A moving coil instrument has a coil resistance of 12Q anda maximum permissible

instrument current of 18 mA. The instrument is converted to a voltmeter with a full

scale deflection of 250 V.

(i) Determine the value of the resistor to give full scale deflection. (6 marks)June/July 2020

To determine the value of the resistor needed to give full scale deflection on the moving coil instrument, we can use Ohm’s law and the definition of full scale deflection.

Ohm’s law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, provided that the temperature and the physical properties of the conductor remain constant. This can be expressed as:

I = V/R

where I is the current, V is the voltage, and R is the resistance.

Full scale deflection (FSD) is the maximum reading that can be displayed on a meter. For a voltmeter, the FSD is the maximum voltage that can be measured.

In this case, we are given that the FSD of the voltmeter is 250 V, the coil resistance is 12 Q, and the maximum permissible instrument current is 18 mA. We need to determine the value of the resistor needed to give full scale deflection.

We can set up the following equation using Ohm’s law and the definition of FSD:

250 V = (18 mA)(R)

Solving for R, we get:

R = 250 V / 18 mA = 13888.9 ohms

Therefore, the value of the resistor needed to give full scale deflection on the moving coil instrument is 13888.9 ohms.