The silicon controlled rectifier (SCR), also known as a thyristor, is a three-terminal semiconductor device used for controlling high power and converting AC current to DC. It operates in three modes: forward blocking, forward conducting, and reverse blocking, with each mode varying based on the voltage and gate conditions. SCRs are advantageous for their ability to handle large voltages and currents, but they have limitations such as conducting only in one direction and needing to be turned on each cycle in AC circuits.

1. • Silicon Controlled Rectifier (SCR)
The Silicon Controlled Rectifier (SCR), also known as a Thyristor, is a fundamental
component in power electronics.
SCR is a three-terminal and four-layer unidirectional current-
controlling semiconductor device. It is made up of silicon materials and is
mainly used for controlling high power and conversion of high AC current
into DC current. Hence the named silicon-controlled rectifier.
INTRODUCTION

2. Silicon Controlled Rectifier (SCR) Symbol:
• A Silicon Controlled Rectifier (SCR) consists of three terminals
namely Anode (A), Cathode (K), and Gate (G) terminal. The device
can be turned ON or OFF by controlling the gate input or biasing
condition of SCR. The schematic symbol of SCR is shown in the
figure below. The arrow of the diode represents the direction of
conventional current flow.

3. The Silicon Controlled Rectifier (SCR) is a powerful switch for controlling high
voltage and current. Its construction involves a unique arrangement of
semiconductor materials to achieve its functionality. Here's a breakdown of its
key aspects:
Structure:
• Four Layers: An SCR is made of four alternating layers of P-type and N-type
semiconductor material, forming a PNPN structure.
Construction of Silicon Controlled
Rectifier (SCR)

4. • Three Junctions: These four layers create three junctions (J1, J2, and J3)
between the different regions.
• Three Terminals: The SCR has three terminals:
• Anode (A): Connected to the outer P-type layer.
• Cathode (K): Connected to the outer N-type layer.
• Gate (G): Connected to a thin layer of P-type material within the structure, providing
control over conduction.

5. STRUCTURE OF SCR

6. The three terminals Anode (A), Cathode (K), and Gate (G) are
arranged in such a way that Gate (G) terminal is attached to the p-
type layer nearer to the Cathode (K) terminal in the PNPN structure.
Anode (A) is a positively charged electrode and the conventional
current enters into the device through this terminal. Cathode (K) is
a negatively charged electrode and conventional current leaves the
device through this terminal. Gate (G) is a control terminal that
controls the flow of current between Anode (A) and Cathode (K).

7. From the structure of SCR, a single SCR is a combination of
one PNP transistor (TR1) and one NPN transistor (TR2). The
emitter of TR1 acts as the Anode terminal and the emitter of TR2 acts
as the cathode terminal of SCR. Further, the base of TR1 is connected
to the collector of TR2 and the collector of TR1 is connected to the
base of TR2.
The Gate terminal of SCR is connected to the base of TR2. This
analogy of SCR as a combination of two transistors is
called two transistor model.
The structure of SCR as two transistor
model

8. The structure of SCR as two transistor
model

9. Depending on the biasing given to SCR, there are three modes of
operation. They are
• Forward Blocking Mode (OFF State)
• Forward Conducting Mode (ON State)
• Reverse Blocking Mode (OFF State)
Modes of operation in Silicon
Controlled Rectifier (SCR):

10. 1. Forward Blocking Mode (OFF State)

11. • Under this condition, junctions J1 and J3 are forward biased
whereas junction J2 is in reverse biased condition. The depletion
region at junction J2 blocks the flow of current from junction J1 to
junction J3 as it acts obstacle or wall between them. However, a
small amount of leakage current flows between these junctions
J2 and J3.
1. Forward Blocking Mode (OFF State)

12. • When the applied voltage across the SCR reaches a breakdown
voltage, the avalanche breakdown occurs due to high energy
minority carriers. The current starts flowing through the SCR at this
breakdown voltage and there is no current flow below the
breakdown voltage because SCR offers very high resistance to the
current below the breakdown voltage and acts as an open
switch by blocking the forward current. Hence it will be in an OFF
state.
1. Forward Blocking Mode (OFF State)

13. 2. Forward Conducting Mode (ON State):
In this mode of operation, the SCR comes into the conduction
mode from blocking mode. It can be done in two ways, i.e. either
by increasing the forward bias voltage (voltage across Anode and
Cathode) beyond the breakdown voltage or by applying positive
pulse or voltage at the Gate terminal. The biasing of SCR in this mode
is shown in the figure below.

