PP-06 Thyristors includes SCR,DIAC,TRIAC 2022.ppt

electrical

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2. THYRISTOR
A thyristor is a four-layer solid-state
semiconductor device that contains 3 PN
junctions in series having 3 terminals
called anode, cathode and gate.
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3. Kinds of Thyristor
1.SCR – Silicon Controlled Rectifier
2.TRIAC – Triode for Alternating Current
3.LASCR – Light Activated SCR
4.GTO – Gate Turn Off Thyristor
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BCT – bidirectional control thyristor
MTO – MOS Turn Off
MCT – MOS controlled
SITH – Static Induction Thyristor
ETO – Emitter turn off
IGCT – integrated Gate commutated thyristor
GATT – Gate Assisted turn off

4. Silicon Controlled Rectifiers (SCRs)
A gate triggered, three terminal thyristor
that switches only for positive anode-to-
cathode voltages and exhibits reverse
blocking state for negative anode-to-
cathode voltages.
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Once switched into the conducting state, the
silicon controlled rectifier will remain
conducting until the forward current is
removed from the device
Like their conventional silicon diode
counterparts,
silicon controlled rectifiers have anode and
cathode connections;
In normal use, a silicon controlled rectifier
(SCR) is triggered into the conducting (on) state
by means of the application of a current pulse
to the gate terminal.

6. SCR construction and schematic symbol.
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Two Transistor Analogy

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SCR I-V Characteristics Curves

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Resetting the SCR

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A typical silicon controlled rectifier for mains
switching applications will require a gate
trigger pulse of about 30mA at 2.5V to
control a current of up to 5A

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In Fig. below, the SCR has a trigger voltage of
0.75 V and a trigger current of 7 mA. What is the
input voltage that turns the SCR on? If the holding
current is 6 mA, what is the supply voltage that
turns it off?

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the minimum input voltage needed to trigger
the SCR is:
the supply voltage that turns off the SCR is:

13. DIACS AND TRIACS
Diacs and triacs are classified
as
 
bidirectional thyristors, which
means that they are capable of
conducting in
 
two directions.
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14. A DIAC is a full-wave or bi-directional
semiconductor switch that can be
turned on in both forward and reverse
polarities.
DIAC construction and schematic symbol.
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15. Bidirectional Switch
The Diac remains in an off state (conducts only a small leakage
current) in either direction until the applied voltage in either direction is
high enough to cause its respective reverse-biased junction to break
down. When this happens, the device turns on and current suddenly
rises to a value which is essentially limited by the resistance in series
with the device.
The Diac therefore functions as a bidirectional switch which will turn on
whenever its breakdown voltage (in either direction) is exceeded.

16. The
 
triac 
a gate triggered, three terminal
thyristor that switches for either positive
anode-to-cathode voltages or negative anode-
to-cathode voltages
Triac schematic
and construction.
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17. Bidirectional Triode (TRIAC)
In applications where it is
necessary to achieve full
control of an ac signal, it is
often much easier to use a
device known as a bidirectional
triode Thyristor.
This device is more commonly
referred to as a TRIAC.
The Triac has basically the
same switching characteristics
as an SCR, however it exhibits
these same characteristics in
both directions.

18. I-V Characteristics of a bidirectional triode (TRIAC)

19. Triggering devices
The primary difference
Triac is triggered into conduction
on both the positive and negative
alternations of each ac input
cycle, while
SCR conducts only on positive
alternations.
Note:
A special triggering device is
generally used to insure that the
Triac turns on at the proper time.
( see figures on the right)

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AC Light Dimmer
This is an AC light dimmer formed by a diac, a triac and some passive
components. The capacitor is charging through the two resistors and
when the voltage on one end of the diac exceeds the breakdown voltage
it goes ON and sends a current to the gate of triac putting the triac to ON
state and thus powering the lamp. After the capacitor is discharged to a
voltage below the breakdown voltage of diac, the diac, triac and lamp
turn off. Then the capacitor is charged again and so on. So the lamp is
only powered for a fraction of time during the full sinewave. This happens
very quickly and the lamp seems dimmed. Brightness is adjusted using
the potentiometer.

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Light-Activated Silicon-Controlled Rectifier
(LASCR)
LASCR 
is triggered by using a 
light source 
such
as
 
LED 
etc. The light (photon) particles upon hitting the
junction produce electron-hole pairs, which triggers the
current flow throw the device.

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Construction of LASCR
The LASCR is made up of
 
silicon material, and the glass
lens in the LASCR is used to focus the light from the
external source on the semiconductor material. The silicon
pellet is used in the bottom of the device, and the light
intensity dislodges electrons in the semiconductor crystal
and contributes to conduction.

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Working of LASCR
The
 LASCR works on the principle of photoconduction that
is conduction due to photon striking the semiconductor
surface. The light rays falling on the device are focused at
one place to intensify it.
The more the intensity of light, the more will be the current
through the LASCR. The internal architecture of LASCR
consists of two transistors in such a way that the collector
of one transistor is connected to the base of another
transistor.

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Applications of the Light Activated SCR
Low Power Applications
Motor Control
Computer Applications
Optical light Controls
Solid State Relay

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Gate Turn Off Thyristor
It is a three terminal, bipolar (current controlled minority
carrier) semiconductor switching device. Similar to
conventional thyristor, the terminals are anode, cathode
and gate. As the name indicates, it has gate turn off
capability.

