type of ohmmeter

S C
E T .
 BRANCH:- ELECTRICAL
 SUBJECT:- Electrical Measurement and Measuring Instruments
 SUBJECT CODE:- 2130903
Topic
Types of Ohmmeter
NAME OF STUDENT: ENROLLMENT NO:
Katargamwala Payal
Parmar Divya D. 140123109007
Rana Parth R. 140423109011
Shaikh Mohammed Jahir R. 140423109013
Vasava Chirag 140123109014

INTRODUCTION
Ohmmeter
 An ohmmeter is an instrument used to measure resistance
and check the continuity of electrical circuits and component.
 It has a low degree of accuracy. The statement regarding
accuracy is not intended in an unfavourable sense
 An ohmmeter is useful for determining the approximate
resistance of circuit components such as heater elements or
machine field coils , measuring and sorting of resistors used in
electronic circuits , checking of semiconductor diodes.

Operation of an Ohmmeter
 The purpose of an ohmmeter, of course, 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 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.
 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. According
to the following equation, derived from Ohm's Law, the value of the
resistance (R) is given by R =V/I.

TYPES OF OHMMETER
According to the construction
 Shunt type ohmmeter
 Series type ohmmeter
According to the display
 Analog ohmmeter
 Digital ohmmeter

SERIES TYPE OHMMETER
1. In this Figure,
R1 is the current limiting resistor,
R2 is the zero adjust resistor,
RX is the unknown resistor,
E is the internal battery voltage and
Rm is the internal resistance of the d’Arsonval movement.
A and B are the output terminals of the ohmmeter across
which an unknown resistor is connected.

2. When RX = 0 (short circuit), R2 is adjusted to get full-scale
current through the movement. Then, I = Ifsd. The
pointer will be deflected to its maximum position on the
scale. Therefore, this full-scale current reading is marked 0
ohms.
3. When RX = ∞ (open circuit), I = 0. The pointer will read
zero. Therefore, the zero current reading is marked ∞
ohms.

SHUNT TYPE OHMMETER
1. Figure shows the basic circuit of the shunt-type ohmmeter where
movement mechanism is connected parallel to the unknown resistance.
In this circuit it is necessary to use a switch, otherwise current will always
flow in the movement mechanism.
2. Resistor ‘Rsh’ is used to bypass excess current.

3. Let the switch be closed. When RX = 0 (short circuit), the
pointer reads zero because full current flows through Rx and
no current flows through the meter and Rsh. Therefore, zero
current reading is marked 0 ohms.
4. When RX = ∞ (open circuit), no current flows through RX.
Resistor R1 is adjusted so that full-scale current flows through
the meter. Therefore, maximum current reading is marked ∞
ohms.

DIGITAL OHMMETER
 Digital ohmmeter s are used to measure the resistance
accurately
 It shows the exact reading of the resistance
 It is mostly used to measure the earth resistance

ANALOG OHMMETER
 Analog ohmmeters are used to
measure the resistance of the circuit with
respect to the current flowing the circuit
 It shows the reading in the analog form
so if there is mistake by the reader in taking
reader there it will cause great difference in
the calculations.
 Many analog ohmmeters will, when
switched to the ohm function, reverse the
polarity of the test leads.

type of ohmmeter

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