DCAC Meter

1. DC and AC meters
Hasmawati Antong
Lecturer
Department of Mechatronics Engineering
International Islamic University Malaysia
hasmawati@iium.edu.my

2. DC
METERS

3.  Introduction to meter
 Galvanometer
 Permanent-magnet moving coil
 D’Arsonval meter movement
 D’Arsonval in DC ammeter
 D’Arsonval – Ayrton Shunt
 D’Arsonval in DC voltmeter
 Voltmeter loading effect
 D’Arsonval in ohmmeter
 Multiple range ohmmeter

4.  A meter is any device built to accurately
detect and display an electrical quantity in a
form readable by a human being
 Usually this “readable form” is visual:
- motion of a pointer on a scale
- a series of lights arranged to form a
“bargraph”
- a display composed of numerical figures
 In the analysis and testing of circuits, there
are meters designed to accurately measure
voltage, current and resistance.

5. AC voltage
Current
Resistance
DC voltage

6.  Most modern meters are DIGITAL  their
readable display is in the form of numerical
digits
 Older designs of meters uses a pointer device
to display the quantity being measured
 In either case, the principle adopted for the
display mechanism are the same
 The display mechanism of meter is called
meter movement
 The first meter movement were known as
GALVANOMETERS

7.  A simple galvanometer is
made from a magnetized
needle suspended from a
string and positioned within
a coil of wire
 Current through the wire coil
produces magnetic field
which deflects the needle
from pointing in the direction
of earth’s magnetic field
 Nowadays, the term galvanometer usually refers to
any design of electromagnetic meter movement
built such as permanent-magnet moving coil (PMMC)

8.  PMMC is a practical electromagnetic meter
made with a pivoting wire coil suspended in
a strong magnetic field and shielded from
the majority of outside influences
Horseshoe magnet
Pointer
Zero position control
Control spring
Counter weight
Pole shoe
Coil

9. Working Principle
 Current to be measured is directed through the
coils of the electromagnet
 Magnetic field produced by the current opposed
the field of the permanent magnet and causes
rotation of the magnet core
 The core is restrained by springs so that the
needle will move in proportion to the current
intensity
 The more current applied to the core, the
stronger the opposing field, and the larger the
needle deflection
 When the current is stopped, the opposing field
collapsed and the needle return to zero

10.  The arc on the meter
display is labeled
with numbers to
indicate the value of
the quantity being
measured
 An increased in
measured current drives the
needle to the right
 A decrease in measured current causes the needle to
drop back to its resting position on the left
 Most meter movements are polarity-sensitive  one
direction of current drive the needle to the right and
the other driving it to the left

11.  Common polarity-sensitive movements include the
D’Arsonval design, which is a PMMC-type instrument.
 Current in one direction through the wire will produce
a clockwise torque on
the needle mechanism
 Current in the other
direction will produce a
counter-clockwise
torque
 D’Arsonval meter
movement is a current
responding device regardless the units (volts, ohms…)
for which the scale is calibrated
 Limitation  measure current less than 1mA

12.  A modification is needed to increase the range of
current that can be measured by the basic meter
movement
 A low resistance, shunt resistance (Rsh) is placed
parallel to the meter movement resistance, Rm
 Rsh will provide an alternate path for the total
metered current around the meter movement
 In most circuit,
Ish > Im

13. Rsh = shunt resistance
Rm = internal resistance of
the meter movement
(R of the moving
coil)
Ish = current through the
shunt
Im = full scale deflection
current of the meter
movement
I = full scale deflection
current for the
ammeter
D’Arsonval meter
movement used in an
ammeter circuit

14.  Voltage drop across the meter movement is
 The voltage drop across the shunt and meter
movement is the same.
 The current through the shunt is
 Therefor Rsh is
m
m
m R
I
V 
m
sh V
V 
m
sh I
I
I 

)
(
R
I
I
I
R
I
I
I
R
I
I
V
R m
m
m
m
sh
m
sh
m
m
sh
sh
sh 







15. Example 1:
Calculate the value of the shunt resistance required
to convert a 1mA meter movement, with a 100Ω
internal resistance into a 0 to 10mA ammeter

16.  The purpose of the shunt circuit is to measure
current, I that is n-time multiple of Im
 The number n is called the multiplying factor
and related I and Im as
I=nIm
1
-
n
R
I
nI
R
I
R
m
m
m
m
m
sh




17. Example 2:
A 100μA meter movement with an internal
resistance of 800Ω is used in a 0 to 100mA
ammeter. Find the value of the multiplying
factor and the required shunt
resistance.

18.  Limitation  only works well on a single-
range ammeter
 To overcome this limitation, Ayrton shunt is
introduced

19.  More suitable design for a
multiple-range ammeter
 Also, it can be used with a
wide range of meter
movements
 The individual resistance
value of the shunts are
calculated by starting with
the most sensitive range
towards the least sensitive
range

20.  In this figure, the most sensitive
range is the 1A range
 The least sensitive range is the
10A range
 When the meter is connected to
the most sensitive range, total
shunt resistance is
 Rsh can also be calculated using
Rsh = Ra + Rb + Rc
1
n
R
R m
sh


* In this case, I is equal to 1A
*

21.  In this figure, Rb+Rc is
parallel with Rm+Ra
 Voltage across parallel
branch must be equal
 Rearranging the equation
gives
V Rb +Rc = V Ra +Rm
(Rb+Rc)(I2-Im)=Im(Ra+Rm) 2
)
(
)
(
2




