JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year ELECTRONIC MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (February - 2022 - R18)

To convert a D'Arsonval movement into a multi-range voltmeter using a potential divider (series string) arrangement, the total resistance R_T for each range is calculated using the formula R_T = V / I_m, where V is the full-scale voltage and I_m = 1 mA = 0.001 A. The series multiplier resistance R_s is then R_s = R_T - R_m, where R_m = 100 Ω.

1. For 0-10V range: R_T = 10 / 0.001 = 10,000 Ω. R_s1 = 10,000 - 100 = 9,900 Ω.
2. For 0-50V range: R_T = 50 / 0.001 = 50,000 Ω. R_s2 = 50,000 - 10,000 = 40,000 Ω.
3. For 0-250V range: R_T = 250 / 0.001 = 250,000 Ω. R_s3 = 250,000 - 50,000 = 200,000 Ω.
4. For 0-500V range: R_T = 500 / 0.001 = 500,000 Ω. R_s4 = 500,000 - 250,000 = 250,000 Ω.

A true RMS responding voltmeter measures the effective heating value of a waveform regardless of its shape. The principle relies on the thermal equivalence of the input signal to a DC voltage.

The circuit typically employs a thermal converter consisting of a heater element and a thermocouple. The input voltage is applied to the heater, generating heat proportional to the square of the RMS value of the input signal (P = I²R). The thermocouple produces a DC output voltage proportional to this heat. This DC signal is then amplified and compared against a reference DC voltage in a feedback loop to maintain the system's accuracy.

Key components include an input attenuator, a high-gain amplifier, the thermal sensing element, and an output display. By using a feedback loop, the system ensures that the output is directly proportional to the true RMS value, effectively linearizing the non-linear thermal response.

Applications:

• Measurement of non-sinusoidal waveforms, such as square waves, pulses, and distorted signals.
• Calibration of high-frequency signal generators.
• Power measurement in industrial electronic circuits where harmonic distortion is present.
• Laboratory standards for voltage and current measurements where high accuracy is required for complex waveforms.