CHAPTER 2B_DC AMMETER_DC VOLTMETER_LOADING EFFECT

DEE10013
MEASUREMENT DEVICES

CHAPTER 2

DC VOLTMETER, LOADING EFFECT
& DC AMMETER METERS
(PART B)

Course Learning Outcome

CLO1:
Apply the concept of measurement in electrical and electronic
equipment using appropriate theorem.

CLO2:
Perform meter calibrating and measuring technique using the
correct measuring equipment.

CLO3:
Demonstrate good communication skill in oral presentation within
a stipulated time frame.

DEE10013_June2019 2

LEARNING OUTCOME

2.2 Understand PMMC meter movement
on DC ammeter and voltmeter

2.3 Apply DC ammeter and voltmeter in
measuring current and voltage

DEE10013_June2019 3

DC VOLTMETER

Single Range
DC Voltmeter

Two(multi range)
DC Voltmeter

4

5

HOW TO BUILD A DC VOLTMETER

Variable D’Arsonval
Resistor

Power
Supply

6

7

HOW TO MEASURE VOLTAGE DROP USING DC
VOLTMETER

8

SINGLE RANGE DC VOLTMETER CIRCUIT

Rs = Multiplier Resistor /Series resistor

Im = Meter current/full scale deflection

current

Rm = Internal resistance/coil resistance9

V = voltage range

DC VOLTMETER

VDC called a  A large resistance is placed
Rs meter in series with a current
multiplier reading meter (D’Arsonval
V (Rs) meter movement)

Ifs = Im  the current multiplied by
Rm the resistance,( Ix R,=V)

 will be the voltage drop across
the resistance.

 This is called a meter
multiplier (Rs) and

V  a meter calibrated in terms
IR of the voltage drop is called
a DC voltmeter.

10

DC VOLTMETER

 The FUNCTION of the
multiplier resistor (RS) are:

 to increase the voltmeter range
 to limit current flowing through

the D’Arsonval to a full scale
current deflection.

11

DC VOLTMETER  RT = RS + Rm

?  RT – Rm = RS

 V( ) – Rm = RS
Ifs @

 RS will find from the Sensitivity, S
of meter

V SERIES CIRCUIT  SSV =- Rm1Ifs= (Ω/V)
IR V= RS
RT =
I=  (1/IFS)V – Rm = RS

12

DC VOLTMETER

• Calculate the sensitivity of a 200A meter movement which is to be used
as a dc voltmeter.

13

DC VOLTMETER

• Calculate the value of the multiplier resistance on the 0-50V range of a dc
voltmeter that used a 500A meter movement with internal resistance of
1k.

14

DC VOLTMETER

• Determine the value of the multiplier resistor RS to be connected in series
to the D’Arsonval meter movement of 1 mA, and internal resistance 850 
in the design of a Dc voltmeter with a range of 10 V.

15

DC VOLTMETER
• A moving coil instrument with a full scale deflection
of 50mA and internal resistance of 300Ω is used as a
voltmeter. Calculate the value of multiplier resistance
needed to measure a voltage range of 0 ~ 15V.

16

DC VOLTMETER

• Given a Permanent Magnet Moving Coil (PMMC) instrument with 900  coil
resistance with, Full Scale Deflection (FSD) of 15mA is to be used as a DC
voltmeter with the range of 0 – 100V. Calculate the multiplier resistance RS?

17

DC VOLTMETER

• A PMMC instrument with full scale deflection (FSD) of a 100µA
and a coil resistance of 1 k is to be converted into a voltmeter.
Determine the required multipler resistance if the voltmeter is
to measure 50V at fullscale. Also calculate the applied voltage
when the instrument indicates 0.8, 0.5, and 0.2 of FSD.

