DirectCurrent and AlternatingCurrent Indicating Instruments

1
LECTURE 3

• Direct-Current and Alternating-Current Indicating
  Instruments

2
CONTENTS

• Series-Type Ohmmeter
• Shunt-Type Ohmmeter
• Multimeter or VOM
• Calibration of DC Instruments
• Introduction to Alternating-current Indicating
  Instruments
• Electrodynamometer
• Moving Iron Instruments

3
Series-Type Ohmmeter CONTENTS

• Certain disadvantage when the battery is old,
  the full-scale current drops and the meter does
  not read "0" when A and B are shorted
• The design can be approach by recognizing that,
  if introducing Rh reduces the meter current to

4
Shunt-Type Ohmmeter CONTENTS

• Particularly suited to the measurement of
  low-value resistors
• When the full-scale meter current will
  be
• The meter current for any value of Rx ,
  expressed as a fraction of the full-scale
  current, is
• At half-scale reading of the meter

5
Multimeter or VOM(Simpson Model 260) CONTENTS
6

• Schematic diagram of the Simpson Model 260
  multimeter
• dc voltmeter section of the Simpson Model 260
  multimeter

7

• dc ammeter section of the Simpson Model 260
  multimeter
• Ohmmeter section of the Simpson Model 260
  multimeter

8
Calibration of DC Instruments CONTENTS

• Potentiometer method of calibrating a dc ammeter
  ---?
• Potentiometer method of calibrating a dc
  voltmeter ---?

9
Introduction to Alternating-current Indicating
Instruments CONTENTS

• The d'Arsonval movement responds to the average
  or dc value of the current through the moving
  coil
• If the movement carries an alternating current
  with positive and negative half cycles, the
  driving torque would be in one direction for the
  positive alternation and other direction for the
  negative alternation
• If the frequency of the ac is very low, the
  pointer would swing back and forth around zero
  point on the meter scale
• At higher frequencies, the inertia of the coil is
  so great that the pointer cannot follow the rapid
  reversals of the driving torque and hovers around
  the zero mark, vibrating slightly.

10
Electrodynamometer CONTENTS

• In the electrodynamometer the flux density (B)
  depends on the current through the fixed coil and
  is therefore directly proportional to the
  deflection current (I)
• Since the coil dimension and the number of turns
  on the coil frame are fixed quantities fo any
  given meter, the developed torque becomes a
  function of the current squared (I2).
• The electrodynanometer is very useful as a
  calibration instrument and is often used for this
  purpose because of its inherent accuracy
• Disadvantage high power comsumption

11
Moving Iron Instruments CONTENTS

• Moving-iron instruments can be classified into
  two types attraction and repulsion instruments
• Radial-vane repulsion moving-iron mechanism