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.
2
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
3
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.
4
5. 5
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.
10. 10
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
11. 11
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?
12. 12
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.
13
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.
14
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.
16
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.
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)
20. 20
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.
21. 21
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.
22. 22
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.
23. 23
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.
24. 24
Applications of the Light Activated SCR
Low Power Applications
Motor Control
Computer Applications
Optical light Controls
Solid State Relay
25. 25
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.
26. 26
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.
27. 27
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.
28. 28
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)
29
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
30. 30
UJT symbol and biasing.
The higher the biasing voltage, the
higher the trigger voltage required to
cause the device to conduct.
31. 31
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
34. 34
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.
39. 39
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
40. 40
1. Photoresistor – it is a light sensor
whose resistance varies inversely with
the light intensity
41. 41
2. Photodiode – is a semiconductor
device that converts light into current. The
current is generated when photons are
absorbed in the photodiode. .
42. 42
3. Phototransistor – phototransistor is a three-
terminal photodetector whose collector current is
controlled by the intensity of the light at its optical
input
43. 43
4. Light Activated SCR (LASCR) – a thyristor
which can be triggered into conduction by
the reaction of photons to the semiconductor
material.
45. 45
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
48. 48
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.
60. 60
Typical LED Characteristics
Semiconductor
Material
Wavelength Colour VF @ 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
61. 61
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.
#2:Thyristors are four-layer pnpn power semiconductor devices. These devices switch between conducting
and nonconducting states in response to a control signal. Thyristors are used in timing circuits, AC motor
speed control, light dimmers, and switching circuits. Small thyristors are also used as pulse sources for
large thyristors
#4:Silicon controlled rectifiers (or thyristors) are three terminal devices which can be used for switching
and a.c. power control. Silicon controlled rectifiers can switch very rapidly from conducting to a nonconducting
state. In the off state, the silicon controlled rectifier exhibits negligible leakage current, while in
the on state the device exhibits very low resistance.
In this section, we consider five: a static switch, a phase-control system, a
battery charger, a temperature controller, and a single-source emergency-lighting system.
#5:(i.e. it is latched in the on state)
In d.c. applications this necessitates the interruption (or disconnection) of the
supply before the device can be reset into its nonconducting state.
control is applied by means of a gate terminal.
#9:Besides reducing VCC, other methods can be used to reset the SCR. Two
common methods are current interruption and forced commutation.
#14:the DIAC, Diode AC bi-directional switch is widely used with TRIACs to improve operation of alternating current power switching systems
The DIAC is widely used to assist even triggering of a TRIAC when used in AC switches. DIACs are mainly used in dimmer applications and also in starter circuits for florescent lamps.
#15:The diode remains in its conduction state until the current through it drops below what is termed the holding current, which is normally designated by the letters IH.
Below the holding current, the DIAC reverts to its high-resistance (non-conducting) state.
#16:However TRIACs are still used for many electrical switching applications:
Domestic light dimmers Electric fan speed controls
Small motor controls
Control of small AC powered domestic appliance
The triac of maximum rating of 16 kw is available in the market.
#17:This makes the Triac equivalent to two SCRs which are in parallel but are connected in opposite directions. (as shown in the figure)
The TRIAC is an electronic component that is widely used in many circuit applications, ranging from light dimmers through to various forms of AC control. It is generally only used for lower power applications, thyristors generally being used for the high power switching circuits. . .
#23:The best thing about Light Activated SCR is that they do not turn off even when the supply of external light is ceased. If you want to turn off the SCR, then you need to reverse the properties of electrodes.
#24:The optical light control use the principle of photoconduction for generating the control signals. Therefore, the LASCR finds extensive application in Optical light control.
In solid state relays, two LASCR are connected in reverse parallel so that they can generate power in both the half cycle of AC.
#25:A gate turn-off thyristor (GTO) is a special type of thyristor, which is a high-power semiconductor device.
These are capable not only to turn ON the main current with a gate drive circuit, but also to turn it OFF. A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off.
#26:The turn ON operation of GTO is similar to a conventional thyristor.
#29:Emergency flasher
The unijunction transistor (UJT) is a three-terminal device whose trigger voltage is proportional to its applied bias voltage.
#33:These are electronic devices which use the principle of light(optics) in the operation.
Convert the flow of electric current to light
Convert light into electric current flow
The usage of both light sensor and light emitter
#34:The first of these, the light−emitting diode (LED), was developed to replace the fragile, short−life incandescent light
bulbs used to indicate on/off conditions on instrument panels.
