SCR Silicon Controlled Rectifier

2. SCR –Silicon Controlled Rectifier
The SCR has three pn – junctions, and four layer
of p and n type semiconductor joined alternatively
to get pnpn device.
The three terminal are taken – one from outer p –
type layer called anode (A), second from the outer
n – type layer called cathode (K) and the third
from the internal p – type layer called gate (G).

3. CONSTRUCTION
 It is a unidirectional device.

4. WORKING OF SCR
In a silicon controlled
rectifier, load is
connected in series with
anode.
The anode is always
kept at positive
potential w. r. t.
cathode.

5. When Gate is Open Circuited
When no voltage to gate terminal,
junction J2, is reverse biased and the
junctions J1 and J3 are forward biased,
Since one of the three junctions is
reverse biased so there is no current
can flow through the load, hence the
SCR is OFF.

6. However if the applied voltage is gradually
increased, a stage is reached, when reverse
biased junction (J2) breaks down.
The SCR now, starts conducting and become ON.
The value of applied voltage at which the reverse
biased junction breaks down and the SCR
becomes ON is known as breakover voltage.

8. When Gate is Positive with Respect to Cathode
The SCR can be turned ON at smaller applied voltage by the
application of a small positive voltage at the gate terminal .
When gate voltage is applied the junction J3 is forward
biased and junction J2 is reverse biased.
Thus, the electrons from n – type layer starts moving across
the junction J3 toward p – type material and the holes from p
– type material towards the n – type material.

9. Due to the movement of holes and electrons across
the junction J3 the gate current starts flowing, Because
of gate current the anode current increases, the
increased anode current makes the more electrons
available at the junction J2.
As a result of this process, in a small time, the
junction J2 breaks down and the SCR is turn ON.

11. Even if the voltage at the gate
terminal is removed, the SCR the
anode current does not decrease,
the SCR can only be turned off by
reducing the applied voltage to
zero.

12. V – I CHARACTERISTICS OF SCR
It is the curve between
anode – cathode voltage (v)
and anode current (I) of an
SCR at constant gate current.

14. FORWARD CHARACTERSTICS
When anode is positive w. r. t. cathode the curve between V
and I is called the forward characteristics.
If the supply voltage is increased from zero, a point reached
( point A ) when the SCR starts conducting.
Under this condition, the voltage across SCR suddenly drops
by dotted curve AB and most of supply voltage appears
across the load resistance R_L.

15. REVERSE CHARACTERISTICS
When anode is negative w. r. t. cathode, the curve between V and I is
known as reverse characteristics.
If the reverse voltage is increased gradually ,at first the anode current
remains small (called leakage current) and at some reverse voltage,
the avalanche breakdown occurs and the SCR starts conducting in the
reverse direction (Shown by the curve in third quadrant ),The
maximum reverse voltage at which the SCR starts conducting in the
reverse direction is called as reverse breakdown voltage.

16. WORKING CHARCTERISTICS
They are
three modes:
1. Forward
blocking
mode ( OFF
state )
2. Forward
conduction
mode ( ON
state )
3. Reverse
blocking
mode ( OFF
state )

17. FORWARD BLOCKING MODE [ OFF STATE ]
The anode is given a
positive while the
cathode is given a
negative voltage,
keeping the gate at
zero potential i.e.,
disconnected.
In this case junction J1 and J3 are
forward biased while J2 is reversed
biased due to which only a small
leakage current exists from the anode to
the cathode until the applied voltage
reaches its breakover value, at which J2
undergoes avalanche breakdown and at
this breakover voltage it starts
conductivity but below breakover
voltage its offers very high resistance to
the current and is said to be in the off
state.

18. FORWARD CONDUCTING MODE [ ON STATE ]
SCR can be brought from blocking mode to conduction
mode in the two ways: either by increasing the voltage
across anode to cathode beyond breakover voltage or
by applying of positive pulse at gate.
Once it starts conducting no more gate voltage is
required to maintain it in the on state.

19. There are two ways to turn it off :
1.Reduce the current through it below a minimum
value called the holding current.
2. With the gate turned off, short out the anode and
cathode momentarily with a push – button switch or
transistor across the junction.

20. REVERSE BLOCKINF MODE [ OFF STATE ]
SCR is reversed biased i.e., when anode is negative
compared to cathode the characteristic of this region are
similar to these of an ordinary PN junction diode.
J1 and J3 are reverse biased and J2 is forward biased the
device behaves as if two diodes are connected in series with
a reverse voltage applied to them, a small leakage current
of the order of micro amperes or micro amperes flow in this
device.

21. This reverse blocking
mode is called the off
state of the thyristor when
the reverse voltage of the
SCR increases to a large
extent breakdown occurs
and the current in the
device increases rapidly.
Thus when the SCR is
biased in this region the
power dissipated is very
high, if the power
dissipated is more than
the rated value of the SCR,
the SCR is permanently
damaged thus in the
reverse bias condition the
voltage should never
cross the breakdown
voltage.

22. Applications Of SCR
Power Switching Circuit.
Controlled Rectifier.
AC Power control circuits.
Speed control of DC shunt motor.
SCR Crowbar.
Computer logic circuits.
Timing Circuits.
Inverters.
Battery Charging Regulators.
Temperature control systems.