Question

A half wave rectifier type AC voltmeter consists of a series resistance ${}^{\prime }{R}_{ext}^{\prime }$, ideal diode 'D' and PMMC instrument as shown in figure. The internal resistance of PMMC meter is $120\Omega$ and full scale deflection is produced by a DC current of 1 mA. The value of ${}^{\prime }{R}_{ext}^{\prime }$ required to obtain full scale deflection with an AC voltage of 10 V (rms), applied to the input terminal is_______$\Omega$.

• A. 4380

Answer 1

The correct option is A 4380
DC sensitivity of meter
$=\frac{1}{I_{fsd}}=\frac{1}{1\times {10}^{-3}}=1k\Omega /V$

For half wave rectifier,

The AC sensitivity $\left(S_{AC}\right)=0.45\times S_{DC}$

$=450\Omega /V$

Resistance of series multiplier ,

$R_S=S_{AC}\times V_{AC}\left(rms\right)-R_m-R_D$

As the diode is ideal, $R_D=0$

$R_s=(450\times 10)-120=4380\Omega$