A light−emitting diode, when forward biased, produces visible light. The light may be red, green, or amber, depending upon the
material used to make the diode.
Laser diodes are LEDs specifically designed to produce coherent light with a narrow bandwidth and
are suitable for CD players and optical communications
#35:LEDs are also very much more reliable than filament lamps. Most LEDs will provide
a reasonable level of light output when a forward current of between 5mA and 20mA is applied.
The LED is designated by a standard diode symbol with two arrows pointing away from the cathode as shown in Figure 2.81 where the
arrows indicate light leaving the diode.
#40:Has high resistance when dark
Resistance decreases as light intensity increases offering the current to flow.
Compose of lead sulphide or cadmium sulphide
#41:Basically, the photodiode is a light−controlled variable resistor. In total darkness, it has a relatively high resistance
and therefore conducts little current.
Photodiodes respond quickly to changes in
light intensity, and for this reason are extremely useful in digital applications such as photographic
light meters and optical scanning equipment.
Silicon photodiodes are utilized in such diverse applications as spectroscopy, photography, analytical instrumentation, optical position sensors, beam alignment, surface characterization, laser range finders, optical communications, and medical imaging instruments.
#42:it is a light sensitive transistor with no base load
A phototransistor is another optoelectronic device that conducts current when exposed to light.
Phototransistors are similar to photoresistors but produce both current and voltage, while photoresistors only produce current.
#43:he LASCRs find many applications including optical light controls, relays, phase control, motor control and a large number of computer applications.
#44:Electric devices that connect two circuits by optical means.
Optocouplers and Opto-isolators are great electronic devices that allow devices such as power transistors and triacs to be controlled from a PC’s output port, digital switch or from a low voltage data signal such as that from a logic gate. The main advantage of opto-couplers is their high electrical isolation between the input and output terminals allowing relatively small digital signals to control much large AC voltages, currents and power.
#51:UJT relaxation oscillator having loudspeaker load produces single click each time flash of light falls on light-activated SCR Setting of R1 determines whether circuit produces series of pulses or tone burst during time light is on Oscillator frequency Increases with light intensity
#52:The battery is being charged through R1 and D1 at a rate determined by R1.
Charging will only take place when the anode of D1 is more positive than its cathode.
The dc level of the full-wave-rectified signal will ensure that the bulb is lit when
the power is on. If the power should fail, the capacitor C1 will discharge through D1,
R1, and R3 until the cathode of SCR1 is less positive than the anode. At the same time,
the junction of R2 and R3 will become positive and establish sufficient gate-to-cathode
voltage to trigger the SCR. Once fired, the 6-V battery would discharge through
the SCR1 and energize the lamp and maintain its illumination. Once power is restored,
the capacitor C1 will recharge and re-establish the nonconducting state of SCR1 as
described above.
#54:Electronics a complete course
A photoresistor (or light-dependent resistor, LDR, or photocell) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity.
#55:Relaxation Oscillator
When on/off switch is close, capacitor c1 charges by R1. When voltage across C1 reaches the UJTs peak value, the UJT will turn ON and its resistance between E and B1 will drop low. This low resistance will allow C1 to discharge through the UJTs E to B1 junction and into the flasher bulb causing it to momentarily flash. As C1 discharges, its voltage decreases and this cause the UJT to turn OFF. The cycle then repeats since the off UJT will allow C1 to begin charging towards Vpeak.
#56:The most common type of LDR has a resistance that falls with an increase in the light intensity falling upon the device (as shown in the image above).
#57:Battery charger
As charging continues, the battery voltage rises to a point where VR is sufficiently
high to both turn on the 11.0-V Zener and fire SCR2. Once SCR2 has fired, the shortcircuit
representation for SCR2 will result in a voltage-divider circuit determined by
R1 and R2 that will maintain V2 at a level too small to turn SCR1 on. When this occurs,
the battery is fully charged and the open-circuit state of SCR1 will cut off the
charging current. Thus the regulator recharges the battery whenever the voltage drops
and prevents overcharging when fully charged
#63:A photoresistor (or light-dependent resistor, LDR, or photocell) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity.
#64:When on/off switch is close, capacitor c1 charges by R1. When voltage across C1 reaches the UJTs peak value, the UJT will turn ON and its resistance between E and B1 will drop low. This low resistance will allow C1 to discharge through the UJTs E to B1 junction and into the flasher bulb causing it to momentarily flash. As C1 discharges, its voltage decreases and this cause the UJT to turn OFF. The cycle then repeats since the off UJT will allow C1 to begin charging towards Vpeak.
