OHMMeters
Electronic measuring instruments
20201120 00:13:31
OHMMeters
The instrument, which is used to measure the value of resistance between any two points in an electrical circuit is called ohmmeter . It can also be used to find the value of an unknown resistor. The units of resistance are ohm and the measuring instrument is the meter. Thus, the word "ohmmeter" is obtained by combining the words "ohm" and "meter" .
Types of ohmmeters
These are the two types of ohmmeters.
 Series ohmmeter
 Shunt ohmmeter
Now let's talk about these two types of ohmmeters one by one.
Series ohmmeter
If the resistance value is unknown and needs to be measured by placing it in series with the ohmmeter, then this ohmmeter is called a series ohmmeter. The electrical diagram of the series ohmmeter is shown below.
The part of the circuit, which is on the left side of terminals A and B, is a serial ohmmeter . So we can measure the value of resistance iKnown by placing it on the right side of terminals A and B. Now let's talk about the calibration scale of the serial ohmmeter.

If $ R_ {x} = 0: Omega $, then terminals A and B will be shorted to each other. So the current from the meter is full scale deviation current . Therefore, this meter full scale deviation current can be represented by $ 0: Omega $.

If $ R_ {x} = infty: Omega $, then terminals A and B will be open circuit with each other. Thus, no current flows through the resistance, $ R_ {1} $. In this case, the meter displays zero deflection current. Therefore, this devizero ation of the meter can be represented by $ infty Omega $.

In this way, considering different values of $ R_ {x} $, the counter displays different deflections. So as a result, we can represent these deflections with the corresponding resistance value.
The serial ohmmeter consists of a calibration scale. It has the indications of 0 $ Omega $ and $ infty: Omega $ at the right hand and left hand ends of the scale respectively. The series ohmmeter is useful for measuring high values of resistances.
Shunt ohmmeter
If the resistance value is unknown and to be measured by placing it in parallel (shunt) with the ohmmeter, then this ohmmeter is called shunt ohmmeter. The electrical diagram of the shunt ohmmeter is shown in the figure below.
The part of the circuit, which is on the left side of terminals A and B is a shunt ohmmeter . Thus, we can measure the value of the unknown resistance by placing it on the right side of terminals A and B.
Now let's talk about the calibration scale of the shunt ohmmeter. Close the switch, S of the above circuit while in use.

If $ R_ {x} = 0: Omega $, then terminals A and B will be shorted to each other. As a result, all the current, $ I_ {1} $, flows through terminals A and B. In this case, no current flows through the PMMC galvanometer. Therefore, the zero deviation of the PMMC galvanometer can be represented by $ 0: Omega $.

If $ R_ {x} = infty: Omega $, then terminals A and B will be open circuit with each other. Thus, no current flows through terminals A and B. In this case, all the current, $ I_ {1} $, passes through the PMMC galvanometer. If necessary, vary (adjust) the resistor value, $ R_ {1} $ until the PMMC galvanometer displays the full scale deflection current. Therefore, thi s the deflection currentfull scale of the PMMC galvanometer can be represented by $ infty: Omega $

In this way, by considering different values of $ R_ {x} $, the meter displays different deflections. So as a result, we can represent these deflections with the corresponding resistance values.
The shunt ohmmeter consists of a calibration scale. It has the indications of $ 0: Omega $ and $ infty: Omega $ at the left hand and right hand ends of the scale respectively.
The shunt ohmmeter is useful for measuring low resistance values . Thus, we can use a series ohmmeter or a shunt ohmmeter based on the values of the resistors to be measured, i.e. high or low.