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Principle of Operation
When the anode terminal is made positive with respect to
cathode by applying a positive gate current, the hole current
injection from gate forward bias the cathode p-base
junction.
This results in the emission of electrons from the cathode
towards the anode terminal. This induces the hole injection
from the anode terminal into the base region. This injection
of holes and electrons continuous till the GTO comes into
the conduction state.

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To turn OFF a conducting GTO, a reverse bias is
applied at the gate by making the gate negative with
respect to cathode. A part of the holes from the P base
layer is extracted through the gate which suppress the
injection of electrons from the cathode.
In response to this, more hole current is extracted
through the gate results more suppression of electrons
from the cathode. Eventually, the voltage drop across
the p base junction causes to reverse bias the gate
cathode junction and hence the GTO is turned OFF.

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Gate Turn-Off Thyristor Applications
It is used as a main control device in choppers and
inverters. Some of these applications are;
AC drives
DC drives or DC choppers
AC stabilizing power supplies
DC circuit breakers
Induction heating
And other low power applications

29. Unijunction Transistors (UJTs)
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The UJT is a three-terminal, semiconductor
device which exhibits negative resistance and
switching characteristics for use as a relaxation
oscillator in phase control applications

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UJT symbol and biasing.
The higher the biasing voltage, the
higher the trigger voltage required to
cause the device to conduct.

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Unijunction Transistor Applications
The most common application of a unijunction transistor is
as a triggering device for
 
SCR’s 
and 
Triacs 
but other UJT
applications include sawtoothed generators, gate pulse,
simple oscillators, phase control, timing circuits and
trigger generator applications to switch and control
either thyristors and triac’s for AC power control type
applications

32. Advantages of a solid state switch or semiconductor
over magnetic switch.
-No mechanical moving parts
-No arcing in contacts
-No contact materials which will wear out in
frequent use
-No induction on control side
-No acoustical noise
-High switching speed
-High reliability
-Resistance to shock and vibration
-No contact which can bounce and stick and
cause intermittent operation
-Long operating life

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OPTOELECTRONIC DEVICES
Optoelectronic devices either produce light or
use light in their operation.
Light Emitters
Light Detectors/Sensor
Optocoupler

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LIGHT EMITTERS
Light emitting diodes (LEDs) can be used as
general purpose indicators and, compared with
conventional filament lamps, operate from
significantly smaller voltages and currents.
Light Emitting Diode (LED) – a semiconductor
device which emits light when forward biased.

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ConstructionElectronic symbol

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LED Application
1.Power indicator
2.Display
Material Used
1.Gallium phosphide
2.Gallium arsenide

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SEVEN-SEGMENT DISPLAY 
is a form of electronic
 
display device 
for
displaying
 
decimal numerals.

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Truth Table

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LIGHT SENSORS/DETECTORS
It is an optoelectronics device which convert
light intensity to current flow.
Photodetectors 
are devices whose electrical
characteristics are controlled by the light they
receive

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1. Photoresistor – it is a light sensor
whose resistance varies inversely with
the light intensity

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2. Photodiode – is a semiconductor
device that converts
 light into current. The
current is generated when photons are
absorbed in the photodiode.
 
.

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3. Phototransistor – phototransistor 
is a three-
terminal photodetector whose collector current is
controlled by the intensity of the light at its
 
optical
input 

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4. Light Activated SCR (LASCR) – a thyristor
which can be triggered into conduction by
the reaction of photons to the semiconductor
material.

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OPTOISOLATOR/OPTOCOUPLER
is a device that uses light to couple a
signal from its input to its output

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This type of optocoupler configuration forms the
basis of a very simple solid state relay application
which can be used to control any AC mains
powered load such as lamps and motors

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end

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Induction heating

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Unijunction Transistor Speed Control

 Using the circuit below, we can control the speed of a
universal series motor (or whichever type of load we
want, heaters, lamps, etc) by regulating the current
flowing through the SCR. To control the motors speed,
simply change the frequency of the sawtooth pulse,
which is achieved by varying the value of the
potentiometer.

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light-activated SCR
 

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Alarm device using a thyristor
and a photo-transistor

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Automatic Night Light using Triac

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Emergency Flasher

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 6.3*10^18
electrons/sec
300 million meters per second.
Testing a Triac

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Typical LED Characteristics
Semiconductor
Material
Wavelength Colour V
F
 
@ 20mA
GaAs 850-940nm Infra-Red 1.2v
GaAsP 630-660nm Red 1.8v
GaAsP 605-620nm Amber 2.0v
GaAsP:N 585-595nm Yellow 2.2v
AlGaP 550-570nm Green 3.5v
SiC 430-505nm Blue 3.6v
GaInN 450nm White 4.0v

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What is Switching Frequency?
The rate at which the DC voltage is switched on and off
during the pulse width modulation process in a switching
power supply.
The switching frequency in an inverter or converter is the
rate at which the switching device is turned on and off.
Typical frequencies range from a few KHz to a few megahertz
(20Khz-2MHz). Increased switching frequency reduces size of
associated components such as the inductors, transformers,
resistors and capacitors in addition to reduced space
requirements on the board and case.

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PLC motor hi low

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