ANALOG OHMMETERS &
• An ohmmeter is an electrical instrument that measures electrical
resistance, the opposition to an electric current
• 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
Operation of an Analog 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
Problems of analog OHM-METER
• 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.
• 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.
• 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
• 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
• 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.
• 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.
• The main part of an analog multi meter is the
D’Arsonval meter movement also known as the
permanent-magnet moving-coil (PMMC) movement.
• 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.
• 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.
• 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
• 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
• In order to make ac measurements, a bridge rectifier
circuit is combined with PMMC as shown
WORKING OF ANALOG
• 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.
• 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
• 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
Ohmmeter battery) must be added to the meter’s circuit to drive the
current necessary for the deflection of the pointer.
Resolution & Accuracy of Analog
• 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
• Accuracy of readings obtained is also often compromised by miscounting division
markings, errors in arithmetic, parallax observation errors, and less than perfect eyesight.
• Mirrored scales and larger meter movements are used to improve resolution and accuracy.
A Course in Electrical and Electronic Measurement and