BASIC ELECTRONICS BASIC ELECTRONICSRectifier.ppt

RECTIFIER

3. 3
Full wave Center–Tapped transformer
Rectifier Circuit
Two diodes and a center-tapped transformer are
required.

4. 4
Operation of the Center–Tapped Transformer
Rectifier Circuit
For the positive half cycle
For the negative half cycle

5. 5
Full-Wave Rectification:
Bridge Network

6. 6
Operation of the Bridge Rectifier Circuit
For the positive half of the AC cycle For the negative half of the AC cycle

7. 7

8. SMOOTHING
A capacitor can be used to filter (remove the
voltage variation) the output voltage.
As the voltage grows the capacitor charges up,
and as the voltage falls the capacitor
discharges through the resistor.
If the capacitance is large enough the voltage
will not fall a lot before the capacitor is charged
up once more. In this way the output voltage is
smoothened.

9. 9
Capacitor Filter
•A half-wave rectifier with a capacitor filter is shown in
Figure . RLrepresents the equivalent resistance of a
load. We will use the half-wave rectifier to illustrate the
principle, and then expand the concept to full-wave
rectification.
•During the positive first quarter-cycle of the input, the
diode is forward-biased , allowing the capacitor to charge
to within 0.7 V of the input peak.

10. 10
•When the input begins to decrease below its
peak, as shown in part (b), the capacitor retains
its charge and the diode becomes reverse-
biased. During the remaining part of the cycle,
the capacitor can discharge only through the
load resistance at a rate determined by the RLC
time constant, which is normally long compared
to the period of the input.

11. 11
•The larger the time constant, the less the capacitor will
discharge. During the first quarter of the next cycle, the
diode will again become forward-biased when the input
voltage exceeds the capacitor voltage by approximately
0.7 V.
•The variation in the output voltage due to the charging
and discharging is called the ripple voltage.
•The smaller the ripple, the better the filtering action

12. 12
•For a given input frequency, the output
frequency of a full-wave rectifier is twice that of a
half-wave rectifier. This makes a full-wave
rectifier easier to filter.
•The full-wave rectified voltage has a smaller
ripple than does a half-wave voltage for the
same load resistance and capacitor values.

13. 13
Transformer Voltage: A transformer's required secondary A.C. voltage varies
greatly with the type of rectifier chosen and filter arrangement.
AllA.C.voltagereferencesareR.M.S.Don‘tforgettotakeintoaccountlosses(not
includedinthisguide),especiallydiodevoltagedrop.Leaveanadequatesafety
marginforD.C.regulatorvoltagerequirementsandminimumoperatinglinevoltage
Transformer Current Ratings: A transformer's A.C. current rating needs to be
recalculated from the D.C. load current. The required current
varieswithtypeofrectifierchosenandfiltertype.Usetheformulasbelowasa
guide,shownforcommonD.C.supplies.Includedintheformulashigherpeakto
peakcapacitorchargingcurrentinthefilter.
Rectifier Selection Notes: When selecting rectifiers remember, average
current in a full wave circuit is .5 x I D.C. per diode.
Inahalfwavecircuit,averagecurrentisequaltoID.C.perdiode.Aratingatleast
twicetheoutputcurrentisrecommendedtocoverturnonsurge.Infullwave
circuits,thereversevoltageratingshouldbeinexcessof1.4xVA.C.Inhalfwave
circuits,thereversevoltageratingshouldbeinexcessof
2.8xVA.C.

14. 14
Capacitor Selection Notes:
Whenchoosingcapacitorvoltage,allowancesshouldbe
madeforD.C.voltageriseduetotransformerregulation.Remember,R.M.S.ripple
currentinafiltercapacitorcanbe2to3timesD.C.loadcurrent.Capacitorlifeisgreatly
increasedbyreducingit'stemperaturevialessR.M.S.currentorreducedambient
temperature.

