“To Observe Waveform of full wave rectification”

 “To Observe Waveform of full wave rectification”

Objectives:

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

Apparatus:

The apparatus used in this experiment is:

1.      Cathode Ray Oscilloscope

2.      4 Diodes

3.      Connecting Wires

4.      Oscillator

5.      Bread Board

6.      Power Supply

7.      Power Cable

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 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). Full Wave rectifier produces an output voltage or current which is purely DC.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. Therefore a full wave rectifier utilizes both half cycles of input AC voltage to produce the DC output.

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:

Full wave rectifier rectifies the full cycle in the waveform i.e. it rectifies both the positive and negative cycles in the waveform. As the center tapped rectifier is expensive and not easily available hence bridge rectifier is mostly used for full wave rectification. In this type four diodes are arranged in the form of bridge. This actually provides same output polarity with either polarity. The average or DC voltage at the output of an unfiltered full wave rectifier can be calculated as:

The DC load current is calculated as:

Since a bridge full wave rectifier provides a full wave output, hence the frequency of the output waveform is found using:

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 the output if the full wave rectifier DC component is more than that of AC .Hence it has less pulsation and its effectiveness is greater.

These are some formulas that are used for the output in full wave rectification. Such as ripple factor, peak voltage. But here we just have to observe the waveform, nothing is calculated. If a simple capacitor filter is connected to the rectifier output the output voltage is smoothed and approaches its ideal DC value. The capacitor charges up while the diodes are conducting and then discharges when the diodes are not conducting to maintain current flow through the resistor. When load current is small ripple are low and the diodes conduct high current for a very short part of the cycle, transferring charge from the source to the capacitor by means of a large current spikes.

Circuit Diagram Description:

Circuit for the experiment of full wave rectification is given by the following diagram:

The circuit as shown above contains four diodes, one low resistor and an AC source. 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 or other electrical components should be placed without making any damage to the board. Diodes is basically a two terminal electrical component with asymmetric conductance. It has low resistance to current ratio in one direction and high resistance to current ratio in another. By using the above circuit completely rectified wave is obtained. During first half cycle of the AC input, the upper portion of the transformer is positive with respect to the lower portion. In first half cycle two diodes are forward biased and two are reversed biased. Current flows through the forward biased diode but not through the reverse biased diode. During the next half cycle the reverse holds true. As now the diodes (which are reverse biased in previous half cycle) are forward biased and vice versa. Thus negative cycle is also rectified and AC is completely converted into DC.

Procedure/Experimentation:

The steps while doing this experiment are given below:

1.      Drawn the circuit diagram.

2.      Insert the diodes in the bread board.

3.      Given the AC input to the circuit through the AC oscillator to the opposite ends of the diode.

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

5.      Connected the CRO to the output of the rectifier. Output is received at the same terminals of the diodes.

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

7.     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 pulsating DC which changes its direction continuously but not reverses.

Conclusion:

Finally the AC input voltage given by the AC oscillator is converted into DC power by the use of bridge rectifier (four diodes rectifier) and it is also plotted.

Short Question:

1.      Why the frequency of output signal in full wave rectifier is twice the frequency of input signal?

The frequency of output signal in full wave rectification is twice the frequency of input signal because the period of output signal is half the input AC .In other words each cycle of input produces two cycles of output.

2.      What will happen in center tapped full wave rectifier, if one diode is shorted?

In a center tapped full wave rectifier if one diode is shorted, the primary fuse will blow. The other diode will behave then as a wire only. This will cause excessive current to flow in the secondary and hence in the primary.

3.      What will happen in Centre tapped full wave rectifier, what will happen if one diode opens?

In Centre tapped full wave rectifier if one diode is opened then the output will be half rectified wave. In this case, measure the forward resistance. Higher resistance means the circuit is opened

4.      Why the Bridge rectifier is preferred over Centre tapped full wave rectifier?

Because in a Centre tapped full wave rectification, each diode utilizes AC voltage appearing across half of the secondary windings. However a bridge circuit utilizes full secondary voltage for rectification

5.      What is the disadvantage of full wave rectifier over half wave rectifier?

The only disadvantage of full wave rectifier is that the peak voltage is half the peak voltage in half wave rectifier. Therefore only half the source goes to each diode.

6.      How Ripple Factor is minimized?

Ripple factor is minimized by using a filter circuit across the rectifier. It consists of capacitor or inductor or both.