of rotation
is proportional to the amount of current flowing through the coil.
• A pointer attached to the coil indicates the position of the coil on
a scale calibrated in terms of current or voltage.
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2.2 Permanent Magnet Moving Coil (PMMC)
2.2.1 Principle Operation of PMMC Meter
• A coil is suspended in the magnetic field of a permanent
magnet in the shape of a horse-shoe.
• A spring is attached to the coil. The function of springs are
To make electrical connection to the coil
To return the coil to its original position when there is no
current through the coil
• The coil is suspended so it can rotate freely in the magnetic
field.
• When current flows in the coil, the developed of torque causes
the coil to rotate.
• The torque is counter-balanced by a mechanical torque of
control springs attached to the movable coil.
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• The balance of torque is indicated by a pointer against a fixed
reference called a “scale”.
• The equation for the developed torque
τ = B x A x N x I
where
τ = torque (Nm)
B= flux density (Wb/m2)
A= coil area (m2)
N= no. of turns of wire coil
I = current in the coil (A)
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CORE
POINTER
COIL
PERMANENT
MAGNET
NON-MAGNETIC
SOPPORT
SPRING
2.2.2 Advantages and Disadvantages of PMMC
• Advantages.
Can be modified using shunts to cover a wide range of
current/voltage
No hysteresis lost
Cheap & robust
Low power consumption
Scale are uniform
• Disadvantages.
Can measure only dc current/voltage
Friction due to jewel pivot suspension
Some errors due to ageing of control springs & the
permanent magnet
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2.3 DC Ammeter
2.3.1 Basic DC Ammeter
• Figure shows a basic DC ammeter
• Since the coil winding of a basic movement is small and light,
it can carry only small currents.
• When large currents are to be measured, it is necessary to
bypass a major part of the current through a resistance called s
shunt.
• The shunt resistance is parallel with the meter, so the voltage
drop across the shunt and meter are same.
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Rsh ImIsh
+
-
+
-
Rm = Internal resistance
Rsh = Shunt resistance
Ish = Current through shunt
resistance
Im = fsd current of moving coil
I = fsd current of meter
DC ammeter
Voltage across shunt and meter
∆ Since
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Rsh ImIsh
+
-
+
-
Example 2.1
Solution 2.1
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2.3.2 Multirange Ammeter
• Multirange ammeter extends the current range by a number of
shunt
• Figure below shows a multirange ammeter with 4 shunts R1,
R2, R3 and R4 which are parallel to the meter to give 4
different current range.
• Switch S is made to protect the meter movement from damage
during range changing (make contact to next terminal before
breaking contact with previous terminal).
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R4
+
-
+
-
Rm
R1 R2 R3
S
2.3.3 The Aryton Shunt / Universal Shunt
• The Aryton shunt eliminates the possibility of having the meter
in the circuit without a shunt.
• Figure shows a basic design of an Aryton shunt with three
current range.
At point A
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+
-
Rm
+
-
Rc
Rb
Ra
I1
I2
I3A B
C
Im
At point B
At point C
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At point C (continue)
The value of Rb can be found by
substitute Eq. 4 into Eq. 3.
The value of Ra can be found by
substitute Eq.3 into Eq. 1.ESE
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+
-
Rm
+
-
Rc
Rb
Ra
I1
I2
I3A B
C
Im
Example 2.2
Design an Aryton shunt to provide an ammeter with current ranges of
1A, 5A and 10 A. A d‟Arsonval movement with an internal
resistance, Rm = 100 Ω and full scale deflection (f.s.d) current of
1mA is used in the configuration of figure below.
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Solution 2.2
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2.3.2 Ammeter Insertion Effects
• All ammeters have internal resistance.
• Inserting ammeter in a circuit will increase the resistance and
reduce the current in the circuit.
• Current flow without ammeter
• Current flow with ammeter
• Insertion error is defined as
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V R
Ie
V R
Im
Rm
Example 2.3
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Solution 2.3
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2.4 DC Voltmeter
2.4.1 Basic DC Voltmeter
• A PMMC can be converted to a voltmeter by connecting a
multiplier, RS in series.
• The purpose is to extend the range of the meter and to limit the
current through the meter to the maximum fsd current.
• The sensitivity of voltmeter
• From figure
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Rs
Im
+
-
+
-
V
Rm
Ifsd
Example 2.4
Solution 2.4
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2.4.2 Multirange Voltmeter
• A DC voltmeter can be converted into multirange voltmeter by
connecting a number of multipliers
• The purpose of multiplier is to provide a number of ranges as
shown in figure below.
i. Multirange Voltmeter ii. Commercial Version
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R3
-
+
- Rm
Ifsd+
R2
R1V1
V2
V3
R3
-
+
-
Rm+
R2
R1
V1
V2
V3
Example 2.4
Refer to figure below (multirange voltmeter), a PMMC meter
movement with full scale deflection current of 2 mA and has
internal resistance of 50 Ω is to be converted into a multirange
voltmeter with voltage range V1 = 0 – 10 V, V2 = 0 – 50 V and V3 =
0 – 100 V. Identify the value of R1, R2 and R3.