15. 15

16. 16
Three Phase Rectifier

17. Marking RMS reverse voltage in
Volts
Current in Amps
1N4001 35 1
1N4002 70 1
1N4003 140 1
1N4004 280 1
1N4005 420 1
1N4006 560 1
1N4007 700 1
1N5400 100 3
1N5401 200 3
1N5402 300 3
1N5404 525 3
1N5406 800 3
1N5407 1000 3
1N5408 1200 3
17

18. 18
S.NoDescription Half Wave F.W C.P F.W.Bridge
1
2 DC Output voltage 0.318 Vs(peak)
3

19. 19
Smoothing Capacitor value

20. 20
Ripple factor
The pulsating output of a rectifier consists of d.c. component and a.c. component (
also known as ripple). The a.c. component is undesirable and account for the
pulsations in the rectifier output.
The effectiveness of a rectifier depends upon the magnitude of a.c. component in the
output : the smaller this component, the more effective is the rectifier.
“The ratio of rms value of a.c. component to the d.c. component in the rectifier output
is known as ripple factor”

21. 21
Ripple factor for Half-wave rectification
By definition the effective (ie rms) value of total load current is given by
Idc = Im/╓

22. 22

23. 23
Types of Filters
1.Capacitor Filter (C-Filter)
2. Inductor Filter
3. Choke Input Filter (LC-filter)
4. PiFilter(-filter)
Capacitor Filter( C-filter)

24. 24
When the Input signal rises from o to a the diode is forward biased therefore it starts
conducting since the capacitor acts as a short circuit for ac signal it gets charged up to
the peak of the input signal and the dc component flows through the load R
L.
When the input signal fall from a to b the diode gets reverse biased . This is mainly
because of the voltage across the capacitor obtained during the period o to a is more
when comapared to V
i. Therefore there is no conduction of current through the diode.
Now the charged capacitor acts as a battery and it starts discharging through the load
R
L. Mean while the input signal passes through b,c,d section. When the signal reaches
the point d the diode is still reverse biased since the capacitor voltage is more than the
input voltage.
When the signal reaches point e, the input voltage can be expected to be more than the
capacitor voltage. When the input signal moves from e to f the capacitor gets charged to
its peak value again. The diode gets reverse biased and the capacitor starts discharging.
The final output across R
Lis shown in Fig.

25. 25
This type of filter is also called choke filter. It consists of an inductor L which is inserted between
the rectifier and the load resistance R
L.
The rectifier contains A.C components as well as D.C components. When the output passes
through the inductor, it offers a high resistance to the A.C component and no resistance to D.C
components.
Therefore, A.C components of the rectified output is blocked and only D.C components reached
at the load.
Inductor Filter

26. 26
LC Filter
In inductor filter, the ripple factor is directly proportional to the load resistance. On the other
hand in a capacitor filter, it is varying inversely with the load resistance. Hence if we combine the
inductor filter with the capacitor the ripple factor will become almost independent of the load
filter. It is also known as inductor input filter, choke input filter, L input or LC-section.
In this circuit a choke is connected in series with the load. It offers high resistances to the AC
components and allows DC component to flow through the load. The capacitor across the load is
connected in parallel which filter out any AC component flowing through the choke. In this way
the reppls are rectified and a smooth DC is provided through the load.

27. 27
CLC or Pie Filter
It consists of one inductor and two capacitor connected across its each end. The three components
are arranged in shape of Greek letter Pi. It is also called capacitor input Pi filter.
The input capacitor C
1is selected to offer very low reactance to the repel frequency hence major
parts of filtering is done by C
1. Most of the remaining repels are removed by the combining
action of L and C
2.
This circuit gives much better filter then LC filter. However C
1is still directly connected across
the supply and would need high pulse of current if load current is large. This filter is used for the
low current equipment’s.

28. 28

29. 29
Power Diodes of largest power rating are required to conduct several kilo amps of
current in the forward direction with very little power loss while blocking several kilo
volts in the reverse direction. Large blocking voltage requires wide depletion layer in
order to restrict the maximum electric field strength below the “impact ionization” level.
Space charge density in the depletion layer should also be low in order to yield a wide
depletion layer for a given maximum Electric fields strength. These two requirements
will be satisfied in a lightly doped p-n junction diode of sufficient width to accommodate
the required depletion layer. Such a construction, however, will result in a device with
high resistively in the forward direction. Consequently, the power loss at the required
rated current will be unacceptably high.
On the other hand if forward resistance (and hence power loss) is reduced by
increasing the doping level, reverse break down voltage will reduce. This apparent
contradiction in the requirements of a power diode is resolved by introducing a lightly
doped “drift layer” of required thickness between two heavily doped p and n layers

30. 30
Fig shows the circuit symbol and the photograph of a typical power diode respectively