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Prof.R.S.Kakade (Sub- Electronics)
Chapter no. 4
RECTIFIERS, FILTERS & REGULATORS
( 20 Marks)
RSK
Rectifier:
Rectifier is an electronic device which is used for converting an alternating
(ac) voltage or current into a unidirectional (dc) voltage or current.
Rectification is the process of converting the alternating voltage or current into
the corresponding direct (dc) quantity (direct voltage or current)
Need of Rectification:
Most of the electronic devices require DC supply for their operation, where as
the electric supply available is almost always AC. A rectifier circuit is, therefore,
commonly included in most of the electronic equipments to convert the alternating
supply into DC.
Types of Rectifier circuits:
Rectifier is a circuit which converts an alternating (ac) voltage into a direct
(dc) voltage.

Half Wave Rectifier:
The circuit configuration of a half wave rectifier is as shown in fig. It is the
input step down transformer. RL is the load resistance.
In the positive half wave cycle of the ac supply, the secondary voltage VAB is
positive i.e. A is positive with respect to B. Hence the diode is forward biased and
starts conducting.
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Prof.R.S.Kakade (Sub- Electronics)
In the negative half cycle the ac supply secondary voltage V AB is negative i.e.
A is negative with respect to B. Hence diode is reverse biased and offers a very high
resistance due to this load voltage and load current both are zero. the o/p waveforms
are as shown in fig.
or
Operation:
The ac voltage across the secondary winding AB changes polarities after every
half cycle. During the positive half cycle of input ac voltage, end A becomes positive
w.r.t. end B. This makes the diode forward biased and hence it conducts current.
during the negative half cycle, end A is negative w.r.t. B. under this condition the
diode is reverse biased and it conducts no current. Therefore, current flows through
the diode during positive half cycles of input ac voltage only. it is blocked during the
negative half cycles. In this way, current flows through load RL is always in the same
direction. Hence d.c.load output is obtained across RL.
Parameters of Half wave Rectifier:
1) Iav- DC or Average Load Current (ILdc):Definition –
The average value of a periodic function is given by, the area under one cycle
of the function divided by the base (period)
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Prof.R.S.Kakade (Sub- Electronics)
Average Load Current Idc =
Im

where Im = peak amplitude of the load current.
2) Vac - DC or Average Load Voltage (VLdc)VLdc = ILdc X RL
VLdc = Vm

3) Irms - AC or RMS Load current (Irms)Consider one complete cycle of the load current waveform (0-2)
Irms = Im
2
4) Vrms - AC or RMS value of load voltage
VLrms  Vm ..... (approximate)
2
5) Ripple factor (RF)The rectifier output consists of AC as well as DC components.
The ripple factor indicates how close the rectifier is to the pure ideal dc voltage
waveform. Small values of ripple factor indicate that the rectifier output waveform is
close to being pure dc. The ideal value is should be as small as possible.
Ripple Factor = Rms values of the AC component of o/p
DC or Average value of the o/p
Ripple factor is defined as the rms value of the ripple voltage (a.c. component
of the output) to the dc output voltage.
6) DC output Power (PLdc)PLdc = V2Ldc
RL
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Prof.R.S.Kakade (Sub- Electronics)
7) AC Input Power - (Pac) Pac = I2s rms X (Rs + RF + RI)
8) Rectification efficiency () It is the ratio of output power to the a.c. input power of a rectifier.
 = DC o/p power
AC I/p power
 = PLdc
Pac
9) Peak Inverse Voltage (PIV)Peak Inverse voltage is the maximum negative voltage which appears across a
no conducting reverse biased diode.
 Full wave rectifier with Center Tapped Transformer:
The full wave rectifier configuration is as shown in fig. A center tapped
secondary winding AB is used with two diodes connected so that each used one half
cycle of input ac voltage. Diode D1 utilizes the ac voltage appearing across the upper
half (OA) of secondary winding rectification while diode D2 uses the lower half
winding (OB).
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Prof.R.S.Kakade (Sub- Electronics)
Operation –
During the positive half cycle of secondary voltage, the end A of the
secondary winding becomes positive and end B negative. This makes the diode D1
forward biased and diode D2 reverse biased. Therefore diode D1 conducts while
diode D2 does not conduct. The conventional current flow is through diodes D1, load
resister RL and the upper half of secondary winding as shown in fig.
