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OHMMETER
OHMMETER
-The purpose of an Ohmmeter is to measure resistance.
-Resistance reading is indicated through a mechanical meter movement which operates on electric current.
-Thus, Ohmmeter must have an internal source of voltage to create current necessary to operate the movement.
-Ohmmeter also have an appropriate ranging resistors to allow just the right amount of current.
-There are 3 types of Ohmmeter:
Series Ohmmeter
Shunt Ohmmeter
Multi-range Ohmmeter
Scale deflection.
SERIES OHMMETER
SERIES OHMMETER
In series ohmmeter, the measuring resistance component or circuit is connected in series with the meter. The value of resistance is measured through the d'Arsonval movement connected in parallel with the shunt resistor R2. The parallel resistance R2 is connected in series with the resistance R1 and the battery. The component whose resistance is used to be measured is connected in series with the terminal A and B.
When the value of unknown resistance is zero the large current flow through the meter. In this condition, the shunt resistance is adjusted until the meter indicates the full load current. For full load current, the pointer deflects towards zero 0 ohms. When the unknown resistance Rx is removed from the circuit the resistance of the circuit becomes infinite and no current flow through the circuit. The pointer of the meter deflects towards the infinity. The meter shows the infinite resistance at zero current and the zero resistance when full range current flows through it. When the unknown resistance is connected in series with the circuit and if their resistance is high, then the pointer of the meter deflects toward the left and if the resistance is low, then pointer deflects toward the right.
To produce full scale current, the battery current must be doubled.
Total current of circuit, IT=2Ih=V/Rh
Shunt current through R2, I2=It-Ifsd
The voltage across R2 or shunt, Vsh=Vm
Therefore Vsh=Vm
I2R2=IfsdRm
Therefore R2=(IfsdRm)/I2
I2=It-Ifsd
R2=(IfsdRm)/(It-Ifsd)
It=V/Rh
Therefore
R2=(IfsdRm)/(V/Rh)-Ifsd
R2=(IfsdRmRh)/(V-IfsdRh)
As,
Rh=R1+(R2Rm)/(R2+Rm)
R1=Rh-[(R2Rm)/(R2+Rm)]
Therefore
R1=Rh-[(IfsdRmRh)/V]
*Sub R2 from previous formula
SHUNT TYPE OHMMETER
SHUNT TYPE OHMMETER
The meter in which the measuring resistance is connected in parallel with the battery is known as the shunt ohmmeter. It is mainky used for measuring the low-value resistance. The battery (E), basic meter (Rm) and the adjustable resistance are the main components of the shunt ohmmeter. The unknown resistance is connected across terminal A and B.
When the value of unknown resistance is zero the meter current becomes zero. And if the resistance becomes infinite (i.e., the terminal A and B are open) then the current passes through the battery and the pointer shows the full-scale deflection toward left. The shunt type ohmmeter has the zero mark (no current) on the left of the scale and the infinity mark on the right side.
MULTI RANGE OHMMETER
MULTI RANGE OHMMETER
The range of this type of ohmmeter is very high. The meter has adjuster which selects the range according to need. For example, consider we use the meter for measuring the resistance under 10 ohms. For this first, we have set the range of 10 ohms. The resistance whose value is used to be measured is connected in parallel with the meter. The magnitude of the resistance is determined through the deflection of the pointer.
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