DC

1
CHAPTER 3

• DC AC METERS
• (12 Hours)

2
CHAPTER OUTLINE

• 3.1 Introduction to DC meters
• 3.2 dArsonval meter movement
• 3.3 DC Ammeter
• 3.4 DC Voltmeter
• 3.5 DC Ohmeter
• 3.6 Introduction to AC meter
• 3.7 dArsonval meter movement (half-wave
  rectification)
• 3.8 dArsonval meter movement (full-wave
  rectification)
• 3.9 Loading effects of AC meter

3
OBJECTIVES

• At the end of this chapter, students should be
  able to
• Explain in detail the principles of operation of
  the pmmc or dArsonval meter movement
• Explain the purpose of shunts across a meter and
  multipliers in series with a meter
• Explain and calculate the voltmeter loading
  effects
• Analyze a circuit in terms of Voltmeter Loading
  Effect
• Explain the purpose of Ohmmeter
• Describe the construction and operation of a
  basic Ohmmeter

4
OBJECTIVES

• Describe the operation of half-wave rectifier
  circuit
• Trace the current path in a full-wave bridge
  rectifier circuit
• Calculate ac sensitivity and the value of
  multiplier resistors for half-wave and full-wave
  rectification

5
3.1 INTRODUCTION TO DC METERS

• A meter is any device built to detect accurately
  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 "bar graph," or some sort of display
  composed of numerical figures
• Most modern meters are "digital" in design
• Older designs of meters are mechanical in nature,
  using some kind of pointer device to show
  quantity of measurement

6
3.1 INTRODUCTION TO DC METERS

• The display mechanism of a meter is often
  referred as a movement, borrowing from its
  mechanical nature to move a pointer along a scale
  so that a measured value may be read.
• Most mechanical movements are based on the
  principle of electromagnetism- 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.

7
3.1 INTRODUCTION TO DC METERS

• 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 the strength of current
• 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 (PMMC) movement or
  dArsonval meter movement

8
3.2 dARSONVAL METER MOVEMENT

• The dArsonval meter movement is a current
  responding device which used very widely nowadays
• Current from a measured circuit passes through
  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. For this
  reason, all DC meter movements show polarity
  markings.
• The moving coil responds to the amount of current
  through its windings.

9
3.2 dARSONVAL METER MOVEMENT

• In the picture above, the meter movement
  "needle" shown is 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
• 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

10
3.2 dARSONVAL METER MOVEMENT

• The arc on the meter display is labeled with
  numbers to indicate the value of the quantity
  being measured
• In other words, if it takes 50 µA current to
  drive the needle fully to the right (making 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
• 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)

11
3.2 dARSONVAL METER MOVEMENT

• 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 shown in figure
  below.

• 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).

12
3.2 dARSONVAL METER MOVEMENT

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

13
3.2 dARSONVAL METER MOVEMENT

• 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

14
3.2 dARSONVAL METER MOVEMENT

• 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 (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.

15
3.2 dARSONVAL METER MOVEMENT

• 2 other features are used to increase the
  accuracy efficiency of this meter.
• First, an iron core is placed inside the coil
  to concentrate with 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.

16
POP QUIZ
Label the figure appropriately
17
3.3 DC AMMETER

• What are meters?
• Meters are used to measure current and voltage.
• Normally the meter 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
  that a coil of wire to which a pointer is
  attached is pivoted between the poles of a
  permanent magnet.
• 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.

18
3.3 DC AMMETER

• The meter pointer deflects in direct proportion
  to the current. This meter is called an ammeter.

Figure 1 A typical 0 to 1mA ammeter.
19
3.3 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
• placing a LOW resistance in PARALLEL with the
  meter movement resistance to increase the range
  of current that can be measured by the meter

20
3.3 DC AMMETER

• DC ammeter
• PMMC galvanometer constitutes the basic movement
  of a dc ammeter
• Since the coil winding of a basic movement is
  small and light, it can carry only very small
  currents
• When large currents are to be measured, it is
  necessary to bypass a major part of the current
  through a resistance called a shunt
• The resistance of shunt can be calculated using
  conventional circuit analysis
• Rsh shunt resistor
• Rm internal resistance of the movement
• Ish shunt current
• Im full-scale deflection current of meter
  movement
• I full-scale current of ammeter shunt (total
  current)

