Download What do resistors do?

RESISTORS


Resistors limit current, create voltage drops
All resistors are rated in both a fixed ohm value of resistance and a power
rating in watts. (Watt = Volts X Amps)

Unit -- Ohm Ώ
resistor in series
with an LED
Enough current flows to make the LED light up, But not so
much that the LED is damaged
TYPES
1. Fixed Resistor
2. Variable Resistor
Fixed Resistor Electric symbol
Generic Variable Resistor Electrical Symbol
FIXED RESISTORS
• Fixed-value resistors are divided into two category types of
resistors: Carbon / Metal Oxide and Wire-Wound.
Carbon and Metal Oxide flm
Wire wound
CARBON RESISTORS
•
•
•
•
Carbon resistors are commonly used in electronic systems.
Carbon is mixed with binder;
the more carbon, the lower the resistance.
Carbon resistors have a fixed resistance value and are used to limit current
flow.
• They are rated in watts and most have color-code bands to show the
resistance value.
• A typical resistor has a watt rating from 0.125W to 2.0 W.
Carbon
Metal Oxide film
WIRE-WOUND RESISTORS
• Made with coils of resistance wire.
• Often enclosed in ceramic to help dissipate heat and protect the
resistor wire,
• Accurate and heat stable.
• The resistance value is often marked.
• Used in higher watt circuits often 2W or higher.
• An ignition ballast resistor is an example of a wire wound resistor.
VARIABLE RESISTORS
• Resistance increases with increasing length. It is possible to
use this effect to build a variable resistor.
• Resistance can be altered by changing the length of resistor in
the circuit. The device below allows just that:
Rotating the knob alters the length, and in turn the resistance.
Types
• Rheostat
• Potentiometer
• Trimmer
RHEOSTAT
•Rheostats have two connections,
•one to the fixed end of a resistor and the other to a sliding
contact on the resistor.
•Turning the control moves the sliding contact away from
or toward the fixed end, increasing or decreasing the
resistance.
•Rheostats control resistance, thus controlling current flow.
RHEOSTAT OPERATION
• As the wiper moves along the rheostat it exposes more or less of the
resistor. Moving the wiper towards the high places a small portion of the
resistor in series with the light, causing the light to glow bright. Moving the
wiper toward the low, places a larger portion of the resistor in series with
the lamp; this increased resistance causes less current to flow lowering the
intensity of the light. Rheostats are not used on computer circuits because
of temperature variations on the resistor when the wiper arm is moved.
POTENTIOMETER
•
•
•
•
•
Used to measure changes in position.
Have three connections or legs: the reference, signal, and ground.
The reference is at one end of a resistor and the Ground is at the other
end.
Current flows from the Reference through the resistor to Ground
creating a voltage drop across the resistor.
The Signal is a sliding contact (movable wiper arm) that runs across
the resistor. Unlike a rheostat, its main purpose is not to vary
resistance but to vary the voltage in a circuit
Potentiometer Symbol
.
Variable Resistor Symbol
POTENTIOMETER OPERATION
•
Remember a potentiometer has three legs, the reference (R), the signal
(S) , and the ground (G) as shown below. 5 volts is supplied to the
reference, current flows from the reference (R) through the entire
resistor to ground (G). The Signal wiper slides across the resistor
changing measure voltage as it moves. As the wiper moves towards
the reference (R), the measured signal voltage at (S) will increase. As
the wiper moves away from the Reference (R) towards ground (G), the
measured signal voltage drops
POTENTIOMETER
APPLICATIONS
• Since potentiometer are used to measure changes in position
they naturally are used for throttle, EGR, AC blend door, and
power seat position sensors. All potentiometers have three
wires and are used to measure position changes
RESISTOR RATING COLOR
BANDS
• The first two bands set the digit or number value of the resistor.
• The third band, also known as the multiplier band, is the
number of zeros added to the number value.
• The last band is the Tolerance band. Example: +/- 10%
RESISTOR COLOR BAND
CHART
• The chart below is used to interpret the color bands on the
carbon resistor. Another chart is used to show the value of
tolerance band colors
READING COLOR BANDS RATING VALUE
• The first color band is Green with a value of "5".
