JNTU B.Tech III ELECTRONIC MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (November/December - 2020 - R16)

A basic DC voltmeter consists of a PMMC (Permanent Magnet Moving Coil) galvanometer connected in series with a high resistance, known as a multiplier. The multiplier limits the current through the meter movement to a safe value, preventing damage and allowing the instrument to measure higher voltages than the basic movement's full-scale deflection (FSD) capacity.

To derive the multiplier resistance, let I_m be the full-scale deflection current of the meter, R_m be the internal resistance of the meter, V be the total voltage to be measured, and R_s be the series multiplier resistance. According to Ohm's Law, the total voltage is the product of the total resistance and the current:

Rearranging to solve for R_s:

This expression shows that the multiplier resistance must be chosen based on the desired voltage range and the meter's internal characteristics.

Performance characteristics of a measurement system are categorized into static and dynamic characteristics.

Static Characteristics: These describe the system's performance when the input is constant or varies very slowly.

• Accuracy: The closeness of the measured value to the true value.
• Precision: The degree of reproducibility among several independent measurements of the same quantity.
• Sensitivity: The ratio of the change in output to the change in input.
• Resolution: The smallest change in the input that can be detected by the instrument.
• Linearity: The ability of the output to vary linearly with the input.
• Hysteresis: The difference in output readings for the same input when approached from increasing versus decreasing directions.
• Threshold and Dead Zone: The minimum input required to produce a detectable output and the range of input for which there is no output, respectively.

Dynamic Characteristics: These describe the system's response to time-varying inputs.

• Speed of Response: How quickly the system reacts to a change in input.
• Dynamic Error: The difference between the true value and the measured value during dynamic conditions.
• Fidelity: The ability of the system to reproduce the input signal without distortion.
• Lag: The time delay between the input change and the corresponding output response.