DC Meters

1
DC Meters

• still in Chapter 02

2
Warm-ups

• stand-up 3x.
• sit down and relax.
• listening to a song by Halim Yazid titled
  ingat sokmo hok mok pesan.
• take a deep breath.

3
Objectives

• At the end of this chapter, the students should
  be able to
• explain the meaning of electrical meters.
• explain the purpose of shunts across a meter.
• calculate the value of shunt resistors across a
  meter.

4
Outlines

• Introduction What is a meter?.
• Pmmc in a DC Ammeter.
• Ammeter Insertion Effects.

5
Introduction

• What are meters?
• Meters are used to measure current and voltage.
• The meter most likely encountered will be a
  single low range meter such as 0 - 1 mA full
  deflection meter of the D'Arsonval type.
• The d'Arsonval type meter works on the principle
  a coil of wire to which a pointer is attached is
  pivoted between the poles of a permanent magnet.

6
Introduction

• What are meters?
• When current flows through the coil, it sets up a
  magnetic field that interacts with the field of
  the magnet to cause the coil to turn.
• The meter pointer deflects in direct proportion
  to the current. This meter is called an ammeter.

7
Introduction
Figure 1 A typical 0 to 1mA ammeter.
8
DC Ammeter

• DC Ammeter
• a device used to measure current.
• put in series/ parallel with the circuit.
• very common in lab.
• use unit Ampere (A)/ mA.
• used the principle of the dArsonval meter
  movement with slight modification.

9
DC Ammeter

• DC Ammeter
• placing a LOW resistance in PARALLEL with the
  meter movement resistance to increase the range
  of current that can be measured by the meter.
• Rsh shunt resistor.
• Rm meter movement resistance.

10
DC Ammeter

• Ish current tru the shunt.
• Im full-scale deflection current of meter
  movement.
• I full-scale deflection current for the
  ammeter.

Figure 2 dArsonval used in DC Ammeter circuit.
11
DC Ammeter

• Vm ImRm
• Vsh Vm
• Ish I Im
• Rsh Vsh /Ish
• ImRm /Ish
• Im/(I -Im)Rm ?

Figure 2 dArsonval used in DC Ammeter circuit.
(single range only).
12
DC Ammeter

• Example 2-1
• Calculate the value of the shunt resistance
  required to convert a 1-mA meter movement, with a
  100-? internal resistance, into a 0- to 10-mA
  ammeter.

13
DC Ammeter

• Example 2-2
• A 100-mA meter movement with an internal
  resistance of 800-? is used in 0- to 100-mA
  ammeter. Find the value of the required shunt
  resistance.

14
Ayrton Shunt

• Also known as the universal shunt.
• Used on a multiple range ammeter.
• It eliminates the possibility of the meter
  movements being in the circuit without any shunt
  resistance.
• It maybe used with a wide range of meter
  movements.

15
Ayrton Shunt
Fig. 3 An Ammeter using Ayrton shunt.
16
Ayrton Shunt
From figure (RbRc)(I2-Im) Im
(RaRm) I2(RbRc) - Im(RbRc) Im Rsh-(RbRc)
Rm) I2(RbRc) - Im(RbRc) ImRsh- Im(RbRc)
ImRm. ThusRbRc Im(RshRm) / I2 (1) Ra Rsh
(RbRc) (2) Rc Im(RshRm) / I3 (3)
Fig. 3 An Ammeter using Ayrton shunt.
17
Ayrton Shunt

• Remember
• n I / Im
• Rsh Rm / (n-1) n 20.

18
Ayrton Shunt
Example 2.3Compute the value of the shunt
resistors for the circuit. Given that Rm 1k?,
Im 100 mA, I110mA, I2100mA, I31A. Check
Rsh Ra Rb Rc always!
Fig. 3 An Ammeter using Ayrton shunt.
19
Ammeter Insertion Effects

• A frequently overlooked source of error in
  measurements.
• All ammeters contain some internal resistance.
• inserting an Ammeter in a circuit always
  increases the resistances of the circuit,
  therefore reduces the measured current in the
  circuit.
• The error caused by the meter depends on the
  value of resistances in Ammeter and also the
  measured circuit.

20
Ammeter Insertion Effects
Fig.4a Expected current in a simple circuit. I
E / R1
Fig.4b Expected current in a series circuit
with an ammeter. I E / (R1Rm)
21
Ammeter Insertion Effects

• Example 2.4
• A current meter having an internal resistance of
  88? is used to measure the current trough Rc in
  figure below. Determine the percentage of error
  due to the ammeter insertion.
• Given that
• RaRbRc 1k?

22
Summary

• In this chapter, we have discussed about
• introduction to electrical meters.
• shunt resistor in a single-range Ammeter.
• universal shunt in multiple-range Ammeter.
• calculation of shunt resistors.
• ammeter insertion effects.

23
Conclusion

• Also known as the universal shunt.
• Used on a multiple range ammeter.
• It eliminates the possibility of the meter
  movements being in the circuit without any shunt
  resistance.
• It maybe used with a wide range of meter
  movements.

24
Evaluation

• Quiz.