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Project Report on Full Wave Rectifier
INTRODUCTION
Although in our daily life we use A.C. current devices. But rectifier is a
‘Electronic device which converts A.C. power into D.C. power’.
The study of the junction diode characteristics reveals that
the junction diode offers a low resistance path, when forward biased, and a
high resistance path, when reverse biased. This feature of the junction
diode enables it to be used as a rectifier.
The alternating signals provides opposite kind of biased
voltage at the junction after each half-cycle. If the junction is forward
biased in the first half-cycle, its gets reverse biased in the second half. It
results in the flow of forward current in one direction only and thus the
signal gets rectified.
In other words, we can say, when an alternating e.m.f.
signal is applied across a junction diode, it will conduct only during those
alternate half cycles, which biased it in forward direction.
TYPE OF RECTIFIER
Mainly we have two types of rectifier :
1.
Full wave rectifier.
JUNCTION DIODE AS A FULL WAVE RECTIFIER
“A rectifier which rectifies both waves of the a.c. input is called a
full wave rectifier”.
Diode Currents
Consider the current path in the diode bridge rectifier. In the positive
half cycle of Vin, diodes D4 and D3 will conduct. During the
negative half cycle, diodes D2 and D1 will conduct. As a result, the
load will pass current in the same direction in each half cycle of the
input.
Design Concerns
•
Reverse current does not exceed the breakdown value
•
Power dissipation limit P = Vd Id is not exceeded
Diode Voltages
•
Forward Bias
o
If we consider a simple, piece-wise linear model for the diode
IV curve, the diode forward current is zero until Vbias >=
Vthreshold, where Vthreshold is 0.6 V to 0.8 V. The current
increases abruptly as Vbias increases further. Due to this turn-on or
threshold voltage associated with the diode in forward bias, we
should expect a 0.6 to 0.8 V voltage drop across each forward
biased diode in the rectifier bridge. In the case of the full wave
rectifier diode bridge, there are two forward biased diodes in series
with the load in each half cycle of the input signal.
o
The maximum output voltage (across load) will be Vin - 2
Vthreshold, or ~ Vin - 1.4 V.
o
Since some current does flow for voltage bias below
Vthreshold and the current rise around is Vthreshold is more
gradual than the piece-wise model, the actual diode performance
will differ from the simple model.
•
Reverse Bias
o
In reverse bias (and neglecting reverse voltage breakdown),
the current through the diode is approximately the reverse
saturation current, Io. The voltage across the load during reverse
bias will be Vout = Io Rload.
o
In specifying a diode for use in a circuit, you must take care
that the limits for forward and reverse voltage and current are not
exceeded.
Principle :-
It is based upon the principle that a junction diode offers
low resistance during forward biased and high resistance, when reverse
biased.
Diagram
b
Difference from half-wave-rectifier :-
The main difference is that
in full wave rectifier we use two diodes. For this when we apply a.c.
current to the rectifier then the first half wave get forward biased due to
first diode. And when the second half wave comes. Then at that time the
second diode comes in action and gets forward biased. Thus output
obtained during both the half cycles of the a.c. input
Arrangement :- The a.c. supply is applied across the primary coil(P) of a
step down transformer. The two diodes of the secondary coil(S) of the
transformer are connected to the P-sections of the junction diodes (D1) and
(D2). A load resistance (RL) is connected across the n-sections of the two
diodes and at centre of the secondary coil. The d.c. output will be obtained
across the load resistance (RL).
Theory :Suppose that during first half of the input cycle, upper end of (S)
coil is at positive potential. And lower end is at negative potential. The
junction diode (D1) gets forward biased, while the diode. (D2) get reverse
biased. When the second half of the input cycle comes, the situation will
be exactly reverse. Now the junction diode (D2) will conduct. Since the
current during both the half cycles flows from right to left through the load
resistance (RL) the output during both the half cycles will be of same
nature.
Thus, in a full wave rectifier, the output is continuous but pulsating
in nature. However it can be made smooth by using a filter circuit.
REVERSE BIASING ON A JUNCTION DIODE
A P-n junction is said to be reverse biased if the positive terminal
of the external battery B is connected to n-side and the negative terminal to
p-side of the p-n junction. In reverse biasing, the reverse bias voltage
supports the potential barrier VB. (Now the majority carriers are pulled
away from the junction and the depletion region become thick. There is
no conduction across the junction due to majority carriers. However, a
few minority carriers (holes in n-section and electrons in p-section) of p-n
Junction diode cross the junction after being accelerated by high reverse
bias voltage. Since the large increase in reverse voltage shows small
increase in reverse current, hence, the resistance of p-n junction is high to
the flow of current when reverse biased.
MATERIAL REQUIRED
1.
Transformer :
It is device which is used to increase or decrease the
alternating current and alternating voltage. For the rectifier, it may be step
down or step up.
2.
Junction Diode :
It is made up of p-type and n-type semiconductor which
conducts when the p terminal of diode to connect to positive terminal of
battery and n region is connected to negative terminal of battery i.e. during
forward biased and does not conduct during reverse biased.