Triac

 Triac

• The triac is another important member of the thyristor family. It is basically two parallel SCRs turned in opposite directions, with a common gate terminal.

Key Point : It is a bidirectional device and can conduct in both the directions.

• The Fig. 5.5.1 (a) shows the basic construction of a triac. The Fig. 5.5.1 (b) shows the symbol of a triac. The Fig. 5.5.1 (c) shows an equivalent circuit of a triac which is antiparallel connection of the two SCRs.

• As triac conducts both ways, anode-cathode terminology is not used. The two main electrodes are called main terminals MT1 and MT2 while common control terminal is called gate G. The gate terminal is near MT1

• The triac can be turned ON by applying either a positive or negative voltage to the gate G with respect to the main terminal MT1.

 

1. Working of Triac

• With gate open, either MT1 is positive with respect to MT2 or MT2 is positive with respect to MT1

• Forward blocking region : When gate is open and MT2 is positive with respect to MT1 but the voltage is less than forward breakover voltage then triac does not conduct. This region is called forward blocking region. If this voltage is increased beyond breakover voltage, the triac conducts in the forward direction.

• Reverse blocking region : When gate is open and MT2 is negative with respect to MT2 but the voltage is less than breakover voltage then triac does not conduct. This region is called reverse blocking region. But note that if this voltage is increased beyond the breakover voltage, triac conducts in reverse direction.

• In forward or reverse blocking, now if gate is made positive or negative with respect to MTX then also the triac conducts. This is the gate control of triac and easy way of switching triac ON.

Key Point : When MT2 is positive and MTX is negative, triac is forward biased while when MT2 is positive and MT2 is negative, triac is reverse biased.

• Operating modes of triac : In each biased state, gate can be positive or negative.

• This gives four different operating modes of triac.

1. Mode I :

• In this mode, triac is forward biased and gate is made positive with respect to MTp This is shown in the Fig. 5.5.2. The terminal MT2 is positive with respect to MTX as triac is forward biased.

• In this arrangement the breakdown occurs as a normal SCR.

• The gate current is positive and the triac is said to be operating in the first quadrant of its V-I characteristics. Hence this mode is also called I+ mode of operation.

2. Mode II :

• In this mode, triac is forward biased and gate is made negative with respect to MTP This is shown in the Fig. 5.5.3. In this arrangement also the breakdown occurs as a normal SCR.

• The sensitivity to gate current is less in this mode. The gate current is negative but triac still operates in first quadrant of its V-I characteristics.

• Due to negative gate, this mode is also called I- mode of operation.

3. Mode III :

• In this mode, triac is reverse biased i.e. MT1 is positive with respect to MT2 and the gate is made positive as shown in the Fig. 5.5.4.

• The gate current initiates the conduction and due to regenerative action, the triac is turned ON.

• In this mode, direction of main SCR i.e. triac current reverses compared to mode I and II and polarities of voltage between MT1 and MT2 are also reversed compared to mode I and II hence triac operates in third quandrant of its V-I characteristics. But due to positive gate, it is called III+ mode.

4. Mode IV :

• In this mode, triac is reverse biased but gate is made negative as shown in the Fig. 5.5.5.

• In this mode, gate current is negative and due to regenerative action, the triac starts conducting.

• Compared to III+ mode, in this mode only gate current direction is reversed. Hence this mode is called III- mode.

• The triac operation is in the third quadrant of its V-I characