UJT, DIAC, TRIAC

1. Triggering Devices
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
12/27/2020 Arpan Deyasi, RCCIIT

2. Content
DIAC
TRIAC
UJT
12/27/2020 Arpan Deyasi, RCCIIT

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DIode for Alternating Current (DIAC)
DIAC
bidirectional semiconductor switch
can be turned ON in both forward
and reverse direction

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DIAC
Arpan Deyasi, RCCIIT

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It does not have a controlling gate terminal because
they can be turned on or off by simply reducing the
voltage level below the avalanche breakdown
voltage and it can be done in both the polarities
DIAC starts conducting electric current if the applied
voltage goes beyond its break-over voltage
DIAC

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DIAC

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Application of DIAC
It can be used in the lamp dimmer circuit
It is used in a heat control circuit
It is used in the speed control of a universal motor

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TRIAC
TRIode for Alternating Current (DIAC)
bidirectional device that can
pass the current in both
forward and reverse direction
can conduct in both the
conditions of the gate signal,
positive and negative

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TRIAC

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1. MT2 is
positive with
respect to MT1
with a gate
polarity
positive with
respect to MT1
TRIAC

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2. MT2 is
positive with
respect to MT1
with a gate
polarity negative
with respect to
MT1
TRIAC

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3. MT2 is
negative with
respect to
MT1 with a
gate polarity
negative with
respect to
MT1
TRIAC

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4. MT2 is
negative with
respect to MT1
with a gate
polarity
positive with
respect to MT1
TRIAC

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TRIAC

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Control circuits like electric fan speed control
and smaller motor controls
High Power lamp switching and light dimmers
AC power control domestic appliances
Application of DIAC

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UJT
Uni Junction Transistor
transistor that has a single PN
junction, but still not a diode
one emitter and two bases

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UJT

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0EV =Initially when no voltage is applied
UJT
If the emitter voltage VE is increased
1
1
1 2
B
B BB
B B
R
V V
R R
 
=  
+ 
Intrinsic resistance 1 2BB B BR R R= +

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UJT
Let
1B
BB
R
R
η
 
=  
 
voltage across VB1 1B BBV Vη=
Emitter voltage 1E D BV V V= +
0.7E BBV Vη= +

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UJT

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Applications of UJT
Relaxation oscillator
Phase Control Circuits
Provide clock for digital circuits
Controlled firing in Thyristors
Sync pulsed for horizontal deflection circuits in CRO