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ME8843
ME 8843
Advanced Mechatronics
Instructor: Professor I. Charles Ume
Power Rectifiers
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ME8843 Outline
Motivation
Rectification Technologies
Types of Rectification
Rectification Circuits
Applications
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ME8843 Motivation
Early experiments with Direct
Current (DC) power relied on Leydenjars (rudimentary batteries) whichhad to be recharged via manuallabor (e.g. grad students)
Due to efficiency and safety reasons,Alternating Current (AC) is used forproviding electrical power
A means to convert AC to DC is
required - called RectificationAC Power
Transmission
Lines
Leyden Jar
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ME8843 Rectification Technologies
Electromechanical
Synchronous rectifier Used motor attached to metal contacts that switched direction ofcurrent flow in time with AC input voltage
Motor-generator set
An AC motor coupled to DC generator
Electrolytic
Two different material electrodes suspended in electrolyte
provide different resistance depending on current flow
Mercury arc rectifier
A sealed vessel with mercury in it provides DC power bytransmitting electricity through ionized mercury vapor
Capable of power on order of hundreds of kilowatts
Vacuum Tube
Capable of high voltages, but relatively low current
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ME8843 Mercury Vapor Rectifiers
From steel manufacturing plant in Germany
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ME8843 Rectification Based on Diode
Diodes provide compact, inexpensive means of
rectification
Can create rectifiers from multiple diodes or purchase
integrated module
Diode Rectifier ModulesDiodes
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ME8843 Types of Rectification
Full Wave Rectifier
Half Wave Rectifier
While output of the rectifiers is now DC (current only
flows in one direction), output oscillates
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ME8843Types of Rectification: Poly-phase
Industrial settings usually have 3-phase
power available for machines
Rectifying 3-phase power results in DC
voltage with less ripple
Three-phase full-wave bridge rectifier circuit
Input and output voltages for three-phase rectifier
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ME8843 Types of Rectification
Half Wave:
Negative components of sinewave are discarded
Full Wave:
Negative components are
inverted
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ME8843 Types of Rectification: Poly-phase
Input and output voltages for 3-phase rectifier
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ME8843 Rectification Circuit: Half-Wave
Rectification is most popular application of diode
It converts alternating current (AC) to direct current (DC).
It involves device that only allows one-way flow of
electrons, and this is exactly what semiconductor diode
does.
Simplest kind of rectifier circuit is half-waverectifier.
It only allows one half of AC waveform to pass through to
load.
Half-wave rectif ier circu it
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ME8843 Rectification Circuit: Half-Wave
For most power applications, half-wave rectification is
insufficient for task.
Harmonic content of rectifier's output waveform is very large andconsequently difficult to filter.
AC power source only supplies power to load once every half-
cycle, meaning that much of its capacity is unused.
Half-wave rectification is, however, very simple way to reduce
power to resistive load.
Two-position lamp dimmer switches apply full AC power
to lamp filament for full brightness and then half-wave
rectify it for a lesser light output.
Half-wave rect i f ier appl icat ion: Two level lamp dimmer.
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ME8843 Rectification Circuit: Half-Wave
In Dim switch position, incandescent lamp receives approximatelyone-half power it would normally receive operating on full-wave AC.
Because half-wave rectified power pulses far more rapidly than filamenthas time to heat up and cool down, lamp does not blink.
Instead, its filament merely operates at lesser temperature than normal,providing less light output.
This principle of pulsing power rapidly to slow-responding load
device to control electrical power sent to it is common in world ofindustrial electronics.
Since controlling device (diode, in this case) is either fullyconducting or fully non-conducting at any given time, it dissipateslittle heat energy while controlling load power, making this method of
power control very energy-efficient.
This circuit is perhaps crudest possible method of pulsing power to aload, but it suffices as a proof-of-concept application.
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ME8843 Rectifier Circuit: Full-Wave
If we need to rectify AC power to obtain full use of both
half-cycles of sine wave, different rectifier circuit
configuration must be used.