I
R
R
I
R
R m
sh
m
c
b

22. 2
)
(
)
( 


 c
b
sh
a R
R
R
R
)
(
)
(
3



I
R
R
I
R m
sh
m
c
)
(

 a
sh
b R
R
R

23. Example 3:
Compute the value of the shunt resistors for the
circuit shown below

24.  The basic D’Arsonval meter movement can be
converted to a DC voltmeter by connecting a
multiplier in series with the meter movement
 Rs extends the voltage range and limit current
through D’Arsonval meter movement to a
maximum full scale deflection
Rs

25.  To find the value of Rs, the sensitivity, S of the
meter movement must be determined
 The sensitivity is determined by taking the
reciprocal of the full-scale deflection current
 To calculate the value of the multiplier for
voltage ranges greater than 1V;
volt
ohms
ohms
volt
V
I
y
Sensitivit
fs





1
amperes
1
)
/
(
1
Rs = S x Range – Internal Resistance

26. Example 4:
Calculate the sensitivity of a 100μA meter
movement which is to be used as a DC voltmeter

27. Example 5:
Calculate the value of the multiplier resistance
on the 50V range of a dc voltmeter that used a
500μA meter movement with an internal r
esistance of 1kΩ

28. Example 6:
Calculate the value of the multiplier resistance
for the multiple ranges DC voltmeter circuit

29. Example 7:
Calculate the value of the multiplier resistor for
the multiple range DC voltmeter circuit shown

30.  When a voltmeter is used to measure the voltage
across a circuit component, the circuit itself is
parallel with the circuit component in the
voltmeter
 The combination of two parallel resistors is less
than any of the two resistor value.
 Therefore:
- The voltage across the component is less when
the voltmeter is connected
- The decrease of voltage is known as voltmeter
loading
- The resulting error is known as loading error

31. Example 8:
Two different voltmeter
is used to measure the
voltage across resistor RB
in the figure. Calculate:
(a) VRB without meter connected
(b) VRB when meter A is used
(c) VRB when meter B is used
(d) Error in voltmeter reading
Meter A: S=1kΩ/V, Rm=0.2k Ω,
range=10V
Meter B: S=20kΩ/V, Rm=1.5k Ω,
range=10V

32. Example 9:
Find the voltage reading and
the percentage of error of
each reading obtained with a
voltmeter on
(a) Its 3V range
(b) Its 10V range
(c) Its 30V range
When connected across RB.
Instrument has 20kΩ/V
sensitivity

33.  A series circuit is
constructed if points X
and Y are connected
 The magnitude of the
current is limited by the
resistors RZ and Rm
 By connecting X and Y
the circuit will be
shorted and RZ will be
adjusted to obtain full
scale deflection of the
meter movement

34.  The amplitude of the
current through the
meter movement can be
determined as
 When measuring
unknown resistor, RX the
circuit current will be
m
z
f s
R
R
E
I


x
m
z R
R
R
E
I




35.  The amplitude of the
current through the
meter movement can be
determined as
 When measuring
unknown resistor, RX the
circuit current will be
m
z
f s
R
R
E
I


x
m
z R
R
R
E
I



* Current I is less than Ifs because of the additional resistance RX

36.  The ratio of the full-scale deflection current Ifs is
equal to the ratio of the circuit resistance and may
be expressed as
x
m
z
m
z
m
z
x
m
z
fs R
R
R
R
R
)
R
R
/(
E
)
R
R
R
/(
E
I
I
P










37. Example 10:
An ohmmeter uses a 1.5V battery and a basic 50μA
movement. The internal resistance is 1kΩ. Calculate
(a) The value of Rz required
(b) The value of Rx that would cause half-scale
deflection in the circuit

38. Example 11:
An ohmmeter is designed around a 1mA meter
movement and a 1.5V call. If the cell voltage decays to
1.3V because of aging, calculate the resulting error at
midrange on the ohmmeter scale

39.  Ohmmeters are usually nonlinear due to,
 to high internal resistance
 however, at half scale the value of Rx is equal to
the total internal resistance, Rm of an ohmmeter
 A variable resistor may be applied to the ohmmeter
probes and set the value required for half-scale
deflection of the pointer in order to mark the scale.

41. AC
METERS

42.  AC meters
 Half-wave rectification
 Full-wave rectification
 Electrodynamometer

43.  Use to measure AC current or voltage
 By far, the widely used meter movement is the
D’Arsonval meter movement
 However, it cannot measure AC current or
voltage directly
 Some modification is required to use the
D’Arsonval meter movement with AC meter:
1) Half-wave rectification
2) Full-wave rectification
 A rectifier is a circuit which converts AC input to
DC output

44.  During each “positive” half-cycle of the AC sine
wave, the diode is forward-biased and current
flow through the diode
 During each “negative” half-cycle, the diode is
reverse-biased and no current flow through the
diode
 Because only one-half of the input waveform
reaches the output, it is very inefficient if used
for power transfer

45.  It converts the whole of the input waveform to
one of constant polarity (+ or -) at its output
 Four diodes (usually in a bridge configuration) is
required for full-wave rectification

46.  A type of AC meter
 It is a current sensitive device
 Able to handle more current than the
D’Arsonval meter movement could handle
without shunt
 A single coil movement could be used to
measure DC / AC current or voltage

47.  It consists of a fixed coil that is divided into two
equal halves, separated by a movable coil
 It has a very low sensitivity (approximately 20 –
100Ω/V)

48. Example 12:
An electrodynamometer movement that has a
full-scale deflection current rating of 10mA is
to be used in a voltmeter circuit. Calculate the
value of the multiplier for a 10V range if
Rm=50Ω

49. Example 13:
An electrodynamometer movement with a full-
scale deflection current rating of 10mA is to be
used as a 1A ammeter. If the resistance of the
moving coil is 40Ω. What is the value of the
shunt?