14. 2. Forward Conducting Mode (ON State):

15. • In the first case, the forward bias voltage applied between Anode and Cathode is increased
beyond the breakdown voltage, the depletion region breakdown occurs at J2, and the
current starts flowing through SCR. In this condition, the SCR will be in an ON state.
• In the second case, a small positive pulse or voltage VG is applied to the Gate terminal of
SCR as shown in the figure above. When the gate voltage is applied to the gate terminal,
the reverse biased junction J2 in forward blocking mode will become forward biased, and
the depletion region width becomes very narrow. In this condition, a small forward bias
voltage between Anode and Cathode can easily penetrate this narrow depletion region.
Therefore on applying a small forward bias voltage, an electric current starts flowing
through the SCR and it will be in an ON state.
2. Forward Conducting Mode (ON State):

16. 3. Reverse Blocking Mode (OFF State):
• In this mode of operation, a positive voltage (+) is given to
Cathode (K) terminal, and a negative voltage (-) is given to Anode
(A), Gate (G) terminal is an open circuit as shown in the figure
below.

17. • Under this condition, junctions J1 and J3 are reverse
biased whereas junction J2 is in forward biased condition. As
junctions J1 and J3 are reverse-biased, there is no current
flow through the SCR. But due to the drift of the charge carrier in a
forward-biased junction J2, there is small leakage current flow in
SCR which is not sufficient to turn ON the device. Hence the SCR
will be in an OFF state and acts as an open switch.
3. Reverse Blocking Mode (OFF State):

18. V-I Characteristics of Silicon Controlled
Rectifier (SCR):
• In this V-I characteristic, the horizontal line represents the amount
of voltage applied VA across the SCR and the vertical line represents
the amount of current flow IA in the SCR. The V-I characteristics of
SCR are shown in the figure below.

19. V-I Characteristics of Silicon Controlled
Rectifier (SCR):

20. • The V-I characteristics of SCR are divided into three regions.
1. Forward Blocking Region
2. Forward Conduction Region
3. Reverse Blocking Region
V-I Characteristics of Silicon Controlled
Rectifier (SCR):

21. 1. Forward Blocking Region
• The region OA in V-I characteristics is called the forward blocking
region. This region represents the forward-blocking mode of SCR
operation. In this region, the forward bias voltage is given to SCR where
positive voltage is given to Anode, the negative is given to SCR and
Gate is open-circuited. In this condition, the junctions J1 and J3 become
forward biased whereas junction J2 becomes reverse biased. A small
leakage current flows from the Anode terminal to the Cathode terminal
of SCR which is known as a forward leakage current. The SCR does not
conduct electric current and the device is in an OFF state in this region.

22. 2. Forward Conduction Region
• The region BC in V-I characteristics is called the forward conduction
region. This region represents the forward conduction mode of SCR
operation. In this region, the current flowing from Anode to
Cathode increases rapidly. When the forward bias voltage applied
between Anode and Cathode is increased beyond the breakdown
voltage, the depletion region breakdown occurs at junction J2 and
the current starts flowing through the SCR and it will be in the ON
state.

23. 2. Forward Conduction Region
• The current flow in this region increases
rapidly after junction J2 breakdown occurs.
The voltage at which the junction
breakdown occurs when the Gate is open is
known as forward breakdown voltage (VBF)
• The region AB in V-I characteristics indicates
that as soon as the SCR becomes ON, the
voltage across the SCR drops to some volts.

24. 3. Reverse Blocking Region
• The region OE in the V-I characteristics is called the reverse
blocking region. This region represents the reverse blocking mode
of SCR operation. In this region, the reverse bias voltage is applied
to SCR where a positive voltage is given to Cathode, a
negative voltage is given to Anode, and the Gate terminal is open-
circuited. In this condition, junctions J1 and J3 are reverse biased
whereas the junction is forward biased. As junction J1 and J3 are in
reverse biased condition, there is no current flow through SCR.

25. But due to the drift of the charge carrier in forward-biased junction J2,
there is small leakage current flow in SCR which is not enough to turn
ON the device. Hence the SCR will be in an OFF state in this region.
• When the reverse bias voltage between Anode and Cathode is
increased beyond the reverse breakdown voltage VBR, an avalanche
breakdown occurs, and the current increases rapidly. The region EF
in V-I characteristics is known as the reverse avalanche region.
3. Reverse Blocking Region

26. Advantages of SCR:
The followings are some advantages of SCR.
• It can handle large voltage, current, and power.
• The voltage drop across conducting SCR is small which will reduce the power dissipation.
• Triggering circuits are simple.
• Easy to turn ON.
• It has a higher switching speed.
• It can be protected with the help of a fuse.
• It is simple to control.

27. Disadvantages of SCR:
The followings are the disadvantages of SCR.
• It can conduct only in one direction. So power control can be done only
during half cycle of ac.
• The gate current cannot be negative
• It cannot be used at high frequency as it can be operated at a maximum
frequency of 400 HZ.
• It needs to be turned on each cycle in ac circuits.

28. Applications of SCR:
The main application of SCR is switching and power control. The
followings are some applications that use switching and power control
properties of SCR.
• It is used as a switch
• It is used b in AC voltage stabilizers
• It is used in choppers (DC to Dc converters)
• It is used for inverters (DC to AC converters)
• It is used in battery charger

29. • It is used for power control circuits
• It is used in DC circuit breaker
• It is used for AC power control with a solid relay
• It is used to control motors speed
• It is used to adjust the light dimmer
• It is used in fan speed regulators.
Applications of SCR: