1. Analog Instruments
Lecture 4
1st year Electrical
Dr. Shimaa Hassan Barakat
1

2. Analog Meters
• The analog device is defined as the instrument
whose output is the continuous function of time,
and they have a constant relation to the input.
• The physical quantities like voltage, current, power
and energy are measured through the analog
devices.
2

3. Principles of Operation
1) Magnetic Effect
• Magnetic effect means the current flows
through the conductor induces the
magnetic field around it.
• Instruments: Ammeters, voltmeters,
watt meters, integrating meters.
3

4. Principles of Operation
2) Thermal Effect
• The current to be measured is
passed through a small element
which heats it. The temperature
rise is converted to an emf by a
thermocouple attached to the
element.
• Instruments:
Ammeters, voltmeters, watt meters.
4
3) Electrostatic Effect
• When two plates are charged, there is a
force exerted between them. This force
is used to move one of the plates.
• Instruments: Voltmeters.

5. Electromechanical Indicating Instruments
5

6. Indicating
instrument
6
Three types of forces are needed for satisfactory
operation of any indicating instrument.
Deflecting
force
Controlling
force
Damping
force
The instruments that give the instantaneous value of the
parameter under measurement.
Ex. Ammeter, Voltmeter.
‫اآلني‬ ‫القيمة‬ ‫تعطي‬ ‫التي‬ ‫األجهزة‬
‫ة‬
‫القياس‬ ‫قيد‬ ‫للمعامل‬
.

7. 1. Deflecting force
7
This force helps in rotating the instrument movement
from its zero position.
The deflecting force’s value is dependent and proportional to the
electrical signal to be measured;
Any instrument’s deflection is found by the total effect of the
deflecting force, control force and damping force.
The system producing the deflecting force is called the deflecting
system or Moving System.

8. 2. Controlling Force
• The act of this force is opposite to the
deflecting force (By Spring or gravity).
• The functions of the controlling system
are:
• To produce a force equal and
opposite to the deflecting force at
the final steady position of the
pointer.
• To bring the moving system back to
its zero position when the force
causing the instrument moving
system to deflect is removed.
8

9. 3. Damping Torque
• A damping force generally works in an
opposite direction to the movement of the
moving system.
1. When the deflecting torque is much
greater than the controlling torque, the
system is called underdamped.
2. If the deflecting torque is equal to the
controlling torque, it is called critically
damped.
3. When deflecting torque is much less than
the controlling torque, the system is under
overdamped condition. 9

10. Analog Ammeters and Voltmeters
• The action of all ammeters and voltmeters, except for electrostatic type of instruments,
depends upon a deflecting torque produced by an electric current.
• Ammeters are connected in series in the circuit whose current is to be measured.
The power loss in an ammeter is I2Ra, where I is the current to be measured and Ra, is
the resistance of ammeter. Therefore, ammeters should have a low electrical resistance
so that they cause a small voltage drop and therefore absorb small power.
• Voltmeters are connected in parallel with the circuit whose voltage is to be
measured. The power loss in voltmeters is V2/Rv, where V is the voltage to be measured
and Rv, is the resistance of voltmeter. Therefore, voltmeters should have a high electrical
resistance, in order that the current drawn by them is small and therefore the power
consumed is small.
10

11. Types of Instruments
• The main types of instruments used as ammeters and voltmeters are:
1) Permanent magnet moving coil (PMMC)
2) Moving iron
3) Electro-dynamometer
4) Hot wire.
5) Thermocouple
6) Induction
7) Electrostatic
8) Rectifier.
11

12. The PMMC type can be used for direct-current (DC) measurements only.
The induction type for alternating-current (AC) measurements only.
The other types of meters can be used with either direct or alternating
currents.
The moving-iron and moving-coil types both depend for their action
upon the magnetic effect of current.
The moving-iron is the most generally used form of indicating instrument,
as it is the cheapest. Used for both DC and AC measurements.
The PMMC instrument is the most accurate type for DC measurements.
12

13. Electrodynamometer type of instruments are used both on a.c. as well
as on d.c.
Thermal instruments have the advantage that their calibration is the
same for both d.c. and a.c. They are particularly suited for AC
measurements since their deflection depends directly upon the heating
effect of the RMS value of the current.
electrostatic instruments have the advantage that their power
consumption is exceedingly small. They can be made to cover a large
range of voltage.
The induction principle is more generally used for watt-hour meters
than for ammeters and voltmeters owing to their comparatively high
cost, and inaccuracy, of induction instruments of the latter types.
13

14. 14
A PMMC meter – also known as a galvanometer
• is an instrument that allows you to measure the
current through a coil by observing the coil’s
angular deflection in a uniform magnetic field.
• A PMMC meter places a coil of wire (a
conductor) in-between two permanent
magnets in order to create stationary magnetic
field.
• Principle: A current carrying coil placed in a
magnetic field experiences a torque.
• The magnitude of this torque will be
proportional to the amount of current through
the coil.
Permanent Magnet Moving Coil Instrument (PMMC)

15. 15

16. Principle of Operation
16
• Three forces operates in electro-
mechanical movement: Deflecting,
Controlling and Damping force.
• Spring attached to the pointer is
used to balance the deflecting force.
• The pointer stops when the
deflecting force = controlling force.

17. Deflecting
Torque
Equation of
PMMC
Instrument
17
Let, B = flux density in the air gap (wb/m2)
i = current in the coil (A)
l = effective axial length of the coil (m)
d = diameter of the coil (m)
n = number of turns of the coil.
Total deflecting torque exerted on the coil,
𝑻𝒅 = 𝑩𝒍𝒊𝒏𝒅 (𝑵. 𝒎)
Control Torque is proportional to the angle θ turned through by the coil.
𝑇𝑐 = 𝑘𝑠 𝜃 (𝑵. 𝑚)
At final steady state position, Control torque = Deflecting torque

18. 18
Given:
n= 60 turns
b or D=18 mm
L= 25 mm
B=0.5 Tesla
Ks=1.5x10-6 Nm/degree
ϴ =100 degree
Find i=?

19. 19

20. 20

21. Ammeter Shunts
21
∴ 𝑰𝒔𝒉𝑹𝒔𝒉 = 𝑰𝒎𝑹𝒎
𝑰 = 𝑰𝒔𝒉 + 𝑰𝒎 → 𝑰𝒔𝒉 = 𝑰 − 𝑰𝒎
𝑹𝒔𝒉 =
𝑰𝒎𝑹𝒎
𝑰𝒔𝒉
∴ 𝑹𝒔𝒉=
𝑰𝒎
𝑰 − 𝑰𝒎
𝑹𝒎
𝑚 =
𝐼
𝐼𝑚
=
𝑰𝒔𝒉 + 𝑰𝒎
𝐼𝑚
= 1 +
𝑰𝑠ℎ
𝑰𝑚
= 1 +
𝑅𝑚
𝑅𝑠ℎ
∴ 𝑹𝒔𝒉=
𝑅𝒎
𝑚 − 1
V

22. Example 4
A PMMC instrument with a 750  coil resistance gives FSD with a 500
µA coil current. Determine the required shunt resistance to convert the
instrument into a dc ammeter with an FSD of 50 mA.
coil resistance 𝑅𝑚 = 750 Ω ,
𝐼𝑚= 500μA (FSD: 𝐹𝑢𝑙𝑙 𝑆𝑐𝑎𝑙𝑒 𝐷𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛)
at FSD = 50 mA → I=50 mA
∴ 𝑹𝒔=
𝑰𝒎
𝑰 − 𝑰𝒎
𝑹𝒎 =
500 × 10−6
× 750
50 × 10−3 − 500 × 10−6
= 7.576 Ω
22