21
3.3 DC AMMETER
DArsonval movement
Therefore,

-

-
Basic dc Ammeter
Since Rsh is in parallel with the meter movement,
the voltage drop across the shunt and movement
must be the same.
22
EXAMPLE 3.1

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

23
3.3 DC AMMETER

• Multirange ammeter
• To obtain a multirange ammeter, a number of
  shunts are connected across the movement with a
  multi-position switch
• Referring to the figure, the circuit has 4 shunts
  Ra, Rb, Rc and Rd which can be placed in parallel
  with the movement to give four different current
  ranges

24
3.3 DC AMMETER
Ayrton Shunt

-

• Also known as universal shunt.
• Used on a multiple range ammeter.
• It eliminates the possibility of the meter
  movements being in the circuit without any shunt
  resistance ?protect the deflection instrument of
  the ammeter from an excessive current flow when
  switching between shunts.
• Advantage - maybe used as a wide range of meter
  movements.
• When the switch is in position 1, Ra is in
  parallel with the series combination of Rb, Rc
  and the meter movement.

m
A
B
2
3
1
S
-

Fig. 3 An Ammeter using Ayrton shunt.
25
3.3 DC AMMETER

• Hence the current through the shunt is more than
  the current through the meter movement,
  protecting the meter movement and reducing its
  sensitivity
• If the switch is connected to position 2, Ra
  and Rb are together in parallel with series
  combination of Rc and the meter movement
• Now the current through the meter is more than
  the current through the shunt resistance

26
3.3 DC AMMETER

• If the switch is connected to position 3, Ra,
  Rb and Rc are together in parallel with the meter
• Hence max current flows through the meter
  movement and very little through the shunt
• ? destroy the meter or blow a fuse
• ? increases the sensitivity

27
EXAMPLE 3.3

• Design an Aryton shunt (figure below) to
  provide an ammeter with a current range of 0-1mA,
  0-10mA, 0-50mA and 0-100mA. DArsonval movement
  with an internal resistance of 100O and full
  scale current of 50µA is used.

28
EXAMPLE 3.3 (solution)
(3.1)
29
EXAMPLE 3.3 (SOLUTION)
(3.2)
(3.3)
(3.4)
30
EXAMPLE 3.3 (SOLUTION)
31
EXAMPLE 3.3 (SOLUTION)

• Hence, the value of shunts are
• R1 0.05263O R3 0.4147O
• R2 0.05263O R4 4.734O

32
EXAMPLE 3.4
Compute 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.
33
3.3 DC AMMETER

• Ammeter insertion effects
• All ammeters contain some external resistance,
  which may range from a low to a greater value
• Inserting ammeter in a circuit always increase
  the resistance of the circuit and therefore
  reduces the current in the circuit.
• Without ammeter, the current flows in the
  circuit shown below can be calculated as

Current without ammeter insertion effect.

34
3.3 DC AMMETER

• However, inserting the ammeter as shown below
  will reduce the current in the circuit to

Circuit with ammeter insertion effect
35
EXAMPLE 3.5

Determine the insertion error in circuit shown
below if E100V, R1100O, and Rm100O.
36
Summary

• In this sub-topic, 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

37
3.4 DC VOLTMETER

• To use the basic meter as a dc voltmeter, it is
  necessary to know the amount of current required
  to deflect the basic meter to full scale known as
  Ifsd
• For example, suppose a 50µA current is required
  for full scale deflection. This full scale value
  will produce a voltmeter with a sensitivity of
  20kO per V.
• ? sensitivity

• Hence, a 0-1mA would have a sensitivity of????