Such circuit is called full-waverectifier.
One kind of full-wave rectifier, called center-tapdesign, usestransformer with center-tapped secondary winding and two
diodes
Full-wave rectif ier, center-tapped d esign .
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ME8843 Rectifier circuit
This circuit's operation is easily understood one half-
cycle at time.
Consider first half-cycle, when source voltage polarity is
positive (+) on top and negative (-) on bottom. Only top diode is conducting; bottom diode is blocking current,
and load sees first half of sine wave.
Only top half of transformer's secondary winding carries current
during this half-cycle.
Ful l -wave center-tap rect i f ier : Top hal f of secon dary win ding cond ucts
dur ing posi t ive hal f -cycle o f input , del iver ing pos i t ive hal f -cycle to load.
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ME8843 Rectifier circuit
During next half-cycle, AC polarity reverses. Now, other
diode and other half of transformer's secondary winding
carry current while portions of circuit formerly carrying
current during last half-cycle sit idle. The load still sees half of sine wave, of same polarity
as before.
Full-wave center-tap recti f ier: During negative inpu t half-cyc le, bo ttom h alf
of secon dary wind ing c ond ucts, del iver ing a pos i t ive half -cycle to th e load.
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ME8843 Rectifier Circuit: Full-Wave
One disadvantage of this full-wave rectifier design is
necessity of transformer with center-tapped secondary
winding.
If circuit in question is one of high power, size and
expense of suitable transformer is significant.
Consequently, center-tap rectifier design is only seen in
low-power applications.
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ME8843 Rectifier circuit: Dual Polarity Full-Wave
The full-wave center-tapped rectifier polarity at load maybe reversed by changing direction of diodes.
Furthermore, reversed diodes can be paralleled withexisting positive-output rectifier.
The result is dual-polarity full-wave center-tappedrectifier.
Note that connectivity of diodes themselves is sameconfiguration as bridge.
Dual polari ty fu l l-wave center tap rect i f ier
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ME8843 Rectifier circuit: Full-Wave Bridge
Another, more popular full-wave rectifier design exists,
and it is built around four-diode bridge configuration.
For obvious reasons, this design is called full-wave
bridge.
Full-wave br idge r ect i f ier.
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ME8843 Rectifier circuit: Full-Wave Bridge Current directions for full-wave bridge rectifier circuit for
positive and negative half-cycles of AC source waveformare shown below and next page respectively.
Note that regardless of polarity of input, current flows insame direction through load.
That is, negative half-cycle of source is positive half-cycle at load.
Ful l -wave bridg e rect i f ier : Electron f low for po si t ive half-cycles
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ME8843 Rectifier circuit: Full-Wave Bridge
Current flow is through two diodes in series for bothpolarities. Thus, two diode drops of source voltage are lost (0.72 = 1.4 V
for Si) in diodes.
This is disadvantage compared with full-wave center-tapdesign. This disadvantage is only problem in very low voltage power
supplies
Full-wave bridg e rect i f ier: Electro n flow fo r negative half=cy cles.
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ME8843 Rectifier circuit: Full-Wave Bridge
Remembering proper layout of diodes in full-wave bridge
rectifier circuit can often be frustrating some times.
An alternative representation of this circuit is easier both
to remember and to comprehend.
It is exact same circuit, except all diodes are drawn in
horizontal attitude, all pointing same direction.
One advantage of remembering this layout for bridge
rectifier circuit is that it expands easily into poly-phase
version shown in next slide.
Alternat ive layout sty le for Ful l-wave b r idge rect i f ier.
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ME8843 Rectifier circuit: Polyphase-Three Phase
Each three-phase line connects between pair of diodes: One to route power to positive (+) side of load, and other to
route power to negative (-) side of load.
Poly-phase systems with more than three phases are
easily accommodated into bridge rectifier scheme. Takefor instance the six-phase bridge rectifier circuit in nextslide
Three-phase ful l-wave b ridg e rect i f ier circ uit .
