The document discusses the triode for alternating current (TRIAC). A TRIAC is a bidirectional thyristor that can conduct current in both directions, unlike an SCR which only conducts in one direction. The TRIAC has symmetrical characteristics in the first and third quadrants of its IV curve. It can be triggered by either a positive or negative gate signal, unlike the SCR which only responds to a positive gate signal. The TRIAC turns on when a positive or negative voltage is applied to its gate terminal to create a triggering current and remains on until the current through its main terminals drops below the holding current.

1. Chapter 2
Triode for Alternating Current (TRIAC)

2. Basic Operation
(a) symbol
(b) Equivalent circuit
 Triac can conduct current in either direction and formally called bidirectional
triode thyristor
 Closely related to SCR
Differences between Triac and SCR
Triac is bidirectional devices (can conduct current in either direction)
while SCR is unidirectional devices (can conduct current only in one
way) force commutation by reversed biasing cannot be employed.
TRIACs can be triggered by either a positive or a negative current
applied to its gate electrode while SCR can only triggered when a
positive current apply to the gate terminal
In order to create a triggering current, a positive or negative voltage has to
be applied to the gate with respect to the A1 terminal (otherwise known as
MT1). EAD 3043 (Industrial Electronic)
Nor Aida Idayu Binti Abdullah. 2012

3. Parameter of TRIAC
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4. Comparison between SCR and
TRIAC
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5. Characteristic
 It can be seen that it is an extension IV characteristic of
SCR
 The characteristic are symmetrical about the first and the
third quadrants
 It shows the gate can be used to control the main current
flow in either direction
 TRIAC will be continuously ON until the current through A1
to A2 are below the holding current EAD 3043 (Industrial Electronic)
Nor Aida Idayu Binti Abdullah. 2012

6. Triggering Method
 The phase shifting method of firing TRIAC is similar to the
firing of the SCR
 The major difference in the waveform is there is a negative
portion in the waveform of TRIAC
 It shows the gate can be used to control the main current
flow in either direction
EAD 3043 (Industrial Electronic)
Nor Aida Idayu Binti Abdullah. 2012

7. Turning the TRIAC ON
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8. Example
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9. Example
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10. Exercise
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11. Exercise
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12. Quiz
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13. Assignment
EAD 3043 (Industrial Electronic)
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