During the negative half -cycle, end A of the secondary winding becomes
negative and end B positive. Therefore diode D2 conducts while diode D1 does not .
The conventional current flows through diode D2, Load RL and lower half winding
referring the fig. it may be seen that current in the load RL is in the same direction for
both half cycles of the input ac voltage. Therefore, d.c. is obtained across the load RL.
1) Average Load Current (ILdc)ILdc =
2Im
π
2) Average Load Voltage (VLdc) –
VLdc = 2Vm .. . Approximate
π
3) RMS Load Current (ILrms)
VLrms =
Im
√2
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Prof.R.S.Kakade (Sub- Electronics)
4) RMS Load voltage (VLrms)
VLrms = Vm .. . Approximate
√2
5) Ripple factor (Rf) –
Ripple Factor =
[ V2Lrms – V2Ldc] 1/2
VLdc
6) Dc output Power (PLdc) –
PLdc = I2Ldc X RL
7) Ac input Power (Pac) –
Pac = I2s rms X (Rs + RF + RL)
8) Ripple Efficiency (η) –
η=
PLdc
Pac
9) Peak Inverse Voltage (PIV) –
PIV = 2 Vm Volts
10) Ripple frequency
The Ripple frequency of full wave rectifier is twice that of the Half wave
rectifier. i.e. 100 HZ.

Full Wave Bridge Rectifier:
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Prof.R.S.Kakade (Sub- Electronics)
Full wave bridge rectifier contains four diodes D1,D2,D3 and D4 connected to
form a bridge as shown in fig. The a.c. supply to be rectified is applied to the
diagonally opposite ends of the bridge through the transformer. And in between other
two ends of bridge, the load resistance RL is connected.
Operation During the positive half – cycle of secondary voltage, the end P of the
secondary windings becomes positive and end Q becomes negative. This makes
diodes D1 & D3 forward biased while diodes D2 & D4 are reverse biased. Therefore,
only diodes D1 & D3 conduct. The conventional current flows from A to B through
load Resistance RL as shown in fig (a) by dotted lines.
During the negative half cycle of secondary voltage, end P becomes negative
and end Q becomes positive. This makes diodes D2 & D4 forward biased where as
diodes D1 & D3 are reverse biased. Therefore only diodes D2 & D4 conducts. These
two diodes will be in series through the load RL. The conventional current flows
through the ckt. As shown in fig. /it may be seen that load RL. i.e. in the same
direction, as for the positive half cycle. Therefore d.c. output is obtained across the
load RL.
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Prof.R.S.Kakade (Sub- Electronics)
Why Bridge Rectifier is better than centre tapped full wave
rectifier?
Ans:- 1) It requires a small size transformer. Centre tap transformer is not required .
This makes the bridge rectifier cost effective.
2) The input transformer is not a must. It is possible to operate the bridge rectifier
directly on the 230V ac supply.
3) This circuit is most suitable for the high voltage applications.
4) High avarae output voltage.
5) Rectifier efficiency is high.
6) Transformer utilization factor TUF is high.
Filters and regulators
Filters: Filter is nothing but electronic circuit which gives ripple free voltage (i.e. no
A.C. Voltage) or pure D.C. Voltage.
Need of Filters:
The purpose of a steady d.c. voltage. Through the output of full wave rectifier
is better unidirectional still it provides pulsating d.c. voltage. (i.e. it contain some
what a.c. voltage). This pulsating d.c. voltage produces losses in load resistor (RL).
Therefore in order to obtain steady d.c. output voltage and remove a.c voltage use of
filter is necessary.
The filter is connected between rectifier and load as shown in fig. Below.
The filter circuits use: Resistors, Capacitors and inductors. Therefore they are
called passive filter.
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Prof.R.S.Kakade (Sub- Electronics)
Types of Filter: Filters are classified depending on the components used and
depending on the configuration in which they are connected.
1) Capacitor filter (Shunt capacitor filter)
2) Choke input filter (Series inductor filter) L Filter
3) LC Filter.
4) π Type filter (CLC filter)
1) Capacitor filter (Shunt capacitor filter) C Filter.
As shown in above fig. Capacitor is connected across the load resistor. O/P of rectifier
is given the load resistor.
As the capacitor is connected in Parallel with load and acts as filter. Therefore
this filter is called as shunt capacitor filter.
Operation:
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Prof.R.S.Kakade (Sub- Electronics)
The initial voltage on capacitor is assumed to be zero. In the first positive half
cycles diode D1 is forward biased and D2 is reverse biased and acts as open switch.
As Diode D1 conducts its supplies the charging current to the capacitor. Capacitors
starts charging through diode D1 and capacitor charges to the peak value of voltage
VM as shown in wave form at point A after point A the secondary voltage starts
decreasing as shown in dotted lines. As secondary voltage decreases. This will reverse
bias the diode D1. Thus diode D1 is turned off. During this period the voltage on
capacitor is higher than rectifier o/p (as shown by dotted lines). The diode D1 and D2
both remains off. Then the capacitor starts discharging exponentially through load
resistor RL as shown in portion AB in o/p wave forms. Again at point B Secondary
voltage starts increasing. Therefore the diode D2 becomes forward biased its starts
conducting and capacitor charges to peak value (VM) through D2 as shown in
waveform by portion of voltage BC. Again at point C both the diodes are reversed
biased and turned off and capacitor starts discharging through load resistor RL as
shown in o/p wave form by the portion CD. Thus we get the pure D.C. o/p shown by
portion ABCD.
Ripple factor:
The expression for the ripple factor of HWR and FWR with center tap
or bridge rectifier are as follows
1) Ripple factor for C filter with FWR or bridge rectifier
r=
1
.
4 √3 fcRL
2) Ripple factor for HWR is
r=
1
.
2√3 fcRL
Where f= supply frequency in Hz
C = Value of filter capacitor in Farad
RL= Load resistance.
The value of ripple factor should be as low as possible.
How to reduce the ripple factor from above expression we can reduce ripple factor by,
1) Increasing value of filter capacitor C or
2) Increasing the load resistance RL
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Prof.R.S.Kakade (Sub- Electronics)
L Filter (Series inductor filter) (Choke input filter)
The circuit diagram for series inductor filter is as shown in figure below
As shown in figure inductor filter is connected in series with load resistor (RL).
Operation:
An inductor has fundamental property of opposing any charge in current
flowing through it whenever the current tends to change a back emf is induced in the
inductor.
As inductor filter is connected in series with load resistor RL. This offers high
reactance to a.c. components of frequency 2w, 4w, 6w etc. and small reactance to
D.C. components. Amplitude of a.c. component in the output voltage are considerably
reduced. If the amplitude of a.c. component increases above the average value
inductor stores energy in its magnetic field and if the current decreases below the
average value the inductor releases the energy. The higher the current flowing through
it better is the filtering.
The expression for ripple factor is given by
Rf =
R
.
3√2 wL
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Prof.R.S.Kakade (Sub- Electronics)
π filter or CLC filter:
The π Type filter is a combination of capacitor input filter and LC filter as
shown in above figure. It consists of two capacitors C1 and C2 along with inductors
L. Generally both the capacitors are of same value.
The o/p of full wave rectifier to given to the π filter. The capacitor C1 comes
first therefore C1 acts as shunt capacitors filter.
The capacitors C1 and C2 provide a low reactance path for the ripple where as
the series inductor L provides a high reactance to the a.c. ripple. The combine effect
of this two filter (L1 and Lc) reduces a.c. o/p and improves D.C. output.
Waveform:
Expression for ripple factor:
The expression for ripple factor with full wave rectifier is given by
r =
1
.
2
2 √3 (w L c1 c2 RL)
and with half wave rectifier,
r =
1
.
2
√3 (w L c1 c2 RL)
From the expression ripple factor of π filter is inversely proportional to the
load. Resistor RL for RL= ∞ i.e. at no load ripple is best as RL decreases the value of
ripple factor increases.
LC Filter:
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Prof.R.S.Kakade (Sub- Electronics)
The above figure shows the LC filter with full wave rectifier. This is
combination of inductor filter and capacitor input filter.
We know that inductor filter is preferred for low values of R and the capacitor
input filter is preferred for high values of load resistance. As it is combination of the
two filters LC filter gives low ripple (i.e. Pure DC o/p) irrespective of load.
The series connected inductor filter offers high reactance to AC component
and small reactance to DC components and parallel capacitor provides allow
reactance by pass path for AC signal. Thus combination of this two filter gives
better DC o/p.
