What is Triac?
Triac is a three terminal bidirectional semiconductor device. Triac stands for Triode for Alternating Current. It conducts current in both directions and gate terminal controls it. Triac belong to the Thyristor family. The difference between thyristor and Triac is SCR conducts current in only one direction, but Triac conducts in both directions. It is used as AC Switch.
Symbol of Triac:
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| Symbol of Triac |
The symbol of Triac has three terminals Anode1, Anode2 and Gate. The Anode1 and Anode2 terminals are commonly called as Main terminal1 and Main terminal2. Gate terminal acts like a trigger to turn the device ON. The symbol looks like two thyristors connected in inverse parallel direction merged together with Gate terminal in common.
Construction of Triac:
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| Construction of Triac |
Triac is a four layer, six doped region and a three terminal device. Gate terminal is connected to both N3 and P2 so that gate triggers the device when both positive and negative voltage is applied. In the Same way MT1 or Anode1 is also connected to N2 and P2 regions, and MT2 or Anode2 is connected to the P1 and N4 regions. So the polarity between the terminals decide the direction of the current through the layers.
Working of Triac:
There are four possible combinations of the potentials applied to the terminals.
Mode1: MT2 is positive and Gate terminal is positive.
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| Mode1: MT1 and Gate positive |
When the MT2 terminal is made positive with respect to the terminal MT1 and when positive voltage is applied at the gate terminal the path of the current flow from MT2 to MT1 will be P1-N1-P2-N2. The junction between P1N1 and P2N2 are forward biased and junction between N1P2 is reverse biased and breakdown occurs at this junction.
Mode2 : MT2 is positive and Gate terminal is Negative.
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| Mode 2: MT2 positive and Gate negative |
When the MT2 terminal is made positive with respect to the terminal MT1 and when negative voltage is applied at the gate terminal, initially the path of the current flow from MT2 to MT1 will be P1-N1-P2-N3. When the voltage applied at the MT2 terminal is further increased the junction P2N2 is forward biased and the path of the current flow will be P1-N1-P2-N2. More Gate current is needed to turn the Triac.
Mode3: MT2 is negative and Gate terminal is positive.
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| Mode 3: MT2 negative and Gate positive |
When the MT2 terminal is made positive with respect to the terminal MT1 and when negative voltage is applied at the gate terminal the path of the current flow from MT2 to MT1 will be P2N1P1. The Junctions P2N1 and P1N4 are forward biased and the junction N1P1 is reverse biased. So in this mode Triac work in a negative biased region.
Mode4: MT2 is negative and Gate terminal is negative.
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| Mode 4: MT2 and Gate negative |
When the MT2 terminal is made negative with respect to the terminal MT1 and when negative voltage is applied at the gate terminal the path of the current flow from MT2 to MT1 will be P2N1P1N4.
Mode2 and mode3 are less sensitive and need more gate current to turn ON the device. Mode1 and mode4 have greater sensitivity when gate polarity and MT2 are of same polarity.
V-I characteristics of Triac:
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| V-I characteristics of Triac |
Triac is made up of two SCRs in inverse parallel. It operates in four modes. Initially the Triac operates in forward and reverse blocking mode and only small leakage current flows through it. When the applied voltage at the MT2 terminal is further increased and when it crosses the breakover voltage Triac starts conduction. The current start to flow and the voltage applied at the Gate terminal controls this current flow.
Advantages:
- Traic needs single fuse for protection
- It can be triggered either with positive or negative voltages
- It has safe breakdown in either directions
- It turns off when the voltage is zero
Disadvantages:
- Triac is not suitable for DC applications
- Since it is triggered at either directions, triggering circuits should be considered carefully
- Reliability is lesser than SCR
Application of Triac:
- Used in high power lamp switching
- Used to Control AC power
- Used in light dimmers
- Used to control the speed of electric fan and small motors
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