18

TWO RANGE DC VOLTMETERS

1. The resistors are placed in series to give
different voltage ranges.

2. The selector switch S is multiposition switch is
used for the range changing.

3. When S =V1 (maximum voltage range) R1 + R2
acts as a multiplier resistance, Rs

4. when the switch S = V2 (lowest voltage range)
only R2 acts as a multiplier resistance, Rs

19

TWO RANGE DC VOLTMETERS

Two Range DC Voltmeter

In position V2 (lowest voltage range), the multiplier

resistance is only R2,

V2 = Im (R2 + Rm)

R2 + Rm = V2/Im

R2 = V2/Im – Rm …………(1) 20

TWO RANGE DC VOLTMETERS

Two Range DC Voltmeter

In position V1 (maximum voltage range), the
multiplier resistance are R1 and R2,

V1 = Im (R1 + R2 + Rm)
R1 + R2 + Rm = V1/Im

R1 = V1/Im – Rm – R2 …………(2)
21

DC VOLTMETER

Can 22

RCSalc=ula(teV/IFS) - Rm
RS = SV - Rm

DEE1012-MEASUREMENT

EXAMPLE 3

A moving coil instrument with an internal resistance of 50Ω
and a full scale deflection current of 2mA is to be used as a
two range voltmeter. Design the multipliers resistance to
obtain the voltage ranges of 0 – 10V and
0 – 50V.

Solution:

For position V 2 = 10V , series multiplier is R2

R2 = V 2 − Rm
Im

= 10 − 50
2 x10−3

= 4.95k

23

CONTINUE

For position V1 = 50V , series multiplier is R1+ R2

R1+ R2 = V1 − Rm
Im

= 50 − 50
2x10−3

= 24.95k

R2 + R1 = 24.95k

R1 = 24.95k − R2

R1 = 24.95k − 4.95k

R1 = 20k 24

DC VOLTMETER

 Calculate all the series resistors needed for the design of a multiple range
DC voltmeter of the Aryton Shunt type with range of 2V, 5V, and 10V. The
full scale current range of the D’Arsonval meter movement is 200 A with
internal resistance of 5 k.

25

LOADING EFFECT

• Ideally, a perfect meter would have no loading
effect, but all meters have some loading effect
on the circuit they are measuring.

• When voltmeter connected to circuit, it will
lowers the effective resistance of the circuit and
change the voltage reading

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

26

LOADING EFFECT

• The resulting error is called a loading
error/effect. The voltmeter loading can be
reduced by using a high sensitivity voltmeter

• For high resistance circuits-use high sensitivity
voltmeter to prevent loading effect

• For Low resistance circuit, loading effect is less

27

EXAMPLE

A simple series circuit of R1 and R2 connected to a 100V
DC source. If the voltage across R2 is to be measured by
voltmeter having
a) A sensitivity of 1000 Ω/V
b) A sensitivity 20000 Ω/V, find which voltmeter will read

the accurate value of voltage across R2
Assume both the meter are used on the 50V range

28

Solution:

True Voltage across the R2 resistance
10K X 100V = 50V

10K+10K

a)Voltmeter having sensitivity of 1000 Ω/V,
Rm = S x V

It has a resistance of 1000 Ω/V X 50V = 50KΩ on its
50V range

29

CONTINUE

Req (Rm//R2)= 10K X 50K =
8.33KΩ
Voltage across V2 = Re1q0K + X50VK

R1 + Req

8.33K X 100V = 45.43V

10K+ 8.33K 30

CONTINUE

b) Voltmeter having sensitivity of 20000 Ω/V,
It has a resistance of 20000 Ω/V X 50V = 1MΩ on its 50V range

Req = 10K X 1M = 9.9KΩ
10K + 1M

Voltage across V2 = Req X V
R1 + Req

9.9K X 100V = 49.74V
10K+ 9.9K
So second voltmeter reads more accurately. Thus the
high sensitivity voltmeter gives more accurate reading,
though the voltage range for both the meter is same.