The second color band is Red with a value of "2".
The third band is Black with a value of "0" zero. (No zeros are
added)
• So the resistor has a base value of 52 ohms.
READING COLOR BANDS TOLERANCE VALUE
• Resistors vary in tolerance (accuracy). Common tolerance
values are 20%, 10%, 5%, 2%, or 1%, simply meaning the
maximum percent allowable difference the resistor value
actually is from the designed value rating. A 1% resistor is a
higher quality resistor than one with a 20% rating.
• The tolerance band (last band) is silver with a value of 10%. So,
the resistance value is "52 ohms plus or minus 5.2 ohms" (46.8
to 57.2 ohms).
CAPACITOR
• A device used to store charge in an electrical circuit
• functions much like a battery but charges and discharges
much more efficiently
• A basic capacitor is made up of two conductors separated by
an insulator, or dielectric
• The dielectric can be made of paper, plastic, mica, ceramic,
glass, a vacuum or nearly any other nonconductive material
Symbol of capacitor
CAPACITANCE
•
•
•
•
•
•
Measure of a capacitor's ability to store charge.
A large capacitance means that more charge can be stored
Measured in farads, symbol F.
However 1F is very large,
so prefixes are used to show the smaller values.
Three prefixes (multipliers) are used, µ (micro), n (nano) and p
(pico):
• µ means 10-6 (millionth), so 1000000µF = 1F
• n means 10-9 (thousand-millionth), so 1000nF = 1µF
• p means 10-12 (million-millionth), so 1000pF = 1nF
TYPES
• Fixed capacitors
• Electrolytic capacitors
– Polarized capacitors
– Nonpolarized capacitors
» Ceramic capacitors
» Plastic capacitors
» Mica capacitors
» Paper capacitors
• Variable capacitors
FIXED CAPACITORS
Electrolytic capacitors
Polarized capacitor
• have implicit polarity
• can only be connected one way
in a circuit
• Symbol
Nonpolarized capacitor
•
•
•
has no implicit polarity
can be connected either way in
a circuit.
Eg Ceramic, mica
VARIABLE CAPACITORS
•
•
•
•
Mostly used in radio tuning circuits
Sometimes called “ tuning capacitors”
Have very small capacitance values
Ttypically between 100pF and 500pF (100pF = 0.0001µF)
Variable Capacitor Symbol
DISSIPATION FACTOR
• Measure of the power factor (or losses) of a capacitor
• DF = 2 P fRC X 100%, where
•
•
•
•
•
R -- Equivalent Series Resistance (ESR) of the capacitor,
f -- frequency,
C -- capacitance.
Dissipation factor varies with frequency and temperature.
ESR -- measure of the total lossiness of a capacitor which
includes the leads, electrodes, dielectric losses, leakage (IR)
and most important, the end spray connecting the leads to the
metallized film.
Inductors
Inductor are used in electrical circuits
because they store energy in their
magnetic fields.
What is an Inductor?
Current
Flux
i
A coil of wire that
can carry current
Current produces a magnetic field
Energy is stored in the inductor
TYPES
• Fixed inductors
– Depending on the type of core used
• Air core inductors
• Iron core inductors
• Ferrite core inductors
• Variable inductors
FIXED INDUCTORS
• Air core inductors
• Consists of few turns of wire, wound
on a hollow former
• Generally used in radio frequency
applications where very low value of
inductance is required
• Iron core inductors
• Contains a number of turns of
copper wire, wound on a hollow
former
• Generally used in low frequency
applications such as filter circuits in
power supplies or chokes in
fluorescent tubes
FIXED INDUCTORS (contd…)
• Ferrite core inductors
• Made up of non-metallic compounds consisting
mainly of ferric oxide in combination with one or
two bivalent metal oxides
• Appliocations
» r.f. chokes for supply decoupling purpose
» Switching regulated type dc power supplies
» Various types of filters used in communication
equipment
VARIABLE INDUCTORS
• symbol
• Hollow former has screw threads in the inner hollow
portion
• Similar matching threads are provided on the ferrite
core which can be screwed in or out of the former
• Because of the change of the position of the ferrite
core, the value of the inductance changes
• Maximum when ferrite core is fully in
Q of an Inductor
• No power loss in an ideal inductor
• But losses do occur in practicalinductor
• 2 types of losses
• Hysteresis and eddy current losses in the core
• I2R loss
Rs
Ls
Equivalent circuit of an Inductor
Q = ω Ls / Rs
THE BIPOLAR JUNCTION TRANSISTOR
• The bipolar junction transistor is a 3-terminal device consisting 2
layers of n-type material sandwiching a thin p-type layer or 2 layers
of p-type material sandwiching a thin layer of n-type material.
