CHAPTER 2B_DC AMMETER_DC VOLTMETER_LOADING EFFECT

Aryton Shunt (2 RANGE)

Rm

RSH 2 ISH Im

RSH 1

I2 I1

+-

51

TWO RANGE DC AMMETERS

1. The resistors are placed in parallel to give
different current ranges.

2. Then, it becomes two shunts Rsh1 and Rsh2
and will give two different ranges of Ish1 and
Ish2 .

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

4. Switch S also protects the meter m52 ovement
from

TWO RANGE DC AMMETERS

1. When switch, S is connected at Rsh1, the shunt
resistance Rsh1 and Rm are connected in parallel.

2. Using Ohm’s law V=IR, since it is in parallel
connection,

Vm= voltage drop across the Rm
IS EQUAL

Vsh1= voltage drop across the Rsh153
Rsh1 = ImRm / I1 - Im

EXAMPLE 6

Design a two range dc miliammeter with a basic
meter having a resistance 75Ω and full scale
deflection for the current of 2mA. The required
ranges are 0-10mA and 0-50mA.

Solution:
The first range is 0-10mA
Give I1 = 10mA, Im = 2mA and Rm =75Ω

Rsh1 = ImRm / I1 - Im

Rsh1 = 2m(75)/(10m- 54
2m)

CONTINUE

The second range is 0-50mA
Give I2 = 50mA, Im = 2mA and Rm =75Ω

Rsh2 = ImRm / I2 - Im
Rsh2 = 2m(75)/(50m-
2m)

= 3.125Ω

55

TWO RANGE DC AMMETERS (AYRTON SHUNT)

R1

1
2

Rm

3 R2

R3

56

EXAMPLE 7

Rm = 100Ω

R2 R1 Im = 50µA
I2 =1A
+ ISH

I1 =10mA

-

57

Solution :

 At range I1 = 10mA Parallel Concept :
❖ VT = Vm = Vsh
Rm = 100Ω ❖ IT = Im + Ish
❖ RT = Rm // Rsh

Im = 50µA

R 2 R 1 ISH 1

+ R SH I1 =10mA

-

58

Rm = 800Ω

Im = 100µA

R 2 R 1 ISH

+ I2 =15mA I1 =20mA

-

59

Solution :

 At range I1 = 10mA Parallel Concept :
❖ VT = Vm = Vsh
Rm = 800Ω ❖ IT = Im + Ish
❖ RT = Rm // Rsh
Im = 100µA

R 2 R 1 ISH 1

R SH I1 =20mA

+

-

60

Solution :

 At range I2 = 1A

Rm = 800Ω R1 Parallel Concept :
❖ VT = Vm = Vsh
Im = 100µA ❖ IT = Im + Ish
❖ RT = Rm // Rsh
R 2 ISH 2

I2 =15mA

+

61

Rm = 800Ω

Im = 100µA

R 2 R 1 ISH

+ I2 =20mA I1 =15mA

-

62

TUTORIAL

1. Calculate the value of the shunt resistance required to
convert a 1mA meter movement, with a 100Ω internal
resistance into a 0 to 10 mA ammeter (Ans: 11.11 Ω)

2. A 100µA meter movement with an internal resistance
of 800Ω is used in 0 to 100mA ammeter. Find the value of
the required shunt resistance (Ans: 0.8Ω)

3. Design a two range DC ammeter with a basic meter
having a resistance 50Ω and full scale deflection for the
current of 2mA. The required ranges are 0-10mA and 0-
25mA.Calculate the value of the required shunt resistances
(Ans: 12.5 Ω, 4.35 Ω)

63

TUTORIAL

4. Design a multirange ammeter by using direct method
(individual shunt) to give the following ranges 10mA and
50mA. If PMMC meter have internal resistance of 10Ω and
full scale current of 3mA (Ans: 4.29 Ω, 0.64Ω)

5. A moving coil instrument gives a full scale deflection
with a current of 40μA, while the internal resistance of the
meter is 500Ω. It is to be used as a voltmeter to measure a
voltage range of 0-10V. Calculate the multiplier resistance
needed (Ans: 249.5K Ω)

6. PMMC instrument with an internal resistance of 50Ω and
a full scale deflection current of 1.5mA is to be used in the
design of multirange DC voltmeter with voltage ranges of
0-10V and 0-25V. Calculate the value of the m64 ultiplier
resistors in the circuit.