ANALOG OHMMETERS & MULTIMETERS

RAHUL RANJAN

11115074

OHM-METER

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•An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current

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•It has a low degree of accuracy. The statement regarding accuracy is not intended in an unfavourable sense.

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•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 Analog OHM-METER

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•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.

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•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.

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•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.

Problems of analog OHM-METER

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•One major problem with this design is its reliance upon a stable battery voltage for accurate resistance reading. If the battery voltage decreases (as all chemical batteries do with age and use), the ohmmeter scale will lose accuracy.

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•With the series range resistor at a constant value and the battery voltage decreasing, the meter will no longer deflect full-scale to the right when the test leads are shorted together.

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•There are design techniques used to compensate for varying battery voltage, but they do not completely take care of the problem and are to be considered approximations at best. For this reason, and for the fact of the nonlinear scale, this type of ohmmeter is never considered to be a precision instrument.

MULTIMETERS

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•A multimeter or a multitester, also known as a VOM (Volt-Ohm meter) is an electronic measuring instrument used to measure voltage, current, and resistance in one unit.

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•Analog multimeters use a microammeter whose pointer moves over a scale calibrated for all the different measurements that can be made. 

•While Digital multimeters display the measured value in numerals.

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•Digital multimeters are now far more common than analog ones, but analog multimeters are still preferable in some cases, for example when monitoring a rapidly-varying value.

ANALOG MULTIMETER vs DIGITAL MULTIMETER

ANALOG MULTIMETER

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•The main part of an analog multi meter is the D’Arsonval meter movement also known as the permanent-magnet moving-coil (PMMC) movement.

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•When the meter current ‘Im’ flows in the wire coil, a magnetic field is produced in the coil. This electrically induced magnetic field interacts with the magnetic field of the horseshoe-type permanent magnet.

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•The result of such an interaction is a force causing a mechanical torque to be exerted on the coil. Since the coil is wound and permanently fixed on a rotating cylindrical drum as shown.

ANALOG MULTIMETER

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•Each meter movement is characterized by two electrical quantities : R.m & I.fs. the electrical circuit symbol of the meter movement that will be as shown

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•The PMMC movement cannot be used directly for ac measurements since the inertia of PMMC acts as an average. Because ac current has zero average value and it produces a torque that has also zero average value.

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•In order to make ac measurements, a bridge rectifier circuit is combined with PMMC as shown

WORKING OF ANALOG MULTIMETER

Ammeter

•The deflection of the pointer in the D’Arsonval meter movement is proportional to the meter current I. Therefore, this instrument can be used to measure current.

Voltmeter

•To increase the full-scale voltage range of the movement when functioning as a voltmeter, the meter movement current Im has to be lowered. This can easily be achieved by inserting a large resistance, called the multiplier resistance

Ohmmeter

•If the meter movement current Im is somehow made to be proportional to the value of an unknown resistance to be measured, the meter's scale can be calibrated to read resistance directly. Here, a voltage source (e.g., a battery) must be added to the meter’s circuit to drive the current necessary for the deflection of the pointer.

Resolution & Accuracy of Analog Multimeter

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•Resolution of analog multimeters is limited by the width of the scale pointer, parallax, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display

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•Accuracy of readings obtained is also often compromised by miscounting division markings, errors in arithmetic, parallax observation errors, and less than perfect eyesight.

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•Mirrored scales and larger meter movements are used to improve resolution and accuracy.

References:

www.Wikipedia.org www.allaboutcircuits.com A Course in Electrical and Electronic Measurement and

Instrumentation

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