Application of P-N Junction Diodes
a) To protect equipment, circuits or devices by a reverse power supply.
b) To rectify ac to dc
c) Enable the Audio Frequency energy carrier by modulated radio waves to be detected.
Rectification and Smoothing
A) Definition
· Rectification is the process of converting a.c current to d.c current.
· A Rectifier is a device that changes a.c to d.c.
b) Reasons for rectification
· The conversion of a.c. to d.c. is often necessary for all electric equipment, such as radios, T.V. sets, computers, musical instruments, e.t.c, use steady d.c.

Types of rectification
There are two types of rectification, namely:-
· Half-wave rectification
· Full-wave rectification.

Half-wave rectification and smoothing
· One diode is used which removes the negative half-wave cycle of the applied a.c.
· It gives a varying but one-way direct current across the load R. R is a piece of electronic equipment
requiring a d.c. supply

• If the Y-input terminals of a CRO are connected first across the input, the waveform on the left will be
  displayed on the screen.
• When a CRO is connected across R, the output waveform is seen to be positive half-wave of the a.c.
  · Smoothing is done using a capacitor connected across R, to give a much steadier varying d.c. supply.
  · The smoothing capacitor provides extra charge so that current flows continuously even as the phase
  current changes and the current go to zero.
• On the positive half-cycle of the a.c. input the diode conducts, current passes through R and also into the capacitor C to charge it up.
  · On the negative half-cycle, the diode is reversing biased and cannot conduct, but C partly discharges
  through R.
  · The charge-storing action of the capacitor, C thus maintains current in R and a steadier p.d across it when
  the diode is not conducting.
  NOTE: - A single diode only allow half of the a.c. to flow through the load R, so far half of the power supply is cut off.

• Full- wave Rectification and smoothing
  There are two methods for obtaining a full-wave rectification namely:-
  · Using two diodes – Full-wave centre-tap transformer.
  · Using four diodes – Full-wave bridge rectifier
  
  

   

   

   

  Using Centre-Tap Transformer
  · In a full-wave rectifier, both halves of the a.c. cycles are transmitted but in the direction, i.e. same side.

  
• During the first half-cycle, when A is positive, DI conducts through the load R at the same time B is
  negative with respect to T, so no current flows in the diode D2.
  · In the next half-cycle when B is positive, D2 conducts through the load R in the same direction as
  before. A is positive with respect to T so no current flows in D1 .

• Using the bridge Rectifier – four diodes
  · In the 1 st half-cycle, diode D2 and D4 conducts.
  · In the 2nd half-cycle, diode D3 and D1 conducts.
  · During both cycles, current passes through R in the same direction, giving a p.d. that varies as shown by the CRO.
  · When a large capacitor is connected across R, the output d.c. is smoothed as shown.

• · During the first half cycle, point A is positive with respect to C, diode D1 and D3 are forward biased while
  diode D2 and D4 are reverse biased. Current therefore flows through ABDCA. During the second half
  cycle, point A becomes negative with respect to point C. diodes D2 and D4 become forward biased
  while D1 and D3 are reverse biased. Conventional current therefore flows through CBDAC.
  · If a capacitor is connected across the resistor, the rectified output is smoothened.
  

What is the work the capacitor?

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