INSTRUMENTS AND ELECTRICAL

MEASUREMENTS
EE 300

Slide Set # 3

Chapter # 3 (Electronic Instrumentation

and measurements by Bell)

Dr. Rizwan Akram

B1-S021
rizwanakram75@qec.edu.sa
OBJECTIVES
At the end of this chapter, students should be able to:

1. Explain the basic contruction and working principle of

D’Arsonval meter movement.

2. Perfom basic electronic circuit analysis for D’Arsonval

meter family.

3. Identify the difference electronic circuit design for

measurement meters using D’Arsonval meter principle.

2
 CHAPTER OUTLINE
1. D’Arsonval Meter Movement
2. DC Ammeter
3. DC Voltmeter
4. Multi-range Voltmeter
5. Voltmeter Loading Effects
6. Ammeter Insertion Effects
7. Ohmmeter
8. Multi-range Ohmmeter
9. Multimeter
10. AC Voltmeter using half-wave rectifier
11. AC Voltmeter Loading Effects

3
DEFLECTION INSTRUMENT
FUNDAMENTALS

Permanent Magnet Moving Coil Instrument

It’s deflection instrument uses a pointer that

moves over a calibrated scale to indicate a
measured quantity.

For this to occur, three forces must operate

inside the instrument.

Deflecting Force

Controlling Force

Damping Force
DEF. INST. FUNDAMENTALS
Deflecting Force

Deflection force causes the

pointer to move from its zero
position when a current flows in
the coil is magnetic force; the
current in the coil sets up a
magnetic field that interacts with
the magnetic field of the
permanent magnet.

When a current flows in a light weigh

moving coil pivoted between the poles of
a permanent magnet, the current sets up
a magnetic field that interacts with the
field of the permanent magnet. A force is
exerted on a current carrying conductor
situated in a magnetic field.
Consequently a force is exerted on the
coil turns as illustrated, causing the coil
to rotate on its pivots. The pointer is
fixed to coil, so it moves over the scale as
the coil rotates.
DEF. INST. FUNDAMENTALS
Controlling Force

The controlling force in the PMMC instrument is provided by spiral springs

-The springs retain the coil and pointer at

their zero position when no current is
flowing in the coil

- When current flows in the coil, the

springs wind up as the coil rotates, and
the force they exert on the coil increases

- The coil and pointer stop rotating when

the controlling force becomes equal to
the deflecting force

- The spring material must be

nonmagnetic to avoid any magnetic field
influence on the controlling force
DEF. INST. FUNDAMENTALS
Damping Force

The Pointer and coil

tend to oscillate fro
some time before
settling down at their
final position.

Damping force is
required to minimize
(or damp out) the
oscillations

- In PMMC instruments, the damping

force is normally provided by eddy
currents.

- Eddy currents induced in the coil

former (or frame) set up a magnetic
flux that opposes the coil motion, thus
damping the oscillations of the coil.
 SUPPORTING METHODS
Two methods of supporting the moving system are

Jeweled Bearing Suspension

- The pointed ends of shafts

or pivots fastened to the
coil are inserted into cone-
shaped cuts in jewel
(sapphire or glass)
bearings

- Least possible friction

- The bearings may be broken by the shock of an instrument being slammed

down heavily upon a bench

- Some jewel bearings are spring supported to absorb such shocks more easily

- The most sensitive jeweled-bearing instruments give full scale deflection (FSD)
with a coil current of 25 µA
 SUPPORTING METHODS
Two methods of supporting the moving system are
Taut Band Method

- Much tougher than jeweled-

bearing suspension. If they
are dropped from bench
height nothing happens

- Two flat metal ribbons

(phosphor bronze or platinum
alloy) are held under tension by
springs to support the coil

- Because of the springs, the metal ribbons behave like rubber under tension

- The ribbons also exert a controlling force as they twist, and they can be used as
electrical connections to the moving coil

- With taut-band suspension FSD may be achieved with as little as 2 µA of coil

current
TORQUE EQUATION AND SCALE
EXAMPLE

If we wish that 1mA current show half scale deflection what should
be the value of K, the deflection constant?

We use
D’ARSORVAL METER MOVEMENT
 Also called Permanent-Magnet Moving Coil (PMMC).
 Based on the moving-coil galvanometer constructed by Jacques d’
Arsonval in 1881.
 Can be used to indicate the value of DC and AC quantity.
 Basic construction of modern PMMC can be seen in Figure 2.1.

• The meter requires low current (~50uA) for a full scale deflection, thus
consumes very low power (25-200 W).
• Its accuracy is about 2% -5% of full scale deflection 12
DC AMMETER
Ammeter is an electrical measuring device, which is used to measure
electrical current through the circuit.

Connection: It is always connected in series to a circuit

Symbol:

Ammeter is a low resistance galvanometer, ideal ammeter has zero

resistance.

- Galvanometer is essentially a PMMC instrument designed to be sensitive to

extremely low current levels

- Center-zero scale

- The scale may be calibrated in microamperes, or it

may be a millimeter scale. In the latter case, the
instrument current sensitivity is stated in
13
DC AMMETER
- Uses taut-band suspension
- Eddy current damping may be
provided as in other PMMC
instruments
- Sometimes a non-conducting coil
frame is employed, and the damping
current is generated by the moving
coil. In this case, a damping resistor is
connected in series with the coil,
which controls the level of eddy
current.
- Frequently, a critical damping
resistance value is stated, which gives
just sufficient damping to allow the
pointer to settle down quickly.
- The light beam behaves as a very
long weightless pointer which can be
substantially deflected by a very small
coil current
 DC AMMETER
 The coil winding of a basic movement is small and light, so it can carry only very
small currents.

 Current Sensitivity (usually stated in A/mm) is used to determine the current

level that produces a measured deflection.

 Pointer galvanometer: 0.1 – 1 A/mm

 Light beam galvanometer: 0.01 – 0.1 A/mm

 Galvanometer Voltage sensitivity is given as microvolts per millimeter.

Voltage sensitivity = damping resistance x current sensitivity

 Mega-Ohm sensitivity is the value of resistance that must be connected in series

with instrument to restrict the deflection to one scale division when a potential
difference of 1V is applied across its terminals.

Mega-ohm sensitivity = (1V/mm) / current sensitivity

15
DC AMMETER
One desirable modification is to
increase the range of current that can
be measured with the basic meter
movement.

This is done by placing a low

resistance in parallel with the meter
movement resistance, Rm.

This low resistance is called a shunt

(Rsh )' and its function is to provide an
alternate path for the total metered
current I around the meter movement.

16
DC AMMETER
The basic dc ammeter circuit is as
shown.

In most circuits Ish is much greater

than Im , which flows in the
movement itself.

The resistance of the shunt is

found by applying Ohm's law

I sh R sh  I m R m Rm = internal resistance of the movement

Rsh = shunt resistance
I sh  I  I m
Ish =shunt current
Im = full scale deflection current of the
I m R movement
R  m
I = full scale current of the ammeter +
I  I
sh
m shunt (i.e. total current) 17
Example:

Calculate the value of the shunt resistance required to convert a 1-mA meter movement. with a

100Ω internal resistance, into a 0 to 10-mA ammeter.

The purpose of designing the shunt circuit is to allow us to measure a current I that is some number n times
larger than 1m. The number n is called a multiplying factor and relates total current and meter current as

So
Multirange Ammeter
 The range of the dc ammeter is extended by a number of shunts, selected
by a range switch.

 The resistors is placed in parallel to give different current ranges.

 Make-before-break switch The instrument is not left without a shunt in

parallel with it. During switching there are actually two shunts in parallel
with the instrument.

 Increase cost of the meter.

19
Aryton shunt or Universal shunt

 Aryton shunt eliminates the

possibility of having the meter in
the circuit without a shunt.
 Reduce cost
 Position of the switch:

a) Range 1: Ra parallel with series combination of Rb, Rc and the meter movement.
Current through the shunt is more than the current through the meter movement,
thereby protecting the meter movement and reducing its sensitivity.
b) Range 2: Ra and Rb in parallel with the series combination of Rc and the meter
movement. The current through the meter is more than the current through the shunt
resistance.
c) Range 3: Ra, Rb and Rc in parallel with the meter. Maximum current flows
20
through the meter movement and very little through the shunt. This will increase the
sensitivity.
Aryton shunt or Universal shunt
On a multiple-range ammeter, the Ayrton shunt, or the universal shunt is frequently a more suitable
design.
One advantage of the Ayrton shunt is that it eliminates the possibility of the meter movement being in the
circuit without any shunt resistance.
Another advantage is that it may be used with a wide range of meter movements.