• These structures are appropriately called npn transistor and pnp
transistor, respectively.
• The terminals are called Emitter, Base and Collector.
• The emitter is heavily doped, the base is lightly doped and collector
is moderately doped.
TRANSISTOR PHYSICAL STRUCTURE AND SYMBOL
• There are two types of BJTs, the npn and pnp
• The two junctions are termed the base-emitter junction and
the base-collector junction
• The term bipolar refers to the use of both holes and electrons
as charge carriers in the transistor structure
• In order for the transistor to operate properly, the two
junctions must have the correct dc bias voltages
– the base-emitter (BE) junction is forward biased
– the base-collector (BC) junction is reverse biased
Forward-Reverse bias of a BJT
TRANSISTOR
OPERATION
For current to flow through the BJT
its two p-n junctions (2 diodes back
to back) must be properly biased.
Typically, one junction is forward
bias while the other is reverse bias.
The figure below shows the biasing
of a pnp transistor
Notice that the emitter-base junction is forward biased while the collector-base junction
is reverse biased.
Majority carriers will flow from emitter to base across the forward biased junction.
Because the base layer is very thin and has a high resistance (lightly doped) most of
these carriers will diffuse across the reverse-biased junction into the collector in the
same direction of the minority charges, and only tiny amounts of current will flow out
of the base terminal. Typically collector currents are of the order of mA while base
currents are μA. Appling Kirchhoff’s current law: IE = IC + IB
TRANSISTOR CURRENTS
• Transistor Currents:
IE = IC + IB
• alpha (αDC)
IC = αDCIE
• beta (βDC)
IC = βDCIB
– βDC typically has a value between 50 and 500
TRANSISTOR VOLTAGES
• DC voltages for the biased transistor:
• Collector voltage
VC = VCC - ICRC
• Base voltage
VB = VE + VBE
– for silicon transistors, VBE = 0.7 V
BJT operating modes
OPERATION MODE
OPERATION MODE
• Active:
– Most importance mode, e.g. for amplifier operation.
– The region where current curves are practically
flat.
• Saturation:
– Barrier potential of the junctions cancel each other
out causing a virtual short.
– Ideal transistor behaves like a closed switch.
• Cutoff:
– Current reduced to zero
– Ideal transistor behaves like an open switch.
BJT - operation in Active mode
Active Mode Circuit Model
TRANSISTOR CONFIGURATIONS
There are three types: Common-Base (CB), Common-Emitter (CE), and CommonCollector (CC).
THE COMMON-BASE CONFIGURATION
The base is common to both the input and the output, usually at ground potential.
Features of CB Connection
• Current Gain <1
• Power Gain>1
• Voltage Gain>1
• Low i/p Impedance
• High o/p Impedance
THE COMMON-EMITTER CONFIGURATION
• Emitter is common or reference to both input and output signal.
• The CE configuration is the one most commonly encountered since it
provides both good current and voltage gain for ac signals.
• In the CE configuration the input is between the base and the emitter.
The output is between the collector and the emitter.
THE COMMON-COLLECTOR CONFIGURATION OR
EMITTER FOLLOWER
• Collector is common to both input and output. Mainly used for current
amplification and impedance matching.
• Characteristics curve is same as CE configuration
• A typical transistor power amp will have a CE stage followed by a CC stage
Features of CC
Connection
• High Input Impedance
• Low Output impedance
• Voltage Gain < 1
• Power + Current Gain
SOME CHARACTERISTICS OF BJTS – A RECAP
1. An equivalent circuit of a NPN transistor is two diodes tied anode to
anode; one cathode being the emitter, the other the collector, and the
junction of the anodes is the base.
