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 15th 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.

The smoothing inductor has a high reactance to block the alternating voltage.  Therefore, most of the ripple voltage is developed across the inductor.  The smoothing capacitor has a high reactance to block the steady voltage. So, most of the steady voltage is developed across the smoothing capacitor and the resistor.  

In a short summary, the smoothing capacitor has a lower reactance when compared with the smoothing inductor, so most of the ripple disappears across the smoothing capacitor.  This is the filtered and smoothed full-wave rectification.  

Procedure:

Half-wave rectifying circuit

1. Set up the apparatus according to Figure C25.5 for testing the operation of the half-wave rectifying circuit.  

2. Use the CRO to confirm that the selected output of the low voltage power supply is an a.c.  If so, measure its peak voltage v1 and frequency f1.  
3. Use the CRO to display the signal across the resistor.  Sketch the waveform and comment on its shape.  Measure its peak voltage v2 and frequency f2.  Compare v2 with v1 and f2 with f1, and account for any difference between them.  

Full-wave rectifying circuit

4. Set up the apparatus according to Figure C25.6 for testing the operation of the full-wave rectifying circuit.  

5. Use the CRO to display the signal across the resistor.  Sketch the waveform and comment on its shape.  Measure its peak voltage v3 and frequency f3.  Compare v3 with v2 and

The capacitance of a storage capacitor should not be too large.  If it is too large, the diodes may be damaged because of the large flow of charge generated by the capacitor.  

Filter circuit

When waveforms E1 and E2 are compared, the former one is just like a straight line, but the latter one has large ripples, even larger than those in waveform D4. The C-cores are used to concentrate the magnetic field in order to produce induced emf with high efficiency.  If C-cores are absent, the efficiency will be too high, and that explains waveform E2.  Therefore, it is better to use the C-cores in the inductor.  

Virtually, all electronic devices need d.c. signal, so rectification of a.c. signal is always necessary.  For example, when using florescent lamp tube, if rectification is not used, the light will keep flashing and make people feel sick.  But if rectification is used, the flashing problem will be better than better, at least the flashing cannot be notice through naked eyes.  Rectification is very important because it can make the power supply as stable as d.c., as all electronic devices require d.c. signal.  

Conclusion:

This experiment is done successfully, an a.c. signal is rectified using a half-wave rectifying circuit, an a.c. signal is rectified using a full-wave rectifying circuit, a full-wave rectified a.c. signal is smoothed using a smoothing circuit, and a smoothed full-wave rectified a.c. signal is filtered using a filter circuit.