Electrical Revolution

The triac is a three terminal bidirectional switching device.
It is an AC switch which can conduct on both of the alternating cycle ( Positive as well as Negative ).
The triac word is combination of two words TRI and AC.
The TRI indicates three terminals and AC means alternating current. Figure A shows structure and symbol of the triac.

Structure

Figure A shows structure of the triac. As the triac can conduct during positive and negative half cycle, it is equivalent to two SCRs in anti parallel with only one gate signal.
The two terminals are MT1 and MT2 not anode and cathode.
The triac may be considered as N₁P₁N₂P₂ section is in parallel with P₁N₂P₂N₃ section. An additional N₄ region serves as gate terminal.

Operation

The AC supply to given to the terminal MT1 and MT2 through load resistance R_L.
The gate circuit consists of DC voltage source, current limiting resistor R and switch S.
When switch S is kept open, the triac can turned on only by providing the supply voltage greater than the breaker over voltage.
When switch S is kept is open, there will be no gate current and triac is not conducting ( considering that the supply voltage less than the break over voltage ).
When the switch S is kept closed, the gate current starts to flow. The triac will start conducting when the MT1 is made positive with respect to MT2.

Now consider the following two cases with proper gate current

( 1 ) When MT1 is made positive with respect to MT2, the triac turns on and current flows through MT1 to MT2 terminal.

( 2 ) When MT2 is made positive with respect to MT1, the triac turns on and current flows through MT2 to MT1 terminal.

Characteristic

The voltage – current characteristic of the triac is shown in the Figure C. The characteristic of the triac is similar to SCR static characteristic in the first quadrant.
The operation of the triac in the first quadrant occurs when the MT1 is made positive with respect to MT2.
This is known as forward conduction mode of the device.
When MT1 is made negative with respect to MT2, the triac operates in the third quadrant. This is known as reverse conduction mode.

Without Gate signal

When the applied voltage is less than the break over voltage, the triac does not conducts considering that the gate current is zero.
The region OA and OA’ is known as blocking or non – conducting state of the device.
When the applied voltage increases up to break over voltage, the device starts conducting and current increases sharply.
The region BC and BC’ is known as conducting state of the device.

With Gate signal

When the gate signal of either polarity is applied, it lowers the break over voltage.
Higher the gate current, lower the break over voltage and vice versa.
The triac is triggered with positive voltage is more sensitive than the negative gate voltage.
Generally the break over voltage of the triac is quite large therefore it is not designed for conduction when the gate current is zero.

Application

The triac is used for control of AC power, control lamp dimmers, heat control of furnaces etc.

Advantages

The triac is equivalent to two SCRs in anti parallel. We shall discuss the following advantages and disadvantages of triac with respect to SCR.

The triac can be triggered with either positive or negative polarity of the supply voltage.
It requires only one heat sink whereas the SCR requires two heat sinks.
It require only one fuse which simplifies it construction.

Disadvantages

It is less reliable than the SCR.
It has low dv / dt as compared to SCR.
As the triac can be triggered with either positive or negative cycle, the trigger circuit needs careful design.
The rating of triac is low as compared to SCR.

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