09-08-2014, 12:18 PM
BRIDGE RECTIFIER
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INTRODUCTION
Rectification is the process of converting an alternating voltage or alternating current into direct voltage or direct current”. The device used for rectification is called rectifier. Rectifiers are mainly two types, half wave rectifier and full wave rectifier.
Half wave rectifier is a circuit which rectifies only one of the halves of the ac cycle. During the half cycles when P is positive and N is negative, the diode is forward biased and will conduct. When P is negative and N is positive, the diode is reverse biased and will not conduct. Efficiency of the half wave rectifier will be about 40.6%.
Full wave rectifier is a circuit which rectifies both half cycles of the a.c. when P of 1st diode is positive; the 1st diode is forward biased and will conduct. Now the 2nd diode will not conduct as it is reverse biased. In all the half cycles either of the two diodes will be conducting. The efficiency of a full wave rectifier is about 81.2 %, twice the efficiency of a half wave rectifier.
Full wave rectifier can be constructed in 2 ways. The first method makes use of a center tapped transformer and 2 diodes. This arrangement is known as Center
HALF WAVE RECTIFIER
A simple Half Wave Rectifier is nothing more than a single pn junction diode connected in series to the load resistor. If you look at the above diagram, we are giving an alternating current as input. Input voltage is given to a step down transformer and the resulting reduced output of transformer is given to the diode ‘D’ and load resistor RL. The output voltage is measured across load resistor RL.
As part of our “Basic Electronics Tutorial” series, we have seen that rectification is the most important application of a PN junction diode. The process of rectification is converting alternating current (AC) to direct current (DC).
Half Wave Rectifier Operation
To understand the operation of a half wave rectifier perfectly, you must know the theory part really well. If you are new to the concepts of pn junction and its characteristics, I recommend you to read the half wave rectifier theory part first.
The operation of a half wave rectifier is pretty simple. From the theory part, you should know that a pn junction diode conducts current only in 1 direction. In other words, a pn junction diode conducts current only when it is forward biased. The same principle is made use of in a half wave rectifier to convert AC to DC. The input we give here is an alternating current. This input voltage is stepped down using a transformer. The reduced voltage is fed to the diode ‘D’ and load resistance RL. During the positive half cycles of the input wave, the diode ‘D’ will be
Explaining Half Wave Rectification in academic words
When a single rectifier diode unit is placed in series with the load across an ac supply, it converts alternating voltage into uni-directional pulsating voltage, using one half cycle of the applied voltage, the other half cycle being suppressed because it conducts only in one direction. Unless there is an inductance or battery in the circuit, the current will be zero, therefore, for half the time. This is called half-wave rectification. As already discussed, diode is an electronic device consisting of two elements known as cathode and anode. Since in a diode electrons can flow in one direction only i.e. from cathode to anode, the diode provides the unilateral conduction necessary for rectification. This is true for diodes of all types-vacuum, gas-filled, crystal or semiconductor, metallic (copper oxide and selenium types) diodes. Semiconductor diodes, because of their inherent advantages are usually used as a rectifying device. However, for very high voltages, vacuum diodes may be employed.
Half Wave Rectifier Theory
Rectification is an application of the pn junction diode. A half wave rectifier is a device which makes use of key properties of a pn junction diode. So to understand the underlying theory behind a half wave rectifier, you need to understand the pn junction and the characteristics of the pn junction diode. We have developed two articles to help you understand both of them.
Advantages and Disadvantages of Half wave rectifier
A half wave rectifier is rarely used in practice. It is never preferred as the power supply of an audio circuit because of the very high ripple factor. High ripple factor will result in noises in input audio signal, which in turn will affect audio quality.
Advantage of a half wave rectifier is only that its cheap, simple and easy to construct. It is cheap because of the low number of components involved. Simple because of the straight forwardness in circuit design. Apart from this, a half wave rectifier has more number of disadvantages than advantages!
FULL WAVE RECTIFIER
a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.
In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, ©. This configuration results in each diode conducting in turn when its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles
FULL WAVE BRIDGE RECTIFIER
Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side.
The Smoothing Capacitor
We saw in the previous section that the single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency. We can therefore increase its average DC output level even higher by connecting a suitable smoothing capacitor across the output of the bridge circuit.
During the first half cycle
During first half cycle of the input voltage, the upper end of the transformer secondary winding is positive with respect to the lower end. Thus during the first half cycle diodes D1 and D3 are forward biased and current flows through arm AB, enters the load resistance RL, and returns back flowing through arm DC. During this half of each input cycle, the diodes D2 and D4 are reverse biased and current is not allowed to flow in arms AD and BC. The flow of current is indicated by solid arrows in the figure above. We have developed another diagram below to help you understand the current flow quickly. See the diagram below – the green arrows indicate beginning of current flow from source (transformer secondary) to the load resistance. The red arrows indicate return path of current from load resistance to the source, thus completing the circuit.
Merits and Demerits of Bridge Rectifier Over Center-Tap Rectifier
A center tap rectifier is always difficult one to implement because of the special transformer involved. A center tapped transformer is costly as well. One key difference between center tap & bridge rectifier is in the number of diodes involved in construction. A center tap full wave rectifier needs only 2 diodes where as a bridge rectifier needs 4 diodes. But silicon diodes being cheaper than a center tap transformer, a bridge rectifier is much preferred solution in a DC power supply. Following are the advantages of bridge rectifier over a center tap rectifier.
A bridge rectifier can be constructed with or without a transformer. If a transformer is involved, any ordinary step down/step up transformer will do the job. This luxury is not available in a center tap rectifier. Here the design of rectifier is dependent on the center tap transformer, which can not be replaced.
Bridge rectifier is suited for high voltage applications. The reason is the high peak inverse voltage (PIV) of bridge rectifier, when compared to the PIV of a center tap rectifier.
Transformer utilization factor (TUF) is higher for bridge rectifier.
INSTRUCTIONS
This circuit provides full-wave rectification without the necessity of a center-tapped transformer. In applications where a center-tapped, or split-phase, source is unavailable, this is the only practical method of full-wave rectification.
In addition to requiring more diodes than the center-tap circuit, the full-wave bridge suffers a slight performance disadvantage as well: the additional voltage drop caused by current having to go through two diodes in each half-cycle rather than through only one. With a low-voltage source such as the one you're using (6 volts RMS), this disadvantage is easily measured. Compare the DC voltage reading across the motor terminals with the reading obtained from the last experiment, given the same AC power supply and the same motor.
CONCLUSION
By doing this project we came to knew that a full ssswave bridge rectifier can be made without centre tapping transformer. And also we came to find the efficiency of full wave rectifier. For bridge type, no centre tap is required on the transformers. Here much smaller transformers are used. Bridge rectifier is used for high voltage application. For the same secondary voltage, the output voltage is twice that of the centre trapped full wave rectifier.
At the same time some disadvantages are there. Here two extra diodes are required and voltage regulation is not satisfactory.
The bridge rectifier with filter section is constructed. A.c and D.C voltages are measured for efficiency is calculated.