23. Multi-range Ammeters
• The current range of a d.c. ammeter may be further
extended by several shunts, selected by a range
switch.
• Multi-range ammeters are used for ranges
from 1 to 50 A.
• When using a multi-range ammeter, first use
the highest current range, then decrease the
current range until good upscale reading is
obtained.
23

24. Ayrton shunt
• The universal shunt or Ayrton shunt is also used for multi-range
ammeters.
• The advantage of an Ayrton shunt is that it eliminates the possibility of
the meter being in the circuit without a shunt.
• But this advantage is gained at the cost of a higher meter resistance.
24

25. Example
Design a multi-range d.c. milli-ammeter using a basic movement with
an internal resistance Rm =50 Ω and a full-scale deflection current Im =
1mA.
The ranges required are 0-10 mA; 0-50 mA; 0-100 mA, and 0-500
mA.
25

26. Solution
(1) 0-10 mA range:
Multiplying power 𝑚 =
𝐼
𝐼𝑚
=
10
1
= 10
resistance of shunt 𝑹𝒔𝒉=
𝑅𝒎
𝑚−1
=
50
9
= 5.55 Ω
(2) 0-50 mA range:
Multiplying power 𝑚 =
𝐼
𝐼𝑚
=
50
1
= 50
resistance of shunt 𝑹𝒔𝒉=
𝑅𝒎
𝑚−1
=
50
49
= 1.03 Ω
26
(3) 0-100 mA range:
Multiplying power 𝑚 =
𝐼
𝐼𝑚
=
100
1
= 100
resistance of shunt 𝑹𝒔𝒉=
𝑅𝒎
𝑚−1
=
50
99
= 0.506 Ω
(4) 0-500 mA range:
Multiplying power 𝑚 =
𝐼
𝐼𝑚
=
500
1
= 500
resistance of shunt 𝑹𝒔𝒉=
𝑅𝒎
𝑚−1
=
50
499
= 0.1 Ω
Rm = 50 Ω and Im = 1mA.

27. 27

28. 28
Voltmeter
Multipliers V = 𝐼𝑚(𝑅𝑚 + 𝑅𝑠𝑐)
𝑅𝑠𝑐 =
𝑉 − 𝐼𝑚𝑅𝑚
𝐼𝑚
=
𝑉
𝐼𝑚
− 𝑅𝑚
𝑅𝑠𝑐 = 𝑚 − 1 𝑅𝑚
𝑚 =
𝑉
𝑣
=
𝐼𝑚(𝑅𝑚 + 𝑅𝑠ℎ)
𝐼𝑚𝑅𝑚
= 1 +
𝑅𝑠𝑐
𝑅𝑚

29. 29

30. Multirange d.c.
Voltmeters
In a multirange voltmeter, different full scale voltage
ranges may be obtained using individual multiplier
resistors.
30

31. Example
Solution. Voltage across the meter movement,
𝑣 = 𝐼𝑚𝑅𝑚 = 1 × 100 = 100 𝑚𝑉
The voltage multiplying factors are:
𝑚1 = 10
100×10−3 =100 , 𝑚2 = 50
100×10−3 =500 , 𝑚3 =2500 , 𝑚4 = 5000
the values of various resistances are:
𝑅1 = 𝑚1 − 1 𝑅𝑚 = (100 − 1) ×100= 9900 Ω
𝑅2 = 𝑚2 − 𝑚1 𝑅𝑚 = (500 − 100) ×100= 40 kΩ
𝑅3 = 𝑚3 − 𝑚2 𝑅𝑚 = (2500 − 500) ×100= 200 kΩ
𝑅4 = 𝑚4 − 𝑚3 𝑅𝑚 = (5000 − 2500) ×100= 250 kΩ
31
A basic PMMC with an internal resistance Rm = 100 Ω and a full-scale
current of Im =1 mA, is to be converted into a multi-range d.c. voltmeter
with ranges of 0 - 10 V, 0 - 50 V, 0 - 250 V and 0 - 500 V.
Find the values of various resistances using the potential divider
arrangement.