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P-N Junction Diode as a Rectifier

Rectifier is a circuit which converts ac to unidirectional pulsating output (Fig. 20). In other words, it converts ac to dc. It is of following two types:
 
108394.png
Fig. 21

Half wave rectifier

When the P-N junction diode rectifies half of the ac wave, it is called half wave rectifier.
  • During positive half cycle,
     
    Diode: forward biased
     
    Output signal: obtained
  • During negative half cycle,
     
    Diode: reverse biased
     
    Output: signal not obtained
  • —The output voltage is obtained across the load resistance RL. It is not constant but pulsating (mixture of ac and dc) in nature.
  • Average output in one cycle,
     
    107027.png and 107033.png
     
    (rf = forward-biased resistance)
  • rms output: 107042.png
  • The ratio of the effective alternating component of the output voltage or current to the dc component is known as ripple factor.
     
    107052.png= 1.21
  • Peak inverse voltage (PIV): The maximum reverse biased voltage that can be applied before commencement of Zener region is called peak inverse voltage. When diode is not conducting, PIV across it = V0.
     
    Efficiency: It is given by
     
    107058.png
     
    If RL >> rf, then η = 40.6%
     
    If RL = rf, then η = 20.3%
  • Form factor = 107064.png
  • The ripple frequency (ω) for half wave rectifier is same as that of ac.
108436.png
Fig. 21

Full wave rectifier

It rectifies both halves of ac input signal.
  • During positive half cycle,
     
    Diode: D1 forward biased
     
    D2 reverse biased
     
    Output signal obtained due to D­1 only
  • —During negative half cycle
     
    Diode: D1 reverse biased
     
    D2 forward biased
     
    Output signal obtained due to D­2 only
     
    —
  • Output voltage is obtained across the load resistance RL. It is not constant but pulsating in nature.
  • —Average output: 107070.png
  • —rms output: 107076.png
  • —Ripple factor, r = 0.48 = 48%.
  • —Ripple frequency: The ripple frequency of full wave rectifier = 2 × (Frequency of input ac)
  • —Peak inverse voltage (PIV): Its value is 2V0.
  • —Efficiency: 107086.png, for rf << RL, 107093.png

Full wave bridge rectifier

Four diodes D1, D2, D3, and D4 are used in the circuit. During positive half cycle D1 and D3 are forward biased and D2 and D4 are reverse biased. During negative half cycle D2 and D4 are forward biased and D1 and D3 are reverse biased (Fig. 22)
 
110317.png
Fig. 22




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