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ME8843 Rectifier circuit: Polyphase-Six Phase
Six-phase ful l -wave br idg e rect i f ier circuit .
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ME8843 Rectifier circuit: Polyphase
When poly-phase AC is rectified, phase-shifted pulses
overlap each other to produce DC output that is muchsmoother
Has less AC content than that produced by rectification of single-
phase AC.
This is decided advantage in high-power rectifier circuits, where
sheer physical size of filtering components would be prohibitivebut low-noise DC power must be obtained.
The Figure in next slide shows full-wave rectification of three-
phase AC.
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ME8843 Rectifier circuit: Poly-phase
Three-phase AC and 3-phase ful l-wave rect i f ier o utp ut.
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ME8843 Rectifier circuit
In any case of rectification -- single-phaseor polyphase -- amount of AC voltage
mixed with rectifier's DC output is called
ripple voltage.
In most cases, since pure DC is desired
goal, ripple voltage is undesirable.
If power levels are not too great, filtering
networks may be employed to reduce
amount of ripple in output voltage.
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ME8843
Output ripple will always be present in circuits shown above
Amplitude of ripple can be reduced by adding smoothing capacitor
Capacitor and load (shown here as resistor) from low pass filter withtime constant T= RC
Time constant should be much longer than one ripple
For given ripple amplitude capacitor size (in microfarads) given by
f: line frequency
Iload: Load Current
Vrip: Amplitude of ripple voltage
NOTE: Voltage rating of the capacitor must be > 1.4*Voutand large
capacitors should have bleeder resistors for safety!
Output Ripple
C IloadfVrip
10 6 (Halfwave) or C Iload2fVrip
10 6 (Fullwave)
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ME8843 Rectifier circuit
Sometimes, method of rectification is referred to bycounting number of DC pulses output for every 360oofelectrical rotation.
A single-phase, half-wave rectifier circuit, then, would becalled 1-pulserectifier, because it produces single pulseduring time of one complete cycle (360o) of AC
waveform. A single-phase, full-wave rectifier (regardless of design,
center-tap or bridge) would be called 2-pulserectifier,because it outputs two pulses of DC during one ACcycle's worth of time.
A 3-phase full-wave rectifier would be called 6-pulseunit.
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ME8843 Rectifier Circuit: Output Voltage
Full wave rectification will produce voltage roughly equal
to
In practice, there will be small voltage drop across
diodes that will reduce this voltage
For accurate supplies, regulation is necessary
Vo 2Vi,RMS
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ME8843 Rectifier circuit
REVIEW:
Rectificationis conversion of alternating current (AC) to
direct current (DC). A half-waverectifier is circuit that allows only one half-
cycle of AC voltage waveform to be applied to load,resulting in one non-alternating polarity across it. The resulting DC delivered to load pulsates significantly.
A full-waverectifier is circuit that converts both half-cycles of AC voltage waveform to unbroken series ofvoltage pulses of same polarity. The resulting DC delivered to load doesn't pulsate as much.
Poly-phase alternating current, when rectified, givesmuch smoother DC waveform (less ripplevoltage) thanrectified single-phase AC.
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ME8843 Applications
DC Power supplies
Used to provide DC power to drive loads Radios
Used to rectify received radio signals as part of AM
demodulation
Signal to be transmitted is multiplied by a carrier wave
Diode in receiver rectifies signal
Carrier Wave
Audio SignalRectified Radio
Wave
Diode
Modulated
Signal Radio Transmission
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ME8843 Applications
Light Dimmer
Sends unrectified or half wave
AC power through light bulb
Automobile Alternators
The output of 3-phase AC
generator is rectified by diode
bridge
More reliable than DC
generator
6 Rectifier Diodes
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ME8843 References
http://en.wikipedia.org/wiki/Rectifier
http://en.wikipedia.org/wiki/Diode_bridge
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
http://my.integritynet.com.au/purdic/power1.html
http://electronics.howstuffworks.com/radio.htm
http://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmTop Related