A silicon-controlled rectifier or semiconductor-controlled rectifier is a four-layer solid-state current-controlling device. Some sources define silicon-controlled rectifiers and thyristors as synonymous,[5] other sources define silicon-controlled rectifiers as a proper subset of the set of thyristors. SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control.

1. SILICON CONTROLLED
RECTIFIER(SCR)
Submitted To – Prof. (Dr.) Neelam Srivastava Submitted By – Diksha Tripathi
B.Sc(Hons) Physics,3rd yea
Exam Roll No.-17229PHY

2. CONTENTS
• Introduction
• What is SCR
• SCR Construction
• Working of SCR
• Characteristic curve
• Importance of SCR
• Applications of SCR
• Disadvantages of SCR
• Conclusion

3. • Introduction
• The silicon controlled rectifier (abbreviated as SCR) is a three-
terminal semiconductor switching device which is probably the
most important circuit element after the diode and the
transistor. Invented in 1957.
• An SCR can be used as a controlled switch to perform various
functions
such as rectification, inversion and regulation of power flow.
• The SCR has assumed paramount importance in electronics
because it can be produced in versions to handle currents upto
several thousand amperes and voltages upto more than 1 kV.
• The SCR has appeared in the market under different names
such as Thyristor, Thyrode transistor.
• It is a unidirectional power switch and is being extensively
used in switching D.C. and A.C., rectifying A.C. to give
controlled D.C. output, converting D.C. into A.C. etc

4. • What is SCR?
A SCR is a three terminal, four
layer solid state device which
controls current flow.
 It’s terminals are:
1. Gate
2.Cathode
3.Anode
(Image source: daenotes.com)

5. • SCR Construction
 SCRs are constructed from silicon and can be
used as controlled switch That’s why it is called
“Silicon Controlled Rectifier”.
 It is a four-layered semiconductor device that
forms NPNP or PNPN structure, which eventually
forms three junctions J1, J2, and J3. Among the
three terminals of the SCR, the Anode is a
positive electrode, it will be on the P-layer and
Cathode is a negative electrode, it will be on the
N-layer of the SCR, the Gate acts as a control
terminal of the SCR.
 The outer P and N layers where the two
electrodes are placed will be heavily doped and
the middle P and N layers will be lightly doped,
the gate terminal will be connected to the P-
layer in the middle.
(Image Source:
learnaboutelectronics.com)

6. • Working of SCR
 The anode terminal of SCR is always kept at positive potential w.r.t. cath
 In a silicon controlled rectifier, load is connected in series with anode.
Depending on the biasing given to the SCR, the operation
of SCR is divided into three modes. They are
1. Forward blocking Mode
2. Forward Conduction Mode and
3. Reverse Blocking Mode

7. 1.Forward Blocking Mode
In this mode of operation, the Silicon
Controlled Rectifier is connected such that the
anode terminal is made positive with respect to
cathode while the gate terminal kept open. In
this state junctions J1 and J3 are forward
biased and the junction J2 reverse biased.
Due to this, a small leakage current flows
through the SCR. Until the voltage applied
across the SCR is more than the break over
voltage of it, SCR offers a very high resistance
to the current flow. Therefore, the SCR acts as a
open switch in this mode by blocking forward
current flowing through the SCR as shown in
the VI characteristics curve of the SCR.

8. 2.Forward Conduction Mode
In this mode, SCR comes into the conduction
mode from blocking mode. It can be done in
two ways as either by applying positive pulse to
gate terminal or by increasing the forward
voltage (or voltage across the anode and
cathode) beyond the break over voltage of the
SCR.
Once any one of these methods is applied, the
avalanche breakdown occurs at junction J2.
Therefore the SCR turns into conduction mode
and acts as a closed switch thereby current
starts flowing through it.
In this mode, maximum current flows through
the SCR and its value depends on the load

9. 3. Reverse Blocking Mode
In the reverse blocking mode, the positive
voltage is applied to the Cathode (-) and the
Negative voltage is given to the Anode (+),
There will not be any pulse given to the gate, it
will be kept as an open circuit.
During this mode of operation the Junctions J1
and J3 will be reverse biased and the junction
J2 will be forward biased. Since the junctions J1
and J3 are reverse biased there will not be any
current flowing through the SCR.
Although there will be a small leakage current
flowing due to the drift charge carriers in the
forward-biased Junction J2, it is not enough to
turn on the SCR.

10. • VI Characteristics of SCR
The VI characteristics of the SCR are obtained by operating the SCR in three
different regions, namely forward blocking region, forward conduction region
and reverse blocking region.
(Image Source : Google)

11. • Importance of SCR
The SCR (silicon controlled rectifier) can handle
large voltage, current and power.
The Triggering circuit for silicon controlled
rectifier (SCR) is simple.
Easy to turn ON.
It can be protected with the help of fuse.
It is able to control AC power.
Simple to control.
Cost is low.
The switching speed is very high as it can
perform 1 nano operations per second.

12. • Applications of SCR
 Silicon Controlled Rectifiers are used as Switch in electronic circuits.
 There is a huge application of Silicon Controlled Rectifier in Inverter
Circuits.
 In controlled Rectifier Circuits SCRs are used.
 SCRs are used in AC Voltage Stabilizer circuits.
 SCRs are also used in Battery Charger circuits.
 SCRs are used to control electrical power.
 SCRs are used for High voltage protection purpose.
 SCRs are used in power regulator devices.

13.  The SCR (silicon controlled rectifier) is
unidirectional devices, so it can control power
only in DC power during positive half cycle of AC
supply, thus only DC power is controlled with the
help of SCR.
 The gate current cannot be negative.
 In AC circuit, it needs to be turned on each cycle.
 It cannot be used at higher frequencies.
• Disadvantages

14. • Conclusion
 An SCR has two states i.e. either it does not conduct or it
conducts heavily. There is no state inbetween. Therefore, SCR
behaves like a switch.
 There are two ways to turn on the SCR. The first method is to
keep the gate open and make the supply voltage equal to the
breakover voltage. The second method is to operate SCR with
supply voltage less than breakover voltage and then turn it on
by means of a small voltage applied to the gate.
 Applying small positive voltage to the gate is the normal way to
close an SCR because the breakover voltage is usually much
greater than supply voltage.
 To open the SCR e. to make it non-conducting, reduce the
supply voltage to zero.

15. Thank You!