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OHMMETERS

The ohmmeter is a direct reading device for the measurement of resistance and is

extremely useful for the measurement of the approximate values of a resistance.

Depending on their construction and their measurement methodology these

instruments are further classified as:

Series type of Ohmmeter

Shunt type of Ohmmeter

Series Type of Ohmmeters:

It consists of a PMMC instrument connected in series with a shunting resistance R2 and

this parallel circuit is in series with a resistance R1 and battery EMF E.

The unknown resistance Rx is connected to the terminals of the circuit as shown in the

figure 1 below.

Figure - 1

Calibration of the Series type of Ohmmeter

To mark the ‘0’ reading on the scale the terminals A and B are shorted allowing a

maximum current (Ifsd) to flow through the circuit. The Resistance R2 is adjusted till the

meter shows a full scale deflection and that point is marked as ‘0’ ohms.

Similarly to mark the “∞” reading on the scale, the terminals A and B are opened no

current flows through the ckt resulting in a 0 deflection of the pointer and that point is

marked as “∞” Ohms.

B

A

V

Rm R2

R1

Rx

The unknown

Resistance

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By connection different known values of resistance across the terminals A and B

intermediate markings on the scale can be done.

The major drawback of this instrument is the decrease in voltage of the internal battery

E with age and time due to which the full scale deflection current falls and the meter

does not read zero when A and B are shorted.

This can be adjusted by varying the shunt resistor R2 which counteracts the drop in

voltage there by bringing the pointer back to 0.

The series type of ohmmeter is a simple design and is extensively used for general

purpose service work

The values of R1 and R2 can be calculated from the value of Rx that gives the full scale

deflection.

Calculation of the values R1 and R2

Let a resistance Rh be connected across the terminals A and B such that the meter gives full

scale deflection

The total resistance presented to the battery is 2Rh and the battery needs to supply a current

that is needed to produce half of the full scale deflection which is

Hence to produce a full scale deflection the battery current must be doubled which is

The current through the resistance R2 is given by

Where Im is the full scale deflection current of the PMMC instrument

The voltage across the R2 and the meter is the same

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But

Therefore

But

Therefore

That is

As

Therefore

Hence

Therefore

Expression 1

Expression 2

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Shunt type Ohmmeter:

The shunt type of Ohmmeter consists of a battery in series with an adjustable resistor R1

and a PMMC instrument as shown in the figure 2 below.

Figure - 2

The unknown resistance is connected across the terminals A and B in parallel with the

PMMC instrument and hence it is called as the shunt type of the ohmmeter.

In this circuit it is required to put and ON – OFF switch in order to disconnect the battery

when the instrument is not used.

To mark the ‘0’ reading on the scale the terminals A and B are shorted thus a zero

current flows through the circuit.

Similarly to mark the “∞” reading on the scale, the terminals A and B are opened the

current finds only through the meter movement and by varying R1, the pointer may be

adjusted to read the full scale.

The shunt type of ohm meter is specially suited for low resistance measurement and is

generally used for Low resistance applications

V

Rx

The unknown

Resistance

B

A

Rm

R1

Switch