2. When a NPN transistor is in operation, there is always a constant 0.6
volt drop between the base and emitter, i.e., the base is always ~ 0.6 volts
more positive than the emitter--always!
3. There is no output at the collector, until the base has reached ~ 0.6 volts
and the base is drawing current, i.e., any signal that appears at the base that
is not up to ~ 0.6 volts (and not drawing base current), is never seen at the
collector.
4. The base requires a current, not a voltage to control the collector
current.
5. The collector is a current source: it does not source a voltage.
6. The collector appears to output a voltage when a resistor is connected
between it and power.
7. The collector is a high impedance when compared to the emitter.
SOME CHARACTERISTICS OF BJTS – A RECAP
8. The transistor can output an amplified signal either from the collector or the
emitter (or both).
9. When operating with a collector resistor (RL): the output voltage from the
collector is an amplified voltage.
10. When operating with only an emitter resistor (Re): the output voltage from
the emitter is not an amplified voltage, because it is always ~ 0.6 volts, below the
input (base) voltage--hence the name voltage follower. But because the emitter
can source large amounts of current to the "LOAD," it can be said, there was
CURRENT amplification.
11. The collector--being high impedance--cannot drive a low impedance load.
12. The emitter--being a low impedance--can drive a low impedance load.
13. The voltage gain from the collector is greater than one (Gv > 1).
14. The voltage gain from the emitter is less than one (Gv < 1).
15. Both the collector and the emitter: output ~ the same power: E x I = P.
Silicon Controlled Rectifiers
Silicon Controlled Rectifier
• A Silicon Controlled Rectifier (or Semiconductor Controlled
Rectifier) is a four layer solid state device that controls current
flow
• The name “silicon controlled rectifier” is a trade name for the
type of THYRISTOR commercialized at General Electric in
1957
Silicon Controlled Rectifier
• An SCR can be seen as a conventional rectifier controlled by a
gate signal
• It is a 4-alternately doped semiconductor layers 3-terminal
device. 3-leads are referred to as the Anode, Cathode, and
Gate.
• When the gate to cathode voltage exceeds a certain threshold,
the device turns 'on' and conducts current
• Which are used as electronically controlled switches
Silicon Controlled Rectifier
• The operation of a SCR can be understood in terms of a pair of
tightly coupled Bipolar Junction Transistors
• used
for the purpose of controlling electrical power while
BJT's and FET's. Since they do not have the power handling
capability
• SCR has three states:
• Reverse blocking mode, forward blocking mode, and
forward conducting mode
TYPES OF DIODES
V-I Characteristics of SCR
• The V-I curve shows the relationship between VF and IF when the SCR's
gate is open.
• When a Forward voltage is applied to the SCR,
– The SCR's cathode-to-anode voltage is designated as VF at this time.
VF increases from zero, the SCR conducts only a small forward current
(IF ) which is due to leakage. As VF continues to increase, IF remains
very low and almost constant but eventually a point is reached where IF
increases rapidly and VF drops to a low value.
– The VF value required to trigger this sudden change is referred to as the
Forward Breakover Voltage (Vp). When this value of Vp is reached
the SCR simply breaks down, and conducts a high IF which is limited
only by the external resistance in series with the device.
– The SCR switches from the off state to the on state at this time. The
drop in VF occurs because the SCR' s resistance drops to an extremely
low value and most of the source voltage appears across the series
resistor.
• When the SCR is in the on state,
– only a slight increase in VF is required to produce a tremendous
increase in IF.
– the SCR will remain in the on state as long as IF remains at a
substantial value. Only when IF drops below a certain minimum value,
will the SCR switch back to its off-state.
– This minimum value of IF which will hold the SCR in the on state is
referred to as the SCR's Holding Current and is usually designated at
IH . The IH value is located at the point where breakover occurs.
• When a reverse voltage is applied to the SCR,
– the device functions in basically the same manner as a reverse-biased PN
junction diode. As the reverse voltage,
– (VR ) across the SCR increases from zero, only a small reverse current
(IR) will flow through the device due to leakage. This current will remain
small until VR becomes large enough to cause the SCR to breakdown.
Then IR will increase rapidly if VR increases even slightly above the
breakdown point.
