Triac working VI characteristic, structure, working and application

Triac structure

A Triac is a bidirectional device it allows current to flow in both the directions. here we will discuss Triac working and vi characteristics, In the SCR family, after the SCR, Triac is the most widely used device for power control. Triac is a three-terminal device, with the terminals named as main terminals 1, 2 (MT1 and MT2) and gate, out of which gate is the control terminal.

Triacs with large voltage and current ratings are now available in the market. Triac is a bidirectional device, i.e. the current can flow in both the directions through it. (Note that SCR is a unidirectional device). The basic structure of the triac is as shown in Figure.

Structure of Triac:

Triac symbol

Its operation is equivalent to two SCRs connected in antiparallel The two main terminals are designated MT1 and MT2 (main terminal 2 and main terminal 1). The gate is near MT1.

When the gate is open, the triac will block both the polarities of the voltage across MT1 and MT2 if the magnitude of voltage is less than the breakover voltage of the device. (See the characteristics of the triac). That means the triac will remain in the OFF state.

Triac structure
Internal structure of triac

Triac working and Vi Characteristics:

The characteristics of the triac are as shown in Figure and they are similar to those of an SCR both in blocking and conducting states. The only difference is that the SCR conducts only in the forward direction (anode-cathode) whereas the triac conducts in both the directions.

The other difference in the operation is the triggering mechanism. The triac can be turned on by applying either a positive or negative voltage to the gate with respect to terminal MT. Whereas the SCR can be triggered only by a positive gate signal.

VI characteristics of Triac
VI characteristics of Triac with four-mode

As seen from the Figure, the characteristics of the triac are the same as that of two back-to-back connected SCRs.

The effect of gate current is also the same i.e. with an increase in the gate current the breakdown voltage decreases. In triac the gate current can be positive or negative whereas in SCR the gate current can be only positive.

triac as two SCR

The triac characteristics can be divided into three regions of operation:

Blocking state or off state.
Transition or unstable state.
Conduction state or on the state.

Depending on the polarity of the voltage applied between its MT2 and MT1 terminals, it will operate in either the first quadrant or the third quadrant as shown in Figure.

MT2 positive w.r.t. MT1: Operation is in the first quadrant

MT2 negative w.r.t. MT1: Operation is in the third quadrant.

 

The different States of Triac working (operation):

The three important states of triac operation are:

Forward blocking state.
Reverse blocking state.
Conduction or on state.

 

Forward blocking state : (MT2 positive with respect to MT1):

When a forward voltage less than the breakover voltage Vn is applied with the gate terminal open the triac can successfully block the forward voltage without getting turned on.

Reverse blocking state: (MT1 positive with respect to MT2):

When the reverse voltage is less than the breakover voltage Vpo with the gate open-circuited, the triac will block the reverse voltage without getting turned on.

Conduction or ON state:

The triac is equivalent to two SCRs connected back to back. Therefore it is a bidirectional device that can conduct in positive as well as negative half cycles of the supply voltage.

The gate current can be positive or negative. The forward and reverse breakover voltage reduces with an increase in the gate current.

Depending on the polarity of supply voltage and the polarity of the gate current the triac can operate in four different modes of operation as follows: l, ll. lll and Vl.

 

Triac working & Modes of Operation:

l, ll, III and VI are the four modes of operation of a TRIAC where I or IIl represents the quadrant of operation and (+) and (-) signs indicate the direction of gate current. The sensitivity of a mode is defined as the minimum gate current required to tur on a TRIAC in that mode. The sensitivity of I mode is the highest while that of III mode is the lowest.

Rating of a triac:

Triac ratings

Advantages of Triac:

  • It is a bi-directional device. So we can control the power delivered to load in both the half cycles of ac supply.
  • It is equivalent to two SCRs connected back to back.
  • We can turn it on by using a positive as well as negative gate current.
  • It is more suitable for resistive loads.
  • Triac is more economical than SCRs since inside the same package we get two SCRs connected back to back.
  • It can control the power delivered to ac loads such as a fan motor.
  • It is not necessary to use a protection diode across the Triac.
  • We can use a single heat sink.

Disadvantages of Triac:

  • We cannot use it as a controlled rectifier.
  • Low dv/dt rating than SCR. So the possibility of an accidental turn on is higher than that of SCR.
  • Low di/dt rating.
  • Suitable for resistive loads only. Not suitable for controlling power to highly inductive loads.
  • Its power rating is lower than that of SCR. Triggering circuits need to be designed more carefully.

Applications of Triac:

  • Lamp dimmer
  • Proximity detector
  • As a static switch
  • In the AC voltage stabilizers
  • Fan speed regulator
  • Flasher circuit
  • Temperature controller.

 

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