Question

A half wave rectifier type AC voltmeter consists of a series resistance $R_s$, ideal diode and PMMC instrument as shown in figure. The internal resistance of instrument is $100\Omega$ and full scale deflection is produced by a DC current of 10 mA. The value of $R_s$ required to obtain full scale deflection with an AC voltage of 100 V (rms), applied to the input terminal is _______ $\Omega$.

• A. 44
• B. 4400
• C. 440
• D. 401

Answer 1

The correct option is B 4400

$\text{DC sensitivity of meter}$
$=\frac{1}{I_{FSD}}=\frac{1}{10\times {10}^{-3}}=0.1k\Omega /V$

$\text{For half wave rectifier,}$

$\text{The AC sensitivity,}$
$S_{AC}=0.45\times S_{DC}=45\Omega /V$

$\text{Resistance of series multiplier,}$
$R_S=S_{AC}\times V_{AC}\left(rms\right)-R_m-R_D$

$R_S=45\times 100-100-0=4400$