JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year ELECTRONIC MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (February - 2022 - R16)

A DC Voltmeter is primarily constructed using a PMMC (Permanent Magnet Moving Coil) movement connected in series with a high resistance, known as a multiplier. The fundamental purpose of the multiplier is to limit the current through the meter movement to a safe value that corresponds to the full-scale deflection of the pointer.

The working principle is based on Ohm's Law. If the meter movement has an internal resistance R_m and requires a full-scale deflection current I_fsd, the total resistance R_t required to measure a voltage V is given by R_t = V / I_fsd. The multiplier resistance R_s is then calculated as R_s = R_t - R_m = (V / I_fsd) - R_m.

• Multiplier Resistor: This is a precision resistor connected in series with the meter movement. It must have high stability and low temperature coefficient to maintain measurement accuracy.
• Sensitivity: The Voltmeter Sensitivity is expressed in ohms per volt (Ω/V) and is defined as the reciprocal of the full-scale deflection current (S = 1 / I_fsd). A higher sensitivity indicates a better voltmeter as it draws less current from the circuit under test, thereby minimizing loading effects.
• Multi-range Voltmeters: To measure different voltage ranges, a range switch is used to select different multiplier resistors in series with the meter movement, allowing the same basic movement to be utilized for various full-scale voltage levels.

The D'Arsonval movement, also known as the Permanent Magnet Moving Coil (PMMC) instrument, operates on the principle of electromagnetic torque. It consists of a light-weight coil of wire suspended in a strong, uniform magnetic field created by a permanent magnet.

The working mechanism involves the following components and physical laws:

• Deflecting Torque (T_d): When a DC current flows through the coil, it experiences a force according to the Lorentz force law (F = BIlN), where B is the magnetic flux density, I is the current, l is the length of the coil, and N is the number of turns. This force creates a torque that causes the coil to rotate. The deflecting torque is directly proportional to the current (T_d ∝ I).
• Controlling Torque (T_c): Two phosphor-bronze hairsprings provide the controlling torque, which acts in opposition to the deflecting torque. This torque is proportional to the angle of deflection (T_c ∝ θ).
• Damping Torque (T_damp): To prevent the pointer from oscillating and to bring it to a steady state quickly, eddy current damping is used. This is achieved by winding the coil on a light aluminum frame; as the frame moves through the magnetic field, eddy currents are induced in it, creating a force that opposes the motion.

At steady state, T_d = T_c, resulting in a linear scale where the pointer deflection is directly proportional to the current passing through the coil. This makes the D'Arsonval movement highly accurate for DC measurements.