SHUNT TYPE OMMETER

 

The circuit diagram of a shunt type ohmmeter is shown in figure 1

                   

Shunt type ohmmeter

Construction:

It consists of a battery in series with an adjustable resistor R₁ and a D’Arsonval movement. The unknown resistance is connected across terminals A and B, in parallel with the meter. In this circuit it is necessary to have an ON/OFF switch to disconnect the battery from the circuit when the instrument is not used.

Case 1:

When the unknown resistor R_x = 0Ω (A and B shorted) the meter current is zero, since it is by passed by the short circuit this pointer is marked as “0” ohms.

Case 2:

When the unknown resistor R_x = ∞ (A and B open) the current flows through the meter movements and by appropriate selection of the values of R1, the pointer can be made to read full scale deflection current. This position of the pointer is marked “∞” ohms.  

Case 3:

Intermediate markings can be done by connecting known values of standard resistor to the terminals A and B. The ohmmeter therefore has the “0” mark at the left side of the scale (no current) and the “∞” at the right side of the scale (full scale deflection current) as shown in figure 2

The shunt type ohmmeter is especially suited to the measurement of low values of resistance. Hence it's used as a test instrument within the laboratory for special low resistance applications.

Analysis: The analysis of the shunt type ohmmeter’s similar to that of the series type ohmmeter. In figure 1

When

R_x = ∞, the full scale meter current

I_fsd = V/ R₁ + R_m                         ……………………     (1)

Where

V = Internal battery voltage

R₁ = Current limiting resistor

R_m = Internal resistance of the movement

Solving for R1, we get

R₁ = V / I_fsd – R_m

For any value of R_x, connected across the meter terminals, the meter current decreases and is given by

 I_m = V / R₁ + R_m R_x /R_m + R_x  *  R_x / R_m + R_x

     = V. R_x / R₁ R_m + R_x (R₁ + R_m)

At half scale reading of the meter (I_m = 0.5 I_fsd)

 R_x = R_h. Then

0.5I_fsd = V. R_h / R₁ R_m + R_h (R₁ + R_m)          …………………          (2)

 Where R_h = external resistance causing half scale deflection.

To determine the relative scale values for a given value of R₁, the half scale reading may be found by dividing (1) by equation (2) and solving for R_h.

R_h = R₁ R_m / R₁ + R_m

The analysis shows that the half scale resistance is determined by limiting resistance R₁ and the internal resistance of the movement R_m. The limiting resistance R₁ is turn determined by the meter resistance R_m and the full scale deflection current I_fsd.