– The reverse voltage (VR) required to breakdown the SCR is referred to as
the SCR's Reverse Breakdown Voltage.
–
– If too much reverse current is allowed to flow through the SCR after
breakdown occurs, the device could be permanently damaged.
– this situation is normally avoided because the SCR is usually subjected to
operating voltages which are well below its breakdown rating.
• When the gate is made positive with respect to the cathode,
– gate current will flow and the SCR's forward characteristics will be affected.
– The Ig = 0 curve shows the relationship between VF and IF when the gate current is zero.
–
The Ig1 curve is plotted for a specific but relatively low value of gate current. Notice that
this curve has the same general shape as the Ig = 0 curve but the forward breakover point
occurs sooner (at a lower VF value).
– The Ig2 curve is plotted for a slightly higher gate current and also has the same general
shape as the other two curves. However, the breakover point occurs even sooner at this
higher for different values of gate value of gate current.
Silicon Controlled Rectifier
• Industrially SCRs are applied to produce DC voltages for
motors from AC line voltage
• Rectifier
– Half-wave rectifier, full-wave rectifier
Half-wave rectifier
Half-wave rectifier
Half-wave rectifier
Reviews
• A SCR is essentially a diode with an extra terminal added
• This extra terminal is called the gate, and it is used to trigger
the device into conduction by the application of a small
voltage
• Widely used in applications where dc and ac power must be
controlled. These devices are often used to apply a specific
amount of power to a load or to completely remove it however,
they are also used to regulate or adjust the amount of power
applied to a specific load.
Application: DC Motor Driver
• DC motor speed generally depends on a combination of the
voltage and current flowing in the motor coils and the motor
loads or braking torque
• The speed of the motor is proportional to the voltage, and the
torque is proportional to the current
Light Emitting Diodes
• Light emitting diodes, commonly called LEDs, are
real unsung heroes in the electronics world.
• They form the numbers on digital clocks, transmit
information from remote controls, light up watches and
they can form images on a jumbo television screen 0r
illuminate traffic light.
• Basically, LEDs are just tiny light bulbs that fit easily
into an electrical circuit. But unlike ordinary
incandescent bulbs, they don't have a filament that will
burn out, and they don't get especially hot. They are
illuminated solely by the movement of electrons in a
semiconductor material, and they last just as long as a
standard transistor.
Inside a LED
• The two wires extending below the LED epoxy enclosure, or the "bulb" indicate
how the LED should be connected into a circuit.
• The negative side of an LED lead is indicated in two ways:
– by the flat side of the bulb,
– by the shorter of the two wires extending from the LED.
– The negative lead should be connected to the negative terminal of a battery.
LED's operate at relative low voltages between about 1 and 4 volts, and
draw currents between about 10 and 40 milliamperes.
– Voltages and currents substantially above these values can melt a LED chip.
– The most important part of a LED is the semi-conductor chip located in the
center of the bulb.
– The chip has two regions separated by a junction. The p region is
dominated by positive electric charges, and the n region is dominated by
negative electric charges.
– The junction acts as a barrier to the flow of electrons between the p and the
n regions. Only when sufficient voltage is applied to the semi-conductor
chip, can the current flow, and the electrons cross the junction into the p
region.
– In the absence of a large enough electric potential difference (voltage)
across the LED leads, the junction presents an electric potential barrier to
the flow of electrons.
LED-Electrical Properties-PN junctions
Operation
•
When the current flows across a diode the negative electrons
move one way and positive holes move the other way.
•
the holes exist at a lower energy level than the free
electrons, so when a free electron falls it loses energy
•
this energy emitted in the form of a light photon. The size
of electron’s “fall” determines the energy level of the
photon, which determines its colour. A bigger fall produces
a photon with the higher energy level and therefore a
higher light frequency.
(Contd…)
• PN junction diode in forward bias, the electron-hole
recombination leads to photon emission
•
I = Is(eeV/kT-1)
• Threshold voltage Vth = Eg/e
•
I = IseeV/ηkT
where η is the ideality factor
Double Heterostructure is used to confine the carriers,
improving
the radiative recombination rate
LED-Optical Properties-Efficiency
• ηint = # of photons emitted from active region per second
# of electrons injected in to LED per second
= Pint / (hν)
I/e
• ηextr = # of photons emitted into free space per second
# of photons emitted from active region per second
= P / (hν)
Pint / (hν)
»Temperature dependence of emission
intensity
• Emission intensity decreases with increasing temperature.
