Question

The figure below shows a circuit for an AC voltmeter. It utilizes a moving-coil meter that gives a full-scale reading when the average current flowing through it is 1 mA. The moving-coil meter has a 50-Ω resistance. The value of R that results in the meter indicating a full-scale reading when the input sine-wave voltage VI is 20 V peak-to-peak is

(a) 3.183 kΩ

(b) 3.133 kΩ

(c) 3.183 Ω

(d) 56.183 Ω

The question was asked in an online interview.

This key question is from The Ideal Diode in portion Diodes of Electronic Devices & Circuits

Answer 1

Right option is (b) 3.133 kΩ

To explain I would say: Average value of input voltage= 20/π or 6.366 V.

Since there is diode half of the time there is no voltage across the voltmeter hence the average value of the input voltage is 10/π or 3.183V.

The total resistance required to generate 1mA current is 3.183 kΩ out of which 50Ω is provided by the moving coil. Therefore, a resistance of 3.133 kΩ is required.