type of ohmmeter

type of ohmmeter ba

1. S ARVAJANIK C OLLEGE OF E NGG& T ECH . 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

2. i ntroduction O hmmeter 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.

3. T he 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. T he 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. O peration of an Ohmmeter

4. Basic Construction of Ohmmeter

5. T ypes of ohmmeter A ccording to the construction Shunt type ohmmeter Series type ohmmeter A ccording to the display Analog ohmmeter Digital ohmmeter

6. Series type ohmmeter In this Figure , R 1 is the current limiting resistor, R 2 is the zero adjust resistor , R X is the unknown resistor , E is the internal battery voltage and R m 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.

7. 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.

8. Shunt type ohmmeter F igure 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. R esistor ‘R sh’ is used to bypass excess current.

9. 3. Let the switch be closed. When R X = 0 (short circuit), the pointer reads zero because full current flows through R x and no current flows through the meter and R sh . Therefore, zero current reading is marked 0 ohms . 4 . When R X = ∞ (open circuit), no current flows through R X . Resistor R 1 is adjusted so that full-scale current flows through the meter. Therefore, maximum current reading is marked ∞ ohms .

10. 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

11. Analog ohmmeter A nalog ohmmeters are used to measure the resistance of the circuit with respect to the current flowing the circuit I t 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 . M any analog ohmmeters will, when switched to the ohm function, reverse the polarity of the test leads.

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