“To Observe Waveform of Half wave rectification”

 “To Observe Waveform of Half wave rectification”

Objectives:

Our main objective is to observe & study the half cycles of DC output by the given AC input voltage through half wave rectification.

Apparatus:

The apparatus used in this experiment is:

1.      Cathode Ray Oscilloscope

2.      1 Diodes

3.      Connecting Wires

4.      Oscillator

5.      Bread Board

6.      Power Supply

7.      Power Cable

8.      Resistor

Introduction/Literature Background:

Most electrical components require DC voltages to operate properly. Since most equipment is connected to the 120-V AC power line, so this AC source must be converted into the required DC value. A circuit that converts the AC power line voltage to the required DC voltage is called a power supply. The most important components in power supplies are rectifier diodes, which convert AC line voltage to Diodes are able to produce a DC output voltage because they are unidirectional devices allowing current to flow through them in only one direction. Mostly diodes used are of silicon.

Rectification is the process in which AC is converted into DC.A direct current flows from the positive to negative terminal of source supply when it is connected by a circuit. The current flows continuously without changing direction. But in case of AC the direction of flow keeps on changing. Because of their ability to conduct current in one direction and block current in the other direction, diodes are used in circuits called rectifiers that convert ac voltage into DC voltage. Rectifiers are found in all dc power supplies that operate from an AC voltage source. A power supply is an essential part of each electronic system from the simplest to the most complex. Rectification is of two types, full wave rectification and half wave rectification.

In full wave rectification, current flows through the load in the same direction for both half cycles of input AC voltage. This can be achieved with two diodes working alternately. For the positive half cycle of input voltage, one diode supplies current to the load and for the negative half cycles, the other diode does so, current being always in the same direction through the load.

There are two main forms of full wave rectifier circuit that can be used. Each type has its own features and is suitable for two different applications. These are

1.      Two Diodes Full wave rectifier

2.      Four Diodes /Bridge Rectifier:

The two diode full wave rectifier circuit is not widely used with semiconductor diodes as it requires the use of center tapped transformer but used when thermionic valves and vacuum tubes are used in excess. The basic circuit is:

In the bridge rectifier the resulting waveform is although same but it does not require center tapped transformer, hence its size and cost is reduced. The single secondary winding is connected to one of the diode bridge network and the load to the other side, the main circuit diagram for this one is as:

In half wave rectification type only half of the cycle is rectified. The wave rectifier consists of a step down transformer, a diode connected to the transformer and a load. The main supply voltage is given to the transformer which increases or decreases the voltage. As it is a known fact that a diode conducts current only when it is forward biased. The input given here is AC. The input voltage is stepped down using a transformer. The reduced voltage is fed to the diode and load resistance R_L During the positive half cycles of the input wave the diode will be forward biased and during the negative half cycles of input the diode will be reverse biased. Output of the waveform is taken across load R_L .Since the diode passes current only during one half cycle of the input wave, we get an output as show below:

The output is positive and significant during the positive half cycles of input wave. At the same time the output is zero during negative half cycles of input wave. Summarizing as that in half wave rectification the rectifier conducts only during the positive half cycles of input AC supply. The negative half cycle of AC supply are suppressed i.e. during negative half cycle, no current conducted hence no voltage appears across the load. Current flows only in one direction the output of a rectifier consists of a DC component and an AC component (also known as ripple).The AC component is undesirable and accounts for the pulsations in the rectifier output. The effectiveness of a rectifier depends upon the magnitude of AC component, the smaller this component the more effective is the rectifier. The ratio of r.m.s value of AC component to the DC component in the rectifier output is known as ripple factor.

In half wave rectification the AC component exceeds the DC component in the output. This results in greater pulsation, and less effectiveness of the rectifier.

Circuit Diagram Description:

The circuit diagram for the half wave rectification is given below:

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQ7n0QJT4Tc9GVo8khu4HPQwVpH7BDWxi8xFXiuYmV2VPYRQP7F

As it can be seen that there is a diode, and a resistor connected in series across the diode. Actual circuit contain a bread board into which the electrical components like diodes and resistors are placed. The sockets of bread board are sturdy and rugged. Diodes, resistors or other electrical components should be placed without making any damage to the board. Four Band resistor is used. Its resistance can be calculated with the help of color coding method. A diode is an electronic device consisting of two elements known as cathode and anode. In a diode current can flow only in one direction. The AC voltage changes the polarity after every half cycle of input value. When a single rectifier diode unit is placed in series with the load across an AC supply. It converts voltage into unidirectional pulsating voltage, using half cycle of the applied voltage, the other half cycle being suppressed because it conducts only in one direction. During the positive half cycle of the input AC voltage, the diode is forward biased and therefore conducts current. But in case of negative half cycle the diode is reverse biased and current is zero.

Procedure/Experimentation:

The steps while doing this experiment are given below:

1.      Drawn the circuit diagram.

2.      Insert the diode in the bread board.

3.      Connect the resistance in series with the diode

4.      AC input is given to the circuit through the AC oscillator.

5.      AC input waveform is seen on CRO by directly connecting CRO with oscillator.

6.     Connected the CRO to the output of the rectifier. Output wave is observed.

7.     Observed the Output Wave form (pulsating DC) on CRO.

8.     Drawn the output waveform on notebook.

Observation & Calculation:

By adjusting the following settings on AC oscillator

Frequency Of the oscillator=50Hz

Amplitude=1

Wave form=Sinusoidal

We observed the half positive cycle which changes its direction continuously but not reverses and half negative cycle of pulsating DC.

Conclusion:

Finally the AC input voltage given by the AC oscillator is converted into DC power by the use of half wave rectification method and it is also plotted.

Short Question:

1.      In a half wave rectification, what will happen if the diode is shorted?

If the diode in the half wave rectifier is shorted, the output from the rectifier will be a sine wave that is identical to the wave of the input primary voltage. Since the diode is shorted. It acts as a piece of wire. Therefore no half cycle of input is eliminated and the output will be a replica of the input.

2.      In a half wave rectification, what will happen if the diode is open?

If diode opens in a half wave rectifier the output from the circuit drop to zero.

3.      Why we use filters?

Filter is used to minimize the pulsation. These are basically used to remove unwanted frequency components.

4.      In filter circuits why capacitor is always used in parallel

Capacitor allows AC and blocks DC signal. In rectifier for conversion of AC into DC, capacitor is placed in parallel with output not in series .If the capacitor is placed in series then there will be no output.

5.      Why diode not operated in the breakdown region in rectifier circuits?

The reason is that if a diode is operated in breakdown region in rectifier circuit then the diode may burn out, if there is no limitation of current