LASCR
Silicon Unilateral Switch
Bilateral Switch
SCR
Silicon
GTO
Unisaction Transistor
Shockley Diode
DIAC
TRIAC
THYRISTORS
- Four layer devices with a control mechanism
- 3 terminals anode(+), cathode(-) , gate terminal (control)
- They are four layer , PNPN device used to control electric power and current by acting as
electronic switches
- Protections to circuits with large voltages and current up to 6000 V, 4500 A
- Control large amount of power using very small input power
- From THYRaton and transistor
 Silicon Controlled Rectifier (SCR)
 Silicon Controlled Switch (SCS)
 Gate Turn off Switch (GTO)
 Light Activated SCR (LASCR)
 Shockley Diode
 Diode Alternating Current (DIAC)
 Triode Alternating Current (TRIAC)
SILICON CONTROLLED RECTIFIER
- Led by Gordon Hall, commercialized by Frank W. Gutzwilled in 1957
- Made of silicon
- Carry leakage current in the off condition
- Used to control quite large current to a load
- Control power up to 10 MW, with individual rating of 2000 A at 1800 V
-
Up to 50 kHz frequency
4 layer PNPN , three terminal, Anode, cathode, gate
SCR EQUIVALENT CIRCUIT
- Two transistors arranged uses positive feedback
- 2 states: non-conducting and conducting
- Uses latch, remains indefinitely
HOW TO TURN ON SCR
- 2 ways : keep the gate open and supply voltage
equal to breakover and operate the
SCR with supply voltage less than the break over then turn it on with small voltage
applied to the gate
HOW TO TURN Off SCR
- anode current interruption and forced commutation
- Natural commutation- thyristor turn off naturally with ac supplies as the voltage
reverses
- Turn off time: 5 ms to 30 ms
Scr characteristics and rating
Forward breakover voltage VBR(F)- is the voltage above which the SCR enters the conduction
region
Holding current IH - is the value of current below which the SCR switches from the conduction
state to the forward blocking region under stated conditions
Forward and reverse blocking regions - are the regions corresponding to the open-circuit
condition for the controlled rectifier that block the flow of charge (current) from anode to
cathode.
Reverse breakdown voltage VBR(R) - is equivalent to the Zener or avalanche region of the
fundamental two-layer semiconductor diode
Gate trigger current, IGT - value of gate current to switch SCR on.
Forward current rating - It is the maximum anode current that an SCR is capable of passing
without destruction
Circuit fusing rating - indicates the maximum forward surge current capability of SCR. It is the
product of square of forward surge current and the time of duration of the surge
ADVANTAGES OF SCR
As a switch, it has no moving parts. Consequently, it gives noiseless operation at high efficiency.
•the switching speed is very high • higher voltage and current ratings • Small size • needs only
a pulse to make it conducting and thereafter it remains conducting
SCR - HALF WAVE RECTIFIER
• Firing angle is α. At this angle in the positive halfcycle, SCR starts conduction.
• The conduction angle is φ = 180° − α
• An ordinary half-wave rectifier will conduct full positive half-cycle, an SCR half-wave rectifier
can be made to conduct full or part of a positive half cycle by proper adjustment of gate
current. Therefore, an SCR can control power fed to the load and hence the name controlled
rectifier.
APPLICATIONS OF SCR
•SCR Crowbar (Overvoltage Protection) •Variable Resistance Phase Control •Battery Charging
Regulator •Temperature Controller •Emergency Lighting System • Inverter
SHOCKLEY DIODE
- William Shockley
- Four layer diode

same as SCR
Same with scr but it has NO GATE TERMNINAL

And the only way to open it is by low current drop out. Current should be reduced below the
holding current
a trigger switch for an SCR.
STOPS when it is reversed biased, CONDUCTS when forward biased
-
COMMON BREAKOVER VOLTAGE IS 60V
Because there are no trigger inputs, the only way to switch the device on is to increase the
anode-to-cathode voltage VAK to the forward breakover voltage.
LASCR
- is the light sensitive equivalent of the normal SCR.
- 3 terminals
• It is an SCR whose state is controlled by the light falling on a silicon semiconductor layer of the
device.
• The LASCR is most sensitive to light when the gate terminal is open( no presence of gate) . Its
sensitivity can be reduced and controlled somewhat by the insertion of a gate resistor
• A gate lead is also provided to permit triggering the device using typical SCR methods.
•It may be triggered with a light source or with a gate signal. Sometimes a combination of both
light source and gate signal is used to trigger an SCR.
• The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate.
Higher the voltage(current) bias, lower the light intensity required
•Note that an increase in junction temperature results in a reduction in light energy required to
activate the device.
• will continue to conduct even if the light source is removed
. • The only way to open it is by low current drop out. Current should be reduced below the holding
current.
• The maximum current (rms) and power (gate) ratings for LASCRs commercially available are about
3 A and 0.1 W
LASCR APPLICATIONS
•HVDC transmission and VAR Compesation •Used in Alarm Circuits • Phase Control by light signal •
Motor Control by light signal
silicon-controlled switch (SCS)
- Like SCR but with addition of another gate ( anode gate)
-
low power device compared with the SCR. It handles currents in milli-amperes rather than
amperes.
Since only small currents are involved, the SCS may be switched off by an appropriate polarity
pulse at one of the gates
-
In general, the triggering (turn-on) anode gate current is larger in magnitude than the required
cathode gate current.
For one representative SCS device, the triggering anode gate current is 1.5 mA, whereas the
required cathode gate current is 1 mA.
An advantage of SCS over an SCR is the reduced turn-off time, typically within the range of 1 to
10 micro seconds for the SCS
Other advantages is increased control and triggering sensitivity
More predictable firing situation (FOR RECTIFIERS ONLY)
DISADVANTAGE:
Limited to low power, current and voltage rating
The device is used in lamp drivers, power-switching circuits, and logic circuits as well as in
essentially any circuit that requires a switch that can be turned ON and OFF by two separate
control pulses.
Gate turn-off switch (GTO)
- is a four layer. three junction semiconductor device with three external terminals
- the transistor equivalent is exactly the same and the characteristics are similar to SCR `
- The most obvious advantage of the GTO over the SCR or SCS is the fact that it can be turned on
or off by applying the proper pulse to the cathode gate
- The device is turned on by a positive gate current and it is turned off by a negative gate
cathode voltage.
-
The turn-off current of a GTO is slightly larger than the required triggering current.
The maximum rms current and dissipation ratings of GTOs manufactured today are limited to
about 3 A and 20 W, respectively.
The majority of the SCR turn-off circuits can also be used for GTOs.
GTO has improved switching characteristics.
- The turn-on time is similar to that of the SCR (typically 1 micro second), but the turn-off time of
about the same duration (1 micro second) is much smaller than the typical turn-off time of an
SCR (5 ms to 30 ms).
- The fact that the turn-off time is similar to the turn-on time rather than considerably larger
permits the use of this device in high-speed applications.
-
Applications include counters, multivibrators, pulse and sawtooth generators.
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FORMULAS
Scr characteristics and ratings
BREAKBACK VOLTAGE
(
(
)
(
( ))
= breakback voltage
= forward breakover voltage
( ) = anode to cathode voltage
SCR - HALF WAVE RECTIFIER
conduction angle is φ = 180° − α
-