S C

E T . BRANCH:- ELECTRICAL

SUBJECT:- Electrical Measurement and Measuring Instruments

SUBJECT CODE:- 2130903

NAME OF STUDENT: ENROLLMENT NO:

Katargamwala Payal

Parmar Divya D. 140123109007

Rana Parth R. 140423109011

Shaikh Mohammed Jahir R. 140423109013

Vasava Chirag 140123109014

Topic

Types of Ohmmeter

INTRODUCTION

Ohmmeter

An ohmmeter is an instrument used to measure resistanceand 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.

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.

Operation of an Ohmmeter

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.