Direct Current Meters

1
Chapter 02

• Direct Current Meters

2
Warm - ups
3
Objectives

• At the end of this chapter, the students should
  be able to
• Describe about the types of suspension used in
  the dArsonval meter movement.
• Explain in detail the principles of operation of
  the pmmc or dArsonval meter movement.

4
Objectives

• Explain the purpose of shunts across a meter and
  multipliers in series with a meter.
• Analyze a circuit in terms of Voltmeter Loading
  Effect and Ammeter Insertion Errors.

5
Objectives

• Describe the construction and operation of a
  basic Ohmmeter.
• Perform calculations to obtain specific meter
  range.
• Apply the concepts related to error to the
  circuits calculation.

6
Outlines

• Introduction
• pmmc dArsonval meter movement
• Ayrton Shunt
• dArsonval used in DC Voltmeter
• dArsonval used in DC Ammeter

7
Outlines

• Voltmeter Loading Effects
• Ammeter Insertion Effects
• Ohmmeter
• Multiple-range Ohmmeter

8
Outlines

• Multi-meter
• Example of applications
• Assignment 1

9
What is a meter?

• A meter is any device built to accurately detect
  and display an electrical quantity in a form
  readable by a human being.
• Usually this "readable form" is visual motion of
  a pointer on a scale, a series of lights arranged
  to form a "bargraph," or some sort of display
  composed of numerical figures.

10
What is a meter?

• Most modern meters are "digital" in design,
  meaning that their readable display is in the
  form of numerical digits.
• Older designs of meters are mechanical in nature,
  using some kind of pointer device to show
  quantity of measurement.

11
What is a meter?

• The display mechanism of a meter is often
  referred to as a movement, borrowing from its
  mechanical nature to move a pointer along a scale
  so that a measured value may be read.
• Mechanical meter movement designs are very
  understandable.

12
What is a meter?

• Most mechanical movements are based on the
  principle of electromagnetism that electric
  current through a conductor produces a magnetic
  field perpendicular to the axis of electron flow.
  
• The greater the electric current, the stronger
  the magnetic field produced.

13
What is a meter?

• If the magnetic field formed by the conductor is
  allowed to interact with another magnetic field,
  a physical force will be generated between the
  two sources of fields.
• If one of these sources is free to move with
  respect to the other, it will do so as current is
  conducted through the wire, the motion (usually
  against the resistance of a spring) being
  proportional to strength of current.

14
What is a meter?

• Practical electromagnetic meter movements can be
  made now where a pivoting wire coil is suspended
  in a strong magnetic field, shielded from the
  majority of outside influences.
• Such an instrument design is generally known as a
  permanent-magnet, moving coil, or PMMC movement .

15
What is a meter?
16
pmmcdArsonval

• In the picture above, the meter movement "needle"
  is shown pointing somewhere around 35 percent of
  full-scale, zero being full to the left of the
  arc and full-scale being completely to the right
  of the arc.
• An increase in measured current will drive the
  needle to point further to the right and a
  decrease will cause the needle to drop back down
  toward its resting point on the left.

17
pmmcdArsonval

• The arc on the meter display is labeled with
  numbers to indicate the value of the quantity
  being measured, whatever that quantity is.
• In other words, if it takes 50 microamps of
  current to drive the needle fully to the right
  (making this a "50 µA full-scale movement"), the
  scale would have 0 µA written at the very left
  end and 50 µA at the very right, 25 µA being
  marked in the middle of the scale.
• In all likelihood, the scale would be divided
  into much smaller graduating marks, probably
  every 5 or 1 µA, to allow whoever is viewing the
  movement to infer a more precise reading from the
  needle's position.

18
pmmcdArsonval

• The basic principle of this device is the
  interaction of magnetic fields from a permanent
  magnet and the field around a conductor (a simple
  electromagnet).
• A permanent-magnet moving-coil (PMMC) movement is
  based upon a fixed permanent magnet and a coil of
  wire which is able to move, as in next figures.

19
pmmcdArsonval

• The basic principle of this device is the
  interaction of magnetic fields from a permanent
  magnet and the field around a conductor (a simple
  electromagnet).
• A permanent-magnet moving-coil (PMMC) movement is
  based upon a fixed permanent magnet and a coil of
  wire which is able to move, as in next figures.

20
pmmcdArsonval

• When the switch is closed, the coil will have a
  magnetic field which will react to the magnetic
  field of the permanent magnet. The bottom portion
  of the coil in Figure 2(a) will be the north pole
  of this electromagnet.
• Since opposite poles attract, the coil will move
  to the position shown in Figure 2(b).

21
pmmcdArsonval

• To use pmmc as a meter, 2 problems must be
  solved.
• First, a way must be found to return the coil to
  its original position when there is no current
  through the coil.
• Second, a method is needed to indicate the amount
  of coil movement.

22
pmmcdArsonval

• The first problem is solved by the
• use of hairsprings attached to each end of the
  coil.
• These hairsprings can also be used to make the
  electrical connections to the coil.
• With the hairsprings, the coil will return to
  its initial position when there is no current.
• The springs will also tend to resist the movement
  of the coil when there is current through the
  coil.

23
pmmcdArsonval

• As the current through the coil increases, the
  magnetic field generated around the coil
  increases.
• The stronger the magnetic field around the coil,
  the farther the coil will move. This is a good
  basis for a meter.
• But, how will you know how far the coil moves?
• If a pointer is attached to the coil and
  extended out to a scale, the pointer will move as
  the coil moves, and the scale can be marked to
  indicate the amount of current through the coil.

24
pmmcdArsonval

• 2 other features are used to increase the
  accuracy efficiency of this meter.
• First, an iron core is placed inside the coil
  to concentrate the magnetic fields.
• Second, curved pole pieces are attached to the
  magnet to ensure that the turning force on the
  coil increases steadily as the current increases.
• The meter movement as it appears when fully
  assembled is shown in this figure.

25
pmmcdArsonval

• The dArsonval meter movement is very widely
  used.
• Current from a measured circuit passes tru the
  windings of the moving coils causes it to behave
  as an electromagnetic.
• The poles of EMT interact with the poles of PM,
  causing the coils to rotate.
• The pointer deflects up scale whenever current
  flows in proper direction in the coil.

26
pmmcdArsonval

• For this reason, all DC meter movements show
  polarity markings.
• dArsonval meter movement is a current responding
  device.
• Regardless of the units (volt,ohm,etc) for which
  the scale is calibrated, the moving coil responds
  to the amount of current thru its windings.

27
summary

• The basic principle and operation of pmmc or
  dArsonval meter movement.
• The two (2) features used to increase the
  accuracy efficiency of this PMMC meters are
• First, an iron core is placed inside the coil to
  concentrate the magnetic fields.
• Second, curved pole pieces are attached to the
  magnet to ensure that the turning force on the
  coil increases steadily as the current increases.
• Regardless of the units (volt,ohm,etc) for which
  the scale is calibrated, the moving coil responds
  to the amount of current thru its windings.

28
conclusion

• The students should be able to describe in detail
  about the basic principles of operation of the
  pmmc or dArsonval meter movement.

29
evaluation
Label the figure appropriately