Download Applications of Capacitors

RECTIFICATION
Normal household power is AC while batteries
provide DC, and converting from AC to DC is
called rectification. Diodes are used so
commonly for this purpose that they are
sometimes called rectifiers, although there are
other types of rectifying devices.
TYPES OF RECTIFIERS
Rectifier
Half-wave Rectifier
Full-wave Rectifier
Centre-tape
full-wave rec.
Full-wave
Bridge rec.
Rectifier
A rectifier is a device which converts a.c. voltage
(bi-directional) to pulsating d.c. voltage (Unidirectional).
Any electrical device which offers a low
resistance to the current in one direction but a
high resistance to the current in the opposite
direction is called rectifier.
Half-wave rectifier
Simplest process used to
convert ac to dc.
Only lets through
positive voltages.
Rejects negative
voltages
A diode is used to clip
the input signal
excursions of one
polarity to zero.
Half-wave rectification
• Only the positive halfcycle of the AC supply is
allowed to pass
TRANSFORMER
A transformer is a static device which transfers the energy from primary winding to
secondary winding through the mutual induction principle, without changing the
frequency.
The transformer winding to which the supply source is connected is called the
primary, while the winding connected to the load is called secondary. If N1,N2 are
the number of turns of the primary and secondary of the transformer then
α =N2/N1 is called the turns ratio of the transformer.
The different types of the transformers are
1) Step-Up Transformer
2) Step-Down Transformer
3) Centre-tapped Transformer
Ideal transformer: VS/VP=NS/NP
Real transformers are ~98% efficient
Half-wave Rectifier with Transformer Coupled Input Voltage
RL
Vin
VPri
VSec
A transformer is often used to couple the ac input voltages from the source to
the rectifier circuits. Transformer coupling provides two advantages. First, it
allows the source voltage to be stepped up or stepped down as needed. Second,
the ac power source is electrically isolated from the rectifier circuit, thus reducing
the shock hazard. Basic ac circuit recall that the secondary voltage of a
N Sec
transformer equals the turns ratio( N Pr i ) times the primary voltage.
N
V sec  ( Sec
N Pr i
) VPr i
If NSec > NPri, the secondary voltage is greater than the primary voltage. If NSec
< NPri, the secondary voltage is less than the primary voltage. If NSec = NPri, then
VSec = VPri.
Full wave rectifier
• The full wave rectifier is the most commonly
used type in DC power supply
• A full wave rectifier allows unidirectional (one
way) current through the load during the
entire 3600 of the input cycle
Full wave Centre–Tapped transformer
Rectifier Circuit
Two diodes and a centre-tapped transformer are
required.
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Full-wave Center-tapped Rectifier
The full-wave center-tapped rectifier uses two diodes connected to the
secondary of a center-tapped transformer, The input voltage is coupled
through the transformer to the center-tapped secondary. Half of the
total secondary voltage appears between the center tap and each end
of the secondary winding.
For a positive half-cycle of the input voltage, the polarities on the
secondary This condition forward-biases the diode D1 and reversebiases the diode D2 .The current path is through D1 and the load resistor
RL, as indicated.
During negative half-cycles, D2 is forward-biased and D1 is reversebiased.
Operation of the Center–Tapped Transformer
Rectifier Circuit
For the positive half of the AC cycle:
For the negative half of the AC cycle:
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Full-wave rectification
Positive AND negative halfcycles converted to ALL
positive
Peak value of output voltage for the full-wave center-tapped rectifier
Vp(out ) 
Vp( sce)
 0.7 V
2
Average value of output for the full-wave center-tapped rectifier
V AGV 
2Vp (out)

Diode peak inverse voltage for the full-wave center-tapped rectifier
PIV  2 VP(out)  0.7 V
Full-Wave Rectification:
Bridge Network
• The dc level obtained from a sinusoidal input can be improved
100% using a process called full-wave rectification.
• The most familiar network is bridge configuration with 4
diodes.
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Operation of the Bridge Rectifier Circuit
For the positive half of the AC cycle:
For the negative half of the AC cycle:
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Bridge rectifier
• Current flow in the bridge
-
vO +
-
vO +
BASIC DC POWER SUPPLY
Power supplies provide the necessary power, voltage and current
requirements for electronic devices.
They usually change ac to dc voltage.
For example, 220 volts ac is changed to 12.0 volts dc.
• Basic DC power supply converts the standard
220 V, 50Hz AC at wall outlets into a constant dc voltage.
AC
Rectifier
VDC
Filtered voltage
0
VDC
Regulated voltage
0
Regulator
Load
220 V, 50 Hz
Filter
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One of the most important applications of a diode is in rectifying
circuits: used to convert an AC signal into the DC voltage required by
most electronics
Power Supplies
Consist of:
1. Transformer – steps ac voltage up or down.
2. Rectifier Diodes – change ac to “bumpy” dc.
3. Filter Network – includes capacitors and
inductors, smoothes out the bumps.
4. Voltage Regulator – keeps the voltage constant.
Power Supplies
• Transformer:- It changes AC voltage based on the
turns ratio of primary and secondary windings.
• Filter:- it eliminates the fluctuation in the rectified
voltage and produces a relatively smooth DC voltage
• Regulator:- it is a circuit that maintain a constant DC
voltage for variation in the input line voltage or in the
load.
• Load:- It is a circuit or device connected to the
output of the power supply and operation from the
power supply voltage and current
Smoothed output
• Un-smoothed
– Large ripple voltage
– Not true DC
• Low-value smoothing
capacitor
– RloadC small
• High-value smoothing
capacitor
– RloadC big
Important Concepts
• Half Wave Rectifier:
– Simplest.
– Hard to filter well.
• Full Wave Center-Tapped Rectifier:
– Easier to filter.
– Requires transformer with center-tap.
– Transformer secondary must be twice intended voltage.
• Full Wave Bridge Rectifier:
–
–
–
–
Easier to filter (just like full wave).
Center-tap transformer not required.
Transformer secondary same as intended voltage.
Higher parts count.