126. In the circuit shown below what is the output voltage (V_out) in Volts if a silicon transistor Q and an ideal op-amp are used?
     
     

1. 1. – 15

   
   
   

   2. – 0.7

   
   
   

   3. + 0.7

   
   
   

   4. +15

   
   
   

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   I = I, e I = I₀e ^{V_RE / V_T} , I = I₀e ^{-V₀ / V_T}
   

   -V0 = VT		ln	I		= VT ln		I3ms
   			I0				Is

   
   V₀ = - 0.7
   R_L = √R_s² + X_s² = √4² + 3² = 5

   Correct Option: B

   
   I = I, e I = I₀e ^{V_RE / V_T} , I = I₀e ^{-V₀ / V_T}
   

   -V0 = VT		ln	I		= VT ln		I3ms
   			I0				Is

   
   V₀ = - 0.7
   R_L = √R_s² + X_s² = √4² + 3² = 5

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127. A single-phase voltage source inverter shown in figure is feeding power to a load. The triggering pulses of the devices are also shown in the figure.
     
     If the load current is sinusoidal and is zero at 0, π, 2π...., the node voltage V_AO has the waveform

1. 1. 
      

   
   
   

   2. 
      

   
   
   

   3. 
      

   
   
   

   4. 

   
   
   

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   NA

   Correct Option: D

   NA

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128. A three-phase fully controlled bridge converter is fed through star-delta transformer as shown in the figure.
     
     The converter is operated at a firing angle of 30°. Assuming the load current (I₀) to be virtually constant at 1 p.u. and transformer to be an ideal one, the input phase current waveform is
     

1. 1. 
      

   
   
   

   2. 
      

   
   
   

   3. 
      

   
   
   

   4. 

   
   
   

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   NA

   Correct Option: B

   NA

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129. A single-phase SCR based ac regulator is feeding power to a load consisting of 5 Ω resistance and 16 mH inductance. The input supply is 230 V, 50 Hz ac. The maximum firing angle at which the voltage across the device becomes zero all throughout and the rms value of current through SCR, under this operating condition, are
     

1. 1. 30° and 46 A

   
   
   

   2. 30° and 23 A

   
   
   

   3. 45° and 23 A

   
   
   

   4. 45° and 32 A

   
   
   

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   V_s = 230V, 50Hz
   R = 5Ω, L = 16mH
   The maximum firing angle at which the volt across device becomes zero is the angle at which device trigger i.e. minimum firing angle to converter.
   

   α = φ = tan-1		XL		= tan-1		ωL
   		R				R

   
   

   α = φ = tan-1		2π × 50 × 16 × 10-3		= 45.1°
   		5

   
   The current flowing SCR is max at their angle ie. when α = φ, γ = π
   
   

   ITrms =	Vm	=	√2 × 230
   	2z		2 × √5² + 5.042

   
   ∴ I_Trms = 22.9 ≈ 23 A
   

   Correct Option: C

   V_s = 230V, 50Hz
   R = 5Ω, L = 16mH
   The maximum firing angle at which the volt across device becomes zero is the angle at which device trigger i.e. minimum firing angle to converter.
   

   α = φ = tan-1		XL		= tan-1		ωL
   		R				R

   
   

   α = φ = tan-1		2π × 50 × 16 × 10-3		= 45.1°
   		5

   
   The current flowing SCR is max at their angle ie. when α = φ, γ = π
   
   

   ITrms =	Vm	=	√2 × 230
   	2z		2 × √5² + 5.042

   
   ∴ I_Trms = 22.9 ≈ 23 A
   

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130. In dc-chopper, per unit ripple is maximum when duty cycle α is ______ .
     

1. 1. 0.5

   
   
   

   2. 5

   
   
   

   3. 0.05

   
   
   

   4. 1.5

   
   
   

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   NA

   Correct Option: A

   NA

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