The following is an RL filter circuit with the inductance L = 400 mH and the input voltage Vin = (20.0 V) sin ω, where ω = 200 rad/s. For what value of R will the output voltage lag behind the input voltage by 30.0°?

  F1 Shubhanshi Ravi 16.10.21 D33

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  1. 119 Ω 
  2. 139 Ω 
  3. 131 Ω 
  4. 121 Ω 

Answer (Detailed Solution Below)

Option 2 : 139 Ω 
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Detailed Solution

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Given that

Input voltage Vin = 20 sin 200t

Inductance L = 400 mH

Angular frequency ω = 200 rad/sec

Inductive reactance XL = ω L = 200 × 400 × 10-3 = 80 Ω 

F1 Shubham Ravi 22.11.21 D1

 \(V_{out}=\frac{R}{R+jX_L}\times V_{in}\)

\(V_{out}=\frac{R}{R+j80}\times V_{in}\)

The output voltage lag behind the input voltage by 30° 

The phase angle of the input is 0°, so the phase angle of the output is -30° (as it is lagging).

The phase angle of the output voltage is given by

ϕ = - tan-1 (80 / R) = - 30° 

⇒ (1/√3) = 80 / R

R = 138.56 ≈ 139 Ω 

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