1kO/V
38
3.4 DC VOLTMETER

• A basic DArsonval movement can be converted into
  a dc voltmeter by adding a series resistor known
  as multiplier (Rs)
• The function of multiplier is to limit the
  current through the movement so that the current
  does not exceed the full scale deflection value
• A dc voltmeter measures the potential between two
  points in a dc circuit or a circuit component

Basic dc voltmeter
39
3.4 DC VOLTMETER

• The value of Rs required is calculated as
  follows

Im full scale deflection current of the
movement (Ifsd)
40
EXAMPLE 3.6
A basic DArsonval movement with a full scale
deflection of 50µA and internal resistance of
500O is used as a voltmeter. Determine the value
of the multiplier resistance needed to measure a
voltage range of 0-10V.
41
3.4 DC VOLTMETER

• To measure the potential difference between two
  points in a dc circuit/component, a dc voltmeter
  is always connected across them with proper
  polarity

42
3.4 DC VOLTMETER

• Multirange voltmeter
• A dc voltmeter can be converted into a multirange
  voltmeter by connecting a number of resistors
  (multipliers) along with a range switch to
  provide a greater number of workable ranges
• Figure below shows a multirange voltmeter using
  four position switch and 4 multipliers R1, R2,
  R3, and R4 for voltage values V1, V2, V3 and V4.

43
EXAMPLE 3.7

• Calculate the values of Rs for the multiple-
  range DC Voltmeter circuit as shown below

44
EXAMPLE 3.7 (solution)
45
3.4 DC VOLTMETER

• Voltmeter loading effects
• When selecting a meter for a certain voltage
  measurement, it is important to consider the
  sensitivity of a dc voltmeter
• A low sensitivity meter may give a correct
  reading when measuring voltages in low resistance
  circuit but produce unreliable reading in a high
  resistance circuit
• A voltmeter when connected across two points in a
  highly resistive circuits, acts as a shunt,
  reducing the total equivalent resistance of that
  portion (Inserting voltmeter always increase the
  resistance and decrease the current flowing
  through the circuit)
• The meter then indicates a lower reading than
  what existed before the meter was connected
• This is called voltmeter loading effect and is
  caused mainly by low sensitivity instrument

46
EXAMPLE 3.8

• Two different voltmeters are used to measure the
  voltage across RB in the circuit below. The
  meters areMeter A S 1k?/VRm0.2k? Range
  10VMeter B S20k?/VRm1.5k? Range 10V
• Calculate
• Voltage across RB without any meter.
• Voltage across RB when meter A is used.
• Voltage across RB when meter B is used.
• Loading Errors in both voltmeter readings.

47
EXAMPLE 3.8 (solution)

• The voltage across the resistance RB, without any
  meter connected is calculated using the voltage
  divider formula
• ii) Starting with meter A, having sensitivity S
  1kO/V. Therefore, the total resistance it
  presents to the circuit is

The total resistance across RB is RB in parallel
with meter resistance, Rm1
48
EXAMPLE 3.8 (solution)

• Therefore, the voltage reading obtained with
  meter 1 using the voltage divider equation is
• iii) The total resistance that meter 2 presents
  to the circuit is
• The parallel combination of RB and meter 2
  gives

49
EXAMPLE 3.8 (solution)

• Therefore, the voltage reading obtained with
  meter 2 using the voltage divider equation is
• iv) The error in the reading of the voltmeter is
  given by

50
EXAMPLE 3.9

• Find the voltage reading and the percentage of
  loading error of each reading obtained with a
  voltmeter on
• Its 5-V range.
• Its 10-V range
• Its 50-V range.
• The meter has a 20-k?/V sensitivity and connected
  across RA.

51
Summary

• In this sub-topic, we have learned about
• The purpose of multipliers put in series with a
  meter movements.
• Calculation of the multiplier resistance of a
  Voltmeter
• Voltmeter loading effects
• The basic dArsonval meter movement can be
  converted to a DC Voltmeter by connecting a
  Multiplier (Rs) with the meter movement.
• Sensitivity, S is the reciprocal of the
  full-scale deflection current.
• Therefore, it is desirable to make the voltmeter
  resistance much-much more higher than the circuit
  resistance.

52
3.5 DC OHMETER

• The purpose of an Ohmmeter is to measure
  resistance
• Resistance reading is indicated trough a
  mechanical meter movement which operates on
  electric current.
• Thus, Ohmmeter must have an internal source of
  voltage to create current necessary to operate
  the movement.
• It also must have an appropriate ranging
  resistors to allow just the right amount of
  current.
• A simple Ohmmeter comprises battery and meter
  movement as in figure below

53
3.5 DC OHMETER

• When there is infinite resistance (no continuity
  between test leads), there is zero current
  through the meter movement, and the needle points
  toward the far left of the scale.
• In this regard, the ohmmeter indication is
  "backwards" because maximum indication (infinity)
  is on the left of the scale

54
3.5 DC OHMETER

• If the test leads of the Ohmmeter are directly
  shorted together (measuring zero O), the meter
  movement will have a maximum amount of current
  through it, limited only by the battery voltage
  and the movement's internal resistance
• With 9 volts of battery and only 500 O of
  internal movement resistance, current will be
  18mA, which is far beyond the full-scale rating
  of the movement ? will likely damage the meter.