Expression for the ripple factor:
The expression for ripple factor of LC filter is given by
Rf
1
r =
.
6 √2 ( w2LC )
Comparison of C,L,Lc and π filter.
Sr. Parameter
1) Component
used
2) Connection
3) Ripple contend
in o/p
4) Size of Filter
5) Suitable for
6)
7
8)
Cost
Expression for
RF
Voltage
regulation
Series
Low
LC Filter
Capacitor &
Inductor
Parallel
Very Low
π Filter
Two Capacitor
and Inductor
Parallel
Negligible
Bulky
Heavy load
application
Moderate
Bulky
Light as well
as heavy
Higher
Bulky
All type of
loads
Highest
C Filter
Capacitor
L Filter
Inductor
Parallel
High
Small
Light load
application
Lowest
r =
1
.
4 √3 FCR
Poor
2nd Sem/CM/IF Rectifiers, Filters & Regulators
r =
1
3 √2 WL
Moderate
.
r =
1
.
6 √2w2 LC
Good
r =
1
.
2√3w2(C1C2RL)
Very Good
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Prof.R.S.Kakade (Sub- Electronics)
Regulator: Definition :
The voltage regulator is a circuit which gives constant o/p voltage irrespective
of change in i/p voltage or change in load current or change in temp.
Need of Regulator:
The combination of rectifier and filter can be used as a d.c. power supply for
certain application. Such as eliminators. But the problem with this type of D.C. power
supply is that its o/p voltage will not remain constant in the event of change in i/p
voltage or change in load current i.e. we get unregulated D.C. voltage. We can not use
this type of unregulated power supply for expensive electronic instruments. Therefore
we need regulator.
Zener Diode as Regulator:
There are two types of regulator that is shunt voltage regulator and series
voltage regulator we can use zener diode as shunt voltage regulator as shown in fig
below. The series resistance Rs is connected to limit the total current drawn from i/p
supply. The zener diode is connected across the load resistance RL.
Operation: The voltage across zener diode remains constant when it is operated in the
zener region in reverse bias condition. This property of zener is utilized for the
regulation of voltage at o/p.
1) If RL is constant and Vin (ip) voltage is changes. If Vin i/p voltage increases then
current I increases. But load current does not increased because Vz zener diode
voltage and RL load resistance is constant there fore the increase in current I will
increase the zener current Iz. If the Zener current Iz is less than IZmax then zener
diode operated in the zener region and o/p voltage remains constant. If Vin (i/p
voltage) decreased the current I decreases but IL remains constant because RL. Vz
is constant and hence naturally Iz is greater than Izmin. The zener diode is
operated in the zener region and o/p voltage remains constant.
2) Vin (i/p voltage is constant) and load Resistor RL is varying.
If RL increases then IL decreases but the total current I is constant.
I = Vin - Vz
Rs
Therefore decrease in load current (IL) the zener current Iz will increase as long as
Iz is less than Izmax. Zener diode is operated in zener region and o/p voltage will
remain constant.
If RL I reduced IL increases but I is constant and zener current Iz will decrease
as long as Iz is higher than Iz min. the zener diode is operated in zener region and
o/p voltage remains constant.
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Prof.R.S.Kakade (Sub- Electronics)
Load regulation:
The load regulation is defined as change in output voltage. When the load
current is changed from zero (no load) to maximum (Full load) value.
LR = VNL - VFL
Where
VNL – o/p voltage on no load
VFL - - o/p voltage on full load
The % load regulation is defined as
% LR = VNL – VFL X 100
VFL
The L R of a power supply should be ideally equal to zero.
Regulation characteristics
Line Regulation:
It is also called as source regulation.
It is defined as the change in regulated load voltage due to change in line voltage.
SR or LR = VLH – VLL
Where
VLH: Load voltage with high line voltage.
VLL : Load Voltage with low line voltage.
Regulated Power Supply:
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Prof.R.S.Kakade (Sub- Electronics)
Waveforms at various points in a regulated dc power supply
A typical Regulated Power supply
A typical regulated dc power supply is as shown in fig. It uses a bridge
rectifier, a capacitor and a zener voltage regulator.
Advantages of the Regulated Power supplies:
1) Very low ripple contents in the output voltage.
2) We get a very high quality (pure) dc output voltage.
3) Output voltage remains constant in spite of variation in input voltage, load current
or temperature.
--o--
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