Working principle of Series-Type Ohmmeter

Working principle of Series-Type Ohmmeter Points : Working principle of Series-Type Ohmmeter When terminals A and B are shorted together and the value of shunt resistor R_sh is adjusted so that the instrument indicates the full scale reading on the scale then this position of the pointer corresponds to zero resistance. When terminal A and B are left open no current flows through the meter and it does not give any movement on the scale and this position of pointer corresponds to resistance.

If the battery voltage V, series resistance R_se and shunt resistance R_sh are constant then marking can placed on the scale for the position of pointer corresponding to different values of R_x x and the accuracy of the reading depends on the calibration process and accuracy of the instrument. Resistance R_sh is a variable resistor and is adjusted to counter act the effect of change in battery voltage due to use and age.

A convenient quantity to use in the design of a series type ohmmeter is the value of R_x which causes half-full scale deflection of the instrument. At this position the resistance across terminals A and B is defined as the half scale position resistance R_H. If full scale deflection current of meter, I_fm, internal resistance of the meter, R_m the battery voltage V and the half scale resistance R_h are given then the circuit can be analysed and the values of R_se and R_sh can be determined.

If introduction of resistance R_h reduces the full scale deflection current I_f to half, the value of R_h must be equal to the total resistance R_sh in series with parallel combination of R_sh and R_m.