Question

An ac voltmeter uses the circuit shown below, where the PMMC meter has an internal resistance of $100\Omega$ and requires a dc current of 1 mA for full scale deflection.
Assuming the diodes to be ideal, the value of $R_s$ to obtain full scale deflection with 100 V (ac rms) applied to the input terminal would be

• A. $80k\Omega$
• B. $89k\Omega$
• C. $89.9k\Omega$
• D. $90k\Omega$

Answer 1

The correct option is C $89.9k\Omega$

It is a full wave rectifier

So current output across PMMC meter will be as


For full wave bridge rectifier,

$R_s=0.9\times \left[S_{DC}\right]\times V_{rms}\left(AC\right)-2R_D-R_M$

$=\frac{1}{I_{DC\left(PSD\right)}}=\frac{1}{{10}^{-3}}={10}^3$

$R_D=\text{Forward diode resistance}=0\Omega$
$R_M=\text{Internal meter resistance}=100\Omega$
$R_S=0.9\times {10}^3\times 100V-100\Omega =89.9k\Omega$