55
3.5 DC OHMETER

• So, to avoid this, add series resistance to the
  meters circuit so that the movement just
  registers full-scale when the test leads are
  shorted together
• To determine the proper value for R, calculate
  the Rtotal needed to limit current to only 1mA
  (full-scale) with 9V of potential from the
  battery, then subtract the movement's internal
  resistance

56
3.5 DC OHMETER

• Now, we're still having a problem of meter range.
• On the left side of the scale we have "infinity"
  and on the right side we have zero.
• One might wonder, What does middle-of-scale
  represent?
• What figure lies exactly between zero and
  infinity?.
• Infinity is more than just a very big amount it
  is an incalculable quantity, larger than any
  definite number ever could be.

57
3.5 DC OHMETER

• If half-scale indication on any other type of
  meter represents 1/2 of the full-scale range
  value, then what is half of infinity on an
  ohmmeter scale?
• The answer to this paradox is a logarithmic
  scale!.
• With a logarithmic scale, the amount of
  resistance spanned for any given distance on the
  scale increases as the scale progresses toward
  infinity, making infinity an attainable goal.

58
3.5 DC OHMETER

• We still have a question of range for our
  ohmmeter, though. What value of resistance
  between the test leads will cause exactly 1/2
  scale deflection of the needle?
• If we know that the movement has a full-scale
  rating of 1 mA, then 0.5 mA (500 µA) must be the
  value needed for half-scale deflection. Following
  our design with the 9 volt battery as a source we
  get

59
3.5 DC OHMETER

• With an internal movement resistance of 500 O and
  a series range resistor of 8.5 kO, this leaves 9
  kO for an external (lead-to-lead) test resistance
  at 1/2 scale.
• In other words, the test resistance giving 1/2
  scale deflection in an ohmmeter is equal in value
  to the (internal) series total resistance of the
  meter circuit.
• Using Ohm's Law a few more times, we can
  determine the test resistance value for 1/4 and
  3/4 scale deflection as well

60
3.5 DC OHMETER

• 1/4 scale deflection (0.25 mA of meter current)
  

• 3/4 scale deflection (0.75 mA of meter current)

61
3.5 DC OHMETER

• So, the scale for this ohmmeter looks something
  like this

• One major problem with this design is its
  reliance upon a stable battery voltage for
  accurate resistance reading.
• If the battery voltage decreases (as all
  chemical batteries do with age and use), the
  ohmmeter scale will lose accuracy.

62
3.5 DC OHMETER

• One thing that needs to be mentioned with regard
  to ohmmeters they only function correctly when
  measuring resistance that is not being powered by
  a voltage or current source.
• In other words, you cannot measure resistance
  with an ohmmeter on a "live" circuit!
• The reason for this is simple the ohmmeter's
  accurate indication depends on the only source of
  voltage being its internal battery. The presence
  of any voltage across the component to be
  measured will interfere with the ohmmeter's
  operation.
• If the voltage is large enough, it may even
  damage the ohmmeter

63
SUMMARY

• In this sub-topic, we have learned about
• Ohmmeters contain internal sources of voltage to
  supply power in taking resistance measurements.
• An analog ohmmeter scale is "backwards" from that
  of a voltmeter or ammeter, the movement needle
  reading zero resistance at full-scale and
  infinite resistance at rest.
• Analog ohmmeters also have logarithmic scales,
  "expanded" at the low end of the scale and
  "compressed" at the high end to be able to span
  from zero to infinite resistance.
• Ohmmeters should never be connected to an
  energized circuit (that is, a circuit with its
  own source of voltage). Any voltage applied to
  the test leads of an ohmmeter will invalidate its
  reading.

64
EVALUATION

• Find the value of R, ¼ scale, ½ scale and ¾ scale
  of this Ohmmeter?

65

• THANK YOU..