The individual resistance values of the shunts are calculated

by starting with the most sensitive range and working
toward the least sensitive range.

On this range the shunt resistance is equal to Rsh and

can be computed by

Rsh = Ra + Rb +Rc
For the given configuration Rsh = Ra + Rm and it is parallel with Rb +Rc so the voltage across each
parallel branch should be equal and can be written as

= Ra + Rb +Rc
In current and resistance terms we can write

Multiplying through by Im

This can be rewritten as

Where as VRc = V(Ra +Rb + Rm)

Than
 Example:
Design a multi range ammeter by computing the values of the shunt resistors
for the current rages of I1 = 10mA, I2 = 100mA and I3 = 1A.
Step 1: Calculate shunt resistance Rsh is found from

This is the shunt for the 10mA range (most sensitive

range).

Step 2: 100-mA range, The resistors Rb and Rc

provide the shunt. The total shunt
Resistance is found by the equation

Step 3: 1A range, The resistors Rc provide the shunt.

Step 4:
SOLVE AS HOME PRACTICE
Design an Aryton shunt to provide an ammeter with a
current range of 0-1 mA, 10 mA, 50 mA and 100 mA. A D’
Arsonval movement with an internal resistance of 100Ω and
full scale current of 50 uA is used.

1mA
+ R4
D’Arsonval
Movement

10mA
+
R3

50mA _
R2

100mA
R1

_ 24
Requirement of a shunt

1) Minimum Thermo Dielectric Voltage Drop

Soldering of joint should not cause a voltage drop.

2) Solder ability
- never connect an ammeter across a source of e.m.f
- observe the correct polarity
- when using the multi-range meter, first use the highest current range.

3) Swamping Resistance
 The moving coil in a PMMC instrument is wound with thin copper wire, and its
resistance can change significantly when its temperature changes.
 The heating effect of the coil current may be enough to produce a resistance change,
which will introduce an error.
 To minimize the error, a swamping resistance made of manganin or constantan is
connected in series with the coil (manganin and constantan have resistance temperature
coefficients very close to zero.

25
 AMMETER INSERTION EFFECTS
 Inserting Ammeter in a circuit always increases the
resistance of the circuit and, thus always reduces the
current in the circuit. The expected current:

E
Ie 
R1
 Placing the meter in series with R1 causes the

current to reduce to a value equal to:

E
Im 
R1  R m
It allows us to determine the error introduced into a circuit caused by ammeter insertion if we know
the value of Thevenin's equivalent resistance and the resistance of the ammeter.

Im R1
 Dividing equation (2-5) by (2-4) yields:

Ie R1  R m
The Ammeter insertion error is given by :
 

I 26
Insertion Error
  1  m  X 100
 Ie 
EXAMPLE: A current meter that has an internal resistance of 78 Ω is used to measure

the current through resistor Rc in Fig. Determine the percentage of error of the

reading due to ammeter insertion.

SOLUTION: Thevenin's equivalent resistance

The ratio of meter current to expected current is

The percentage of error attributable to ammeter

insertion as

27
Concept of DC Voltmeter
 The deflection of PMMC is directly proportional to current passing
through the moving coil, while the current through the coil is
proportional to voltage across it  so the scale of PMMC can be
calibrated to measure voltage.
To use the basic meter as a dc voltmeter, must know the amount of
current (Ifsd) required to deflect the basic meter to full scale.
1
 The sensitivity is based on the fact that the full scale current should S
results whenever a certain amount of resistance is present in the meter
I fsd
circuit for each voltage applied.

Example: Calculate the sensitivity of a 200 A

meter movement which is to be used as a dc
voltmeter. 1 1
S   5k / V
Solution: I fsd 200uA

 The resistance of the moving coil is very small  so small

voltage can be detected
To avoid this we need to add a series resistance, which is called 28

multiplier resistance
 DC Voltmeter
A basic D’Arsonval movement can be converted into a DC voltmeter
by adding a series resistor (multiplier) as shown in Figure 2.3.
Im = full scale deflection current of
Rs the movement (Ifsd)
+
Im
Multiplier Rm= internal resistance of the
Rm movement
V
Rs = multiplier resistance
_
V = full range voltage of the
instrument
Basic DC Voltmeter

V  I m ( Rs  Rm )
V  I m Rm V
Therefore, Rs    Rm
Im Im 29
EXAMPLE
A basic D’ Arsonval movement with a full-scale deflection of 50 uA and
internal resistance of 500Ω is used as a DC voltmeter. Determine the value
of the multiplier resistance needed to measure a voltage range of 0-10V.