31

Meter Voltage True Rm Vdrop
Sensitivit meter Voltage, (Rm = S x across R2
y VR2 V)
(Voltage
1000 Ω/V 50V divider) 50KΩ 45.43V

50V

20000 50V 50V 1MΩ 49.74V
Ω/V

32

DC AMMETER

DC AMMETER

Single Range
DC Ammeter

Two(multi Range)
DC Ammeter

34

SINGLE RANGE DC AMMETER CIRCUIT

+ ISH Vm Im
Rm
I RSH

VSH

RSH = Shunt resistance

Rm = Internal resistance/coil resistance movements

(movable coil)

ISH = Shunt current

Im /Ifsd = Meter current/ Full scale deflection current of the

meter movement 35

I = Current range/Full-scale deflection current for
ammet*eRrSH is smaller than Rm.

FUNCTION OF SHUNT (PARALLEL) RESISTANCE,
RSH IN DC AMMETER

1. The basic DC ammeter circuit can be constructed
using a PMMC by connecting a resistor in parallel
with the meter movement.

2. The parallel resistor is called a shunt resistor and
its function to control the amount of current
passing through the meter movement.

3. Thus the shunt resistance and PMMC can be thought
of as forming a current dividing circuit.

A low value resistor (shunt resistor) is used in DC ammeter to
measure large current. Basic DC ammeter:

36

DERIVATION EQUATION FOR SHUNT RESISTOR IN SINGLE RANGE DC

AMMETER Im • Voltage equation:

I ISH • Vm = ImRm Vsh = Ish Rsh

+ VSH RSH Vm Vm = VSH

Rm Im .Rm = ISH.RSH

PARALLEL CIRCUIT RSH = Im Rm equation 1
ISH
V V=
IR R= • Current equation:

I= I = ISH + Im

ISH = I − Im to equation 2

Substitute : eq (2)into eq (1)

RSH = Im .Rm 37
I − Im

DERIVATION EQUATION FOR SHUNT RESISTOR IN SINGLE RANGE DC
AMMETER

I Im RSH = Im Rm equation 1
ISH
+ VSH ISH
Vm ISH = I − Im to equation 2
RSH
Rm Substitute : eq (2)into eq (1)

RSH = Im .Rm to equa3t8ion 3
I − Im
Purpose I >>n Im , n = multiplying factor

EXAMPLE 3

A 100uA meter movement with an internal resistance of
800Ω is used in a 0 - 100 mA ammeter . Find the value of
the required shunt resistance.
Solution:

39

EXAMPLE 4

A 1mA meter movement with an internal resistance of 100Ω
is to be converted into a 0-100 mA. Calculate the value of
shunt resistance required.
Solution:

40

EXAMPLE 4

A 2mA meter movement with an internal
resistance of 100Ω is to be converted into a 0-
150 mA. Calculate the value of shunt resistance
required.
Solution:

41

EXAMPLE 4_ANSWER

A 2mA meter movement with an internal
resistance of 100Ω is to be converted into a 0-
150 mA. Calculate the value of shunt resistance
required.
Solution:

42

EXAMPLE 5

43

EXAMPLE 6 : DC AMMETER D’ARSONVAL

44

EXAMPLE 7

 A PMMC movement having Im = 1mA & Rm = 500Ω is to be converted
into ammeter. Calculate shunt resistances for current ranges 50mA

ANS: 10.2 Ohm

ANS: 9.05 Ohm

45

ANS: 0.50 Ohm

ANS: 5 Ohm
ANS: 0.05 Ohm

46

(1) Multirange Ammeter :
(Individual Shunt Circuit)

(2) Ayrton Shunt:
(Individual Ayrton Shunt

Circuit)

47

Multirange DC Ammeter

+
+

R1 R2 R3 R4 Rm
_

D’Arsonval
Movement

S

_

Figure 2.3: Multirange Ammeter

48

Aryton Shunt (2 RANGE)

Total shunt resistor, RshT = R2 + R1

Total resistor, RT = Rsh2 + Rsh1

# To calculate the total shunt resistor,
determine from the lowest range.

# To calculate another RSH, start
from the highest range.

49

Aryton Shunt (2 RANGE)

I1 Im
ISH
Rm
R2

I2

R1

50