• Causes include non-radiative recombination via deep levels, surface
recombination, and carrier loss over heterostucture barriers.
»High internal efficiency LED designs
• Radiative recombination probability needs to be increased and non-radiative
recombination probability needs to be decreased.
• High carrier concentration in the active region, achieved through double
heterostructure (DH) design, improves radiative recombination.
R=Bnp
• DH design is used in all high efficiency designs today.
High internal efficiency designs
• Doping of the active regions and that of the cladding regions strongly
affects internal efficiency.
• Active region should not be heavily doped, as it causes carrier spill-over in
to the confinement regions decreasing the radiative efficiency
• Doping levels of 1016-low 1017 are used, or none at all.
• P-type doping of the active region is normally done due to the larger
electron diffusion length.
• Carrier lifetime depends on the concentration of majority carriers.
• In low excitation regime , the radiative carrier lifetime decreases with
increasing free carrier concentration.
• Hence efficiency increases with doping.
• At high concentration, dopants induce defects acting as recombination
centers.
High extraction efficiency structures
• Shaping of the LED die is critical to improve their efficiency.
• LEDs of various shapes; hemispherical dome, inverted cone, truncated
cones etc have been demonstrated to have better extraction efficiency
over conventional designs.
• However cost increases with complexity.
Visible spectrum LEDs
The plot charts the gains made in luminous efficiency till date.
White-light LEDs
• White light can be generated in several different ways.
• One way is to mix to complementary colors at a certain power
ratio.
• Another way is by the emission of three colors at certain
wavelengths and power ratio.
• Most white light emitters use an LED emitting at short
wavelength and a wavelength converter.
• The converter material absorbs some or all the light emitted by
the LED and re-emits at a longer wavelength.
• Two parameters that are important in the generation of white
light are luminous efficiency and color rendering index.
• It is shown that white light sources employing two
monochromatic complementary colors result in highest
possible luminous efficiency.
White-light LEDs( Contd…)
• Wavelength converter materials include phosphors,
semiconductors and dyes.
• The parameters of interest are absorption wavelength,
emission wavelength and quantum efficiency.
• The overall energy efficiency is given by
η = ηext(λ1/ λ2)
• Even if the external quantum efficiency is 1, there is always an
energy loss associated with conversion.
• Common wavelength converters are phosphors, which consist
of an inorganic host material doped with an optically active
element.
• The optically active dopant is a rare earth element, oxide or
another compound.
• Common rare earth elements used are Ce, Nd, Er and Th.
White-light LEDs
• Phosphors are stable materials and can have quantum efficiencies
of close to 100%.
• Dyes also can have quantum efficiencies of close to 100%.
• Dyes can be encapsulated in epoxy or in optically transparent
polymers.
• However, organic dyes have finite lifetime. They become optically
inactive after 104-106 optical transitions.
LED Advantages
• While all diodes release light, most don't do it very effectively.
• In an ordinary diode, the semiconductor material itself ends up
absorbing a lot of the light energy. LEDs are specially
constructed to release a large number of photons outward.
• Additionally, they are housed in a plastic bulb that
concentrates the light in a particular direction.
• Most of the light from the diode bounces off the sides of the
bulb, traveling on through the rounded end.
• Remote control operation (Advantages of LED)
• the basic operation of the remote goes like this: You press a
button. When you do that you complete a specific connection.
The chip senses that connection and knows what button you
pressed. It produces a morse-code-line signal specific to that
button. The transistors amplify the signal and send them to the
LED, which translates the signal into infrared light. The sensor in
the TV can see the infrared light and "seeing" the signal reacts
appropriately.
RECTIFIERS
•INTRODUCTION
•HALF WAVE RECTIFIERS
•FULL WAVE RECTIFIERS
•BRIDGE RECTIFIERS
• Rectification is the conversion of alternating current (AC) to
direct current (DC).
• A rectifier converts a sinusoidal voltage to a constant
voltage.