Solution: V 10V
Rs   Rm   500  199.5k
Im 50uA
 Sensitivity and voltmeter range can be used to calculate the multiplier
resistance, Rs of a DC voltmeter. Rs = (S x Range) - Rm

EXAMPLE
If Im = 50uA, Rm = 500Ω, for the range of 10V find Rs
Sensitivity,

1 1
S   20k / V
I m 50uA
So,
Rs = (20kΩ/V x 10V) – 500 Ω = 199.5 kΩ 31
EXAMPLE
Calculate the value of the
multiplier resistance for the
multiple range dc voltmeter
circuit .

The sensitivity is

32
Multi-Range Voltmeter

A DC voltmeter can be converted into a multi-range voltmeter by

connecting a number of resistors (multipliers) in series with the
meter movement.
A practical multi-range DC voltmeter is shown in Figure 2.6

Figure: Multirange voltmeter

33

The advantage of this circuit is except R4 all other resistances are standard.
EXAMPLE
Calculate the value of multiplier
resistances for multiple range dc
voltmeter

34
 VOLTMETER LOADING EFFECTS
 When a voltmeter is used to measure the voltage across a circuit component, the voltmeter
circuit itself is in parallel with the circuit component.

 Total resistance will decrease, so the voltage across component will also decrease. This is called
voltmeter loading.

 The resulting error is called a loading error.

 The voltmeter loading can be reduced by using a high sensitivity voltmeter.

Exampe: Two different voltmeters are used to measure the voltage across resistor RB in the circuit.
The meters are as follows.

Meter A: S = 1 kΩ/V. Rm = 0.2 k Ω.

range = 10 V

Meter B: S = 20 k Ω /V. Rm = 1.5 k Ω.

range = 10 V
Calculate

(a) Voltage across Rb without any meter

connected across it
(b) Voltage across Rb when meter A is used. 35
(c) Voltage across Rb when meter B is used.
(d) Error in voltmeter readings.
 (a) The voltage across resistor Rb without either meter connected is found using the voltage divider
equation:

(b) Starting with meter A the total resistance it presents to the circuit is

The parallel combination of Ra and meter A IS

Therefore. the voltage reading obtained with meter A. determined by the voltage divider equation. is

36
= 3.53 V
(c) The total resistance that meter B presents to the circuit is

The parallel combination of Rb and meter B is

Therefore. the voltage reading obtained with meter B. determined by use of the voltage divider equation. is

(Expected value - Measured value)

(d) Voltmeter A error  100%
Expected value

37
Although the reading obtained with meter B is much closer to the correct value, the voltmeter still
introduced a 2% error due to loading of the circuit by the voltmeter.
EXAMPLE: Find the voltage reading and the percentage of error of each reading obtained with a voltmeter on

(a) Its 3-V range.

(b) Its 10-V range.

(c) Its 30-V range

The instrument has a 20-kΩ/v sensitivity and is connected across RB

SOLUTION: The voltage drop across Ra without the voltmeter connected is computed as

(a) On the 3-V range

The voltmeter reading is

38
The percentage of error on the 3-V range is

(b) On the 10-V range.

The voltmeter reading is

39
(c) On the 30-V range.

The voltmeter reading is

The percentage of error on the 30-V range is

We can experimentally determine whether the voltmeter is

introducing Appreciable error by changing to a higher range. If the
voltmeter reading does not change. The meter is not loading the
40
circuit appreciably. If loading is observed, select the range with the
greatest deflection and yielding the most precise measurement.
Deflection Instrument Error

Reading Errors:

• Bearing Friction
• Improperly adjusted zero
• Incorrect reading of the pointer indication.
Specified Accuracy:

• In High Quality instrument accuracy might be specified as a percentage of

the actual scaling or measured quantity.
• In general manufacturers specify the accuracy as percentage of FSD.
42
HW