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R3
-
+
- Rm
Ifsd+
R2
R1V1
V2
V3
Solution 2.4
For 0 - 10 V range
For 0 – 50 V range
For 0 – 100 V range
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Example 2.5
Refer to figure below (commercial version of multirange
voltmeter), if V1 = 30 V, V2 = 10 V and V3 = 3 V, full scale
deflection current of 50 uA and has internal resistance of 1 kΩ,
calculate the value of multiplier resistance for the multirange
voltmeter.
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R3
-
+
-
Rm+
R2
R1
V1
V2
V3
Solution 2.5
For 0 - 3 V range
For 0 – 10 V range
For 0 – 30 V range
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2.4.3 Loading Effects
• A voltmeter when connected across two points in a highly
resistive circuits, act as shunt for that portion, reducing the
total equivalent resistive of that portion.
• The meter indicates a lower reading than what existed before
the meter connected.
• This phenomena called loading effect.
• Sensitivity is define as
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Example 2.6
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Solution 2.6
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Solution 2.6 (cont)
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Solution 2.6 (cont)
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2.5 AC Voltmeter
2.5.1 Basic AC Voltmeter
• The PMMC can be used to measure AC current and voltage.
• In order to measure AC current /voltage, we need to rectify the
AC source using rectifier to produce unidirectional current
flow.
• Figure shows an general rectifier type ac voltmeter.
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+
-
Rm
Im
Rsh
D1
AC
InputD2
RsIt
Ish
AC Input
Meter Output
• Diode D1 conducts during the +ve half of the cycle cause the
meter deflect according to the average value of this half cycle.
• The meter movement is shunted by resistor Rsh in order to
draw more current through the D1.
• In -ve half cycle, diode D2 conducts. The current flow in
opposite direction and bypass PMMC Meter.
• Refer to figure above, the sensitivity
• Since the meter will only response to the average value of the
AC sine wave or DC value.
• AC sensitivity
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2.5.2 AC Voltmeter Using Half Wave Rectifier
• If a diode D1 is added to the dc voltmeter, we have an ac
voltmeter using half wave rectifier capable of measuring ac
voltages..
• Figure shows an ac voltmeter using half wave rectifier.
• In general
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Example 2.7
Calculate the value of the multiplier resistor for a 10 Vrms range on
the voltmeter shown. Given Ifsd = 1 mA and Rm = 200 Ω.
Solution 2.7
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Example 2.8
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Solution 2.8
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Solution 2.8 (cont)
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2.5.3 AC Voltmeter Using Full Wave Rectifier
• The full wave rectification is used to improve the sensitivity
on AC voltmeter.
• The construction of full wave rectifier can be seen as figure
below.
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Rs
Rm
AC
D1 D2
D3 D4
Ifsd
AC Input Meter Output
• Diode D2 and diode D3 conduct during the half +ve input cycle
cause the meter deflect according to the average value of this
half cycle.
• In –ve half cycle, diode D1 and diode D4 conduct. The current
flows in an opposite direction cause the meter to deflect
according to the average value of this second half cycle.
• Since the meter will only response to the average value of the
AC sine wave or DC value.
• AC sensitivity.
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Example 2.9
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Solution 2.9
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Solution 2.9 (cont)
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Quiz 2.1
An AC voltmeter is to be used to measure the voltage across 20 kΩ as
shown in figure below. If the voltmeter uses half wave
rectification and 200 μA d‟Arsonval meter movement se on its 20
V range with an internal resistance of 1.5 kΩ. Identify
i) Reading of the voltmeter
ii) If the loading effect will be reduced by using full wave
rectification for the same d‟Arsonval meter movement.
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Vo
R1
30 kΩ
R2
20 kΩ
40 Vrms
Solution Quiz 2.1
Voltmeter impedance
Since the voltmeter uses half wave rectification
Total resistance of voltmeterESE
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Solution Quiz 2.1 (cont)
Voltmeter reading (half wave rectification)
Voltmeter reading
Voltmeter reading (full wave rectification)
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2.6 Ohmmeter (series type ohmmeter)
2.6.1 Basic Ohmmeter
• A series type ohmmeter is a D‟Arsonval meter connected in
series with a resistance R1 and battery E.
• An unknown resistor Rx is connected across point AB.
• The current flow through the movement meter depends on the
magnitude of the Rx.
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Rm
ItR1
Rx
A
B
Im
I2
R2
E
2.6.2 Calibration of Series Type Ohmmeter
• To mark „0‟ reading.
Terminal AB is shorted (Rx = 0 Ω)
Maximum current flows in the circuit.
R2 is adjusted until the movement indicates fsd current ,
Ifsd.
The position of the pointer on the scale is marked 0 ohm.
• To mark „∞‟ reading.
Terminal AB is opened (Rx = ∞ Ω)
No current flows in the circuit & no deflection of the
pointer.
The position of the pointer on the scale is marked ∞ ohm.
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• To mark other value reading.
Terminal AB is connected with different known value of
resistance.
The position of the pointer on the scale is marked with the
value of known resistor.
• Disadvantages.
Decrease the internal battery voltage, hence the fsd current
drops and meter does not read „0‟ when AB is shorted.
To overcome this problem, a zero adjust resistor can be
added in parallel to the meter to counteract the drop in the
voltage and bring back the pointer to „0‟.
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Example 2.10
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Solution 2.10
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Solution 2.10 (cont)
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Solution 2.10 (cont)
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