• This involves a device that only allows one-way flow of
electrons.
• As we have seen, this is exactly what a semiconductor
diode does.
• A Diode switches polarity from + to - many times a second,
into a straight DC supply.
• Rectifier is a type of ac-dc converter
•􀂃 Single phase
•􀂃 Three phase
HALF WAVE RECTIFIERS
• A half-wave rectifier is a circuit that allows only one half-cycle
of the AC voltage waveform to be applied to the load,
resulting in one non-alternating polarity across it.
• The resulting DC delivered to the load “pulsates” significantly.
• This circuit takes an AC signal in and chops off anything that
falls below 0 Volts.
• The half-wave rectifier is used in AM radios to rectify the
signal.
CIRCUIT DIAGRAM
OUTPUT WAVEFORM
WORKING PRINCIPLE OF THE
HALF WAVE RECTIFIER
•The Half wave rectifier is a circuit, which converts an ac
voltage to dc voltage.
•In the Half wave rectifier circuit the transformer serves
two purposes.
•It can be used to obtain the desired level of dc voltage
(using step up or step down transformers).
•It provides isolation from the power line.
•The primary of the transformer is connected to ac supply.
•This induces an ac voltage across the secondary of the
transformer.
• During the positive half
cycle of the input voltage
the polarity of the voltage
across
the
secondary
forward biases the diode.
• As a result a current IL
flows through the load
resistor, RL.
•
The forward biased diode
offers a very low resistance
and hence the voltage drop
across it is very small.
•
Thus the voltage appearing
across the load is practically
the same as the input
voltage at every instant.
•
During the negative half
cycle of the input voltage
the
polarity
of
the
secondary voltage gets
reversed.
•
As a result, the diode is
reverse biased.
•
Practically no current flows
through the circuit and
almost
no
voltage
is
developed
across
the
resistor.
•
All input voltage appears
across the diode itself.
•Thus when the input voltage is going through its positive half
cycle, output voltage is almost the same as the input voltage and
during the negative half cycle no voltage is available across the
load.
•This explains the unidirectional pulsating dc waveform obtained
as output.
• The process of removing one half the input signal to establish a
dc level is aptly called half wave rectification.
•The diode only conducts on every other half cycle.
•There is one pulse for every cycle in. i.e 50 pulses per second (in
the UK).
•The rectified voltage is DC (it is always positive
value).However, it is not a steady DC but PULSATING DC.
•It needs to be smoothed before it becomes useful.
•If the diode is reversed then the output voltage is negative.
in
Signal In
Signal Out (Half-wave):
The simplest rectifier uses one diode, like this:
PARAMETERS
Peak Inverse Voltage
• When the input voltage reaches its
maximum value Vm during the
negative half cycle the voltage
across the diode is also maximum.
• This maximum voltage is known as
the peak inverse voltage.
• Thus for a half wave rectifier
Ripple Factor
• Ripple factor is defined as the ratio of
RMS value of ac component to the dc
component in the output.
• RMS voltage at the load resistance :
Efficiency
• Efficiency, h is the ratio of the dc output
power to ac input power:
Transformer Utilization Factor
• Transformer Utilization Factor, TUF can be
used to determine the rating of a
transformer secondary.
• In half wave rectifier the rated voltage of
the transformer secondary is
• But actually the RMS current
flowing through the winding is only
Form Factor
• Form factor is given by,
Peak Factor
• Peak factor is given by,
FULL WAVE RECTIFIERS
• A full-wave rectifier is a circuit that converts both half-cycles of
the AC voltage waveform to an unbroken series of voltage
pulses of the same polarity.
• The resulting DC delivered to the load doesn't “pulsate” as
much.
• A full-wave rectifier flips the negative half of the signal up into
the positive range.
• When used in a power supply, the full-wave rectifier allows us
to convert almost all the incoming AC power to DC.
CIRCUIT DIAGRAM OF A
FULL WAVE RECTIFIER
OUTPUT WAVEFORM
WORKING PRINCIPLE OF A FULL
WAVE RECTIFIER
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•
•
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•
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A Full Wave Rectifier is a circuit,
which converts an ac voltage into
a pulsating dc voltage using both
half cycles of the applied ac
voltage.
