This document discusses various methods for turning on or triggering silicon controlled rectifiers (SCRs). It describes seven main triggering methods: forward voltage, temperature, dv/dt, light, gate, AC, and pulse triggering. For each method, it provides details on the triggering mechanism and notes advantages and disadvantages. The gate triggering method, which uses a positive gate current to turn on the SCR, is highlighted as the most reliable, simple and efficient triggering technique. AC triggering is also identified as the most commonly used method for AC applications.

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THYRISTOR
Turn on/Triggering
methods
Dr. A. Ravi
Professor/EEE
FRANCIS XAVIER ENGINEERING COLLEGE
TIRUNELVELI
Tamil Nadu - India

2. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
What is triggering
Process of Turn
on the SCR
To Break the Junction J2
which is Reverse Bias

3. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
•Forward voltage triggering
•Temperature triggering
•dv/dt triggering
•Light triggering
•Gate triggering
•AC triggering
•Pulse triggering
Different Methods

4. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
Forward voltage triggering
by increasing the forward
anode to cathode voltage.
By doing this, the
depletion layer width is also
increasing at junction J2.
This further leads to an
avalanche breakdown of the
junction J2 at a forward
breakover voltage VBO.
In practice this is not
employed because it needs a
very large anode to cathode
voltage.

5. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
Temperature Triggering
 The reverse leakage current depends on the
temperature. If the temperature is increased to a certain
value, the number of hole-pairs also increases.
 Increase the leakage current and further it increases the
current gains of the SCR.
This starts the regenerative action inside the SCR since
the (α1 + α2) value approaches to unity
practically not employed because it causes the
thermal runaway . SCR may be damaged.

6. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
dv/dt Triggering
 In forward blocking state junctions J1 and J3 are forward
biased and J2 is reverse biased.
 The junction J2 behaves as a capacitor. I = C dv/ dt
produce very high voltage spikes across the SCR so
there will be considerable damage to it.
dv/dt is the rate of change of applied voltage
C is the junction capacitance.
Rate of change of the applied voltage is large that leads to
increase the charging current which is enough to increase the
value of α. Hence SCR ON

7. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
Light Triggering
An SCR turned ON by light radiation is also
called as Light Activated SCR (LASCR).
light rays with appropriate wavelength and
intensity are allowed to strike the junction J2.
It consist a niche in the inner p-layer. Therefore, when the
light struck on this niche, electron-hole pairs are generated at
the junction J2 which provides additional charge carriers at the
junction leads to turn ON the SCR.

8. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
Light Triggering
It consist a niche in the inner
p-layer. Therefore, when the
light struck on this niche,
electron-hole pairs are
generated at the junction J2
which provides additional
charge carriers at the junction
leads to turn ON the SCR.

9. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
DC Gate Triggering
Gate triggering is the method in which positive gate
current is flown in forward biased SCR to make it ON.
reliable,
simple and
efficient way
to turn on
SCR
positive gate voltage between gate and cathode
terminals are applied in forward biased SCR which
establishes gate current from gate terminal to cathode

10. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
DC Gate Triggering
• When the gate current Ig is zero, the forward breakover voltage is VBO.
• As gate current increases from zero to Ig1, the forward breakover voltage
reduces from VBO to V1. Similarly, its value reduces from V1 to V3 as the gate
current increases from Ig1 to Ig3.

11. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
AC Triggering
most commonly used method
for AC applications.
With the proper isolation
between the power and
control circuit, the SCR is
triggered by the phase-shift
AC voltage derived from the
main supply.

12. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN ON METHODS (SCR TRIGGERING)
Pulse Triggering
Gate is supplied with single pulse or a train of pulses is
given into the SCR
ADVANTAGE
gate drive is discontinuous or doesn’t need continuous
pulses to turn the SCR.

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