Objective: l To rectify an a.c. signal using a half-wave rectifying circuit. l To rectify an a.c. signal using a full-wave rectifying circuit. To smooth a full-wave rectified a.c. signal using a smoothing circuit.

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20th October 2009

YMCA of Hong Kong Christian College

A-Level Physics Lab Report

Wong Hoi Sun 7Y20

Experiment C25: Rectification of an A.C. Signal
(Done on 15
th October 2009)

Objective:

  • To rectify an a.c. signal using a half-wave rectifying circuit.
  • To rectify an a.c. signal using a full-wave rectifying circuit.
  • To smooth a full-wave rectified a.c. signal using a smoothing circuit.
  • To filter a smoothed full-wave rectified a.c. signal using a filter circuit.

Apparatus:

  • Semiconductor diode        4
  • Capacitor (10 μF, 100 μF ×2, 500 μF, 1000 μF)        5
  • 1100-turn coil with double C-core
  • Resistor (100 Ω)
  • CRO
  • Low voltage power supply (a.c./d.c.)
  • Clip component holders        3
  • Connecting leads (plug leads, alligator clip leads)

Theory:

Rectification is a process of changing an alternating current to a direct current by a diode.  The diodes have low resistance to allow a current flow in one direction (forward bias) and a high resistance to block the current flow or allow an extremely small current flow in opposite direction (reverse bias).  They are unidirectional circuit conductors.  Many electrical appliances and applications require the change of an a.c. signal to its d.c. form for proper operation.  Therefore, diodes are used to rectify the circuit.  

Half-wave Rectification (Figure C25.1)

If one diode is used, only half of the a.c. signal is allowed to get to the load.  For the first half of the a.c. cycle, the diode is in forward bias, so there is a current flowing through the diode and the resistor.  For the second half of the a.c. cycle, the diode is in reverse bias, so there is no current flowing through the diode and the resistor.  As only half of the a.c. cycle is used, so it is called half-wave rectification.  

Full-wave Rectification (Figure C25.2)

If four diodes connected in the form of bridge are used, there is no blockage of the a.c. signal.  During the first half-cycle, diodes P and Q are in forward bias, so a current flows through diode P, resistor, and diode Q.  During the second half-cycle, diodes R and S are in forward bias, so a current flows through diode R, resistor and diode S.  For both the half-cycles, the current flows through the resistor in one direction only.  Therefore the rectified voltage across the resistor is unidirectional, and a full-wave rectification is produced.  

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Smoothed Full-wave Rectification (Figure C25.3)

The rectified a.c. has become a d.c. signal already, but it is not steady enough for many operations, therefore further shaping of the signal is required.  In order to smooth the voltage, a storage capacitor is used.  It stores charge while the input is increasing and discharges slowly while the input is decreasing.  

During the first half-cycle, a current flows through diode P, capacitor, resistor and diode Q.  The capacitor is charged up and stores charge.  The voltage across the capacitor is qual to the voltage across the resistor.  This occurs ...

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