3.0
RESISTANCE MEASUREMENT
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Resistance Measurement
 The resistances are classified as ;
 1. Low Resistance : All resistances of the order of 1 ohm
and below. Example: Machine armature, series field
winding shunt etc.
 2. Medium Resistance : All resistances of the order of 1
ohm to 100,000 ohms. Example: Winding resistance,
multiplier resistance.
 3. High Resistance : All resistances of the order of 100,000
ohm and above. Example: Insulation resistance of machines,
cables,
porcelain insulator etc.
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Ohmmeter
 The purpose of an ohmmeter, is to measure the
resistance placed between its leads.
 This resistance reading is indicated through a mechanical
meter movement which operates on electric current.
 The ohmmeter must then have an internal source of
voltage to create the necessary current to operate the
movement, and also have appropriate ranging resistors to
allow just the right amount of current through the
movement at any given resistance.
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 The original design of an ohmmeter provided a small battery
to apply a voltage to a resistance. It used a galvanometer to
measure the electric current through the resistance.
 The scale of the galvanometer was marked in ohms, because
the fixed voltage from the battery assured that as resistance
decreased, the current through the meter would increase.
 A more accurate type of ohmmeter has an electronic circuit
that passes a constant current I through the resistance, and
another circuit that measures the voltage V across the
resistance.
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 The standard way to measure resistance in ohms is to supply
a constant voltage to the resistance and measure the current
through it.
 That current is of course inversely proportional to the
resistance according to Ohm's law, so that you have a nonlinear scale.
 The current registered by the current sensing element is
proportional to 1/R, so that a large current implies a small
resistance.
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Series Type Ohmmeter
 It contains galvanometer connected
in parallel with shunting resistor R2.
 This parallel circuit is in series with
a resistance R1 and a battery of EMF,
E.
 This series circuit is connected to
the terminals A and B of the
unknown resistance R
 R1 current limiting resistor
 R2 zero adjusting resistor
 Rm internal resistance of
galvanometer
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Operation
 Supply is given through the battery B and voltage E is supplied.
 If terminal A and B are short circuited by shunting the resistance R,
maximum power flows through the meter. Under this condition,
resistor R2 is adjusted until the pointer indicates full scale current
Ifs. The full scale current position of the pointer is marked ‘0’ Ω on
the scale.
 When R is removed from the circuit, the terminals are open, the
movement indicates zero, which is marked infinite ‘ ’ for zero
current.
 Intermediate scale markings may be placed on the scale by different
known values of resistance ‘R’ in the instrument. The accuracy of
these scale markings depends on the movement and the tolerance of
the calibrating resistors.
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 Although the series type ohmmeter is a popular design and it is used
extensively is portable instrument for general service work.
 The important disadvantages of ohmmeter is internal battery, whose
voltage decreases gradually with time and age. So the full scale drops down
and meter does not read zero when the terminals are short circuited.
 Rectified AC given to some ohmmeter is the power source. In this case,
the voltage must be regulated and given.
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Shunt Type Ohmmeter
 It consists of a battery in series with an adjustable resistor R1 and a basic
d’Arsonval movement (meter)
 The unknown resistance is connected across terminals A and B, parallel with the
meter.
 In this circuit it is necessary to have an ON-OFF switch to disconnect the battery
from the circuit when the instrument is not in use.
 When the unknown resistor, Rx = 0 (shorted), the meter current is zero. If the
unknown resistor
Rx =  (open), current find path only through the meter
and selecting a proper value for
resistance R1, the pointer may
be made to read full scale.
LHS( 0 = no current) and
RHS (  = full scale deflection current )
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Megger
 The megger is an instrument used for the measurement
high resistance and insulation resistance.
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Construction
 There are two pairs of permanent magnets, one set for ohmmeter
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and the other for the generator.
 The moving coil consists of three coils namely control coil,
deflecting coil and compensating coil.
 The control coil and deflecting coil are fixed at right angles to one
another and free to move on a stationary C shaped iron core.
 The compensating coil in series with a control coil and protection
resistance R is connected across the generator terminals. The coils
are connected to the circuits system through flexible leads. These
leads do not exert any force on the moving system at anytime which
will therefore take up any position when the generator is not driven.
A resistor R is connected in series with deflecting coil to protect the
deflecting coil under short circuit condition. The guard ring bypasses
the leakage current, if any to negative terminal of the generation and
prevents leakage current from entering the deflecting coil.
Operation
 The unknown resistance is connected between the terminal L (line)
& E (earth).
 The generator handle is then steadily turned at uniform speed. There
is a slip mechanism in the drive which ensures a limited speed.
 When the resistance value is small, the current through the
deflecting coil will be high, its deflecting torque will be very high
and hence the pointer will move to the extreme clock wise position
‘0’ or very low resistance value.
 When the resistance value is high, the current through the current
coil will be high, its deflecting torque will be very high and hence
the pointer will be taken to the extreme anticlockwise position
indicating infinity or very high resistance value.
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Multimeter
 A multimeter or a multitester is an
electronic measuring instrument that
combines several functions in one
unit.
 Multimeter is basically a PMMC
meter.
 The most basic instruments include an
ammeter, voltmeter, and ohmmeter
with a function switch.
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DC Ammeter Section
 DC currents are measured making use a suitably designed
shunt resistors.
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Multi-range Ohm-meter
 Multi-range ohm-meter is built with the meter movement,
battery cells, shunt and series resistors
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DC Voltmeter Section
 The meter movement has a resistance of 2000 ohms.
 Suitable resistor are added as multiplier to get voltage
range from 2.5V to 250V.
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Merits/Demerits of Multimeter
 Merits
 It is single meter that performs several measuring
functions.
 It has a small size and portable
 It can made measurements with reasonable accuracy
 Demerits
 It cannot make precise and accurate measurements
due to the loading effect.
 Technical skill is required to handle it
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AC Voltmeter Section
 To measure AC voltage the output voltage is rectified
before the current passes through the meter using
half wave rectifier.
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Multimeter – Capabilities
 DC Voltage Measurements

 AC Voltage RMS Measurements
 DC and AC Current Measurements
 Resistance Measurements
 Capacitance/Inductance Measurements
 Frequency/Period Measurements
 Diode Measurements
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- End –
be continue…Chapter 3
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