It uses two diodes of which one
conducts during one half cycle
while the other conducts during
the other half cycle of the applied
ac voltage.
During the positive half cycle of
the input voltage, diode D1
becomes forward biased and D2
becomes reverse biased.
Hence D1 conducts and D2
remains OFF.
The load current flows through
D1 and the voltage drop across
RL will be equal to the input
voltage.
During the negative half cycle of
the input voltage, diode D1
becomes reverse biased and D2
becomes forward biased.
Hence D1 remains OFF and D2
conducts.
The load current flows through
D2 and the voltage drop across
RL will be equal to the input
CURRENT FLOW DURING
ONE HALF CYCLE
PARAMETERS
Ripple Factor
The ripple factor for a Full Wave
Rectifier is given by
RMS value of the voltage at the load resistance is
Efficiency
• Efficiency, h is the ratio of the dc
output power to ac input power
• The maximum efficiency of a Full
Wave Rectifier is 81.2%.
Transformer Utilization Factor
• Transformer Utilization Factor, TUF can be
used to determine the rating of a
transformer secondary.
• It is determined by considering the primary
and the secondary winding separately and
it gives a value of 0.693.
Form Factor
• Form factor is defined as the ratio of the
rms value of the output voltage to the
average value of the output voltage.
Peak Factor
• Peak factor is defined as the ratio of
the peak value of the output voltage
to the RMS value of the output
voltage.
• Peak inverse voltage for Full Wave
Rectifier is 2Vm because the entire
secondary voltage appears across
the non-conducting diode.
BRIDGE RECTIFIERS
• The Bridge rectifier is a circuit, which converts an ac
voltage to dc voltage using both half cycles of the
input ac voltage.
• A bridge rectifier can be made using four individual
diodes.
• It is called a full-wave rectifier because it uses all
the AC wave (both positive and negative sections).
• 1.4V is used up in the bridge rectifier because each
diode uses 0.7V when conducting and there are
always two diodes conducting.
• Bridge rectifiers are rated by the maximum current
they can pass and the maximum reverse voltage
they can withstand (this must be at least three times
the supply RMS voltage so the rectifier can
withstand the peak voltages).
• One advantage of a bridge rectifier over a
conventional full-wave rectifier is that with a
given transformer the bridge rectifier produces
CIRCUIT FOR A BRIDGE
RECTIFIER
OUTPUT OF A BRIDGE
RECTIFIER
Output: full-wave varying DC
(using all the AC wave)
WORKING PRINCIPLE
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The circuit has four diodes
connected to form a bridge.
The ac input voltage is applied
to the diagonally opposite ends
of the bridge.
The
load
resistance
is
connected between the other
two ends of the bridge.
For the positive half cycle of the
input ac voltage, diodes D1 and
D3 conduct, whereas diodes D2
and D4 remain in the OFF state.
The conducting diodes will be
in
series
with
the
load
resistance RL and hence the
load current flows through RL.
• For the negative half cycle
of the input ac voltage,
diodes D2 and D4 conduct
whereas, D1 and D3 remain
OFF.
•
The conducting diodes D2
and D4 will be in series with
the load resistance RL and
hence the current flows
through RL in the same
direction as in the previous
half cycle.
• Thus a bi-directional wave
is
converted
into
a
unidirectional wave.
PARAMETERS
Peak Inverse Voltage
• Peak inverse voltage represents the maximum
voltage that the non- conducting diode must
withstand.
• At the instance the secondary voltage reaches its
positive peak value, Vm the diodes D1 and D3 are
conducting, where as D2 and D4 are reverse biased
and are
non-conducting.
• The conducting diodes D1 and D3 have almost zero
resistance.
• Thus the entire voltage Vm appears across the load
resistor RL. The reverse voltage across the
non-conducting diodes D2 (D4) is also Vm.
• Thus for a Bridge rectifier the peak inverse voltage
Ripple Factor
• The ripple factor for a Bridge Rectifier
is given by
• RMS value of the voltage at the load
resistance is
Efficiency
• Efficiency, h is the ratio of the dc output power to ac input
power.
• The maximum efficiency of a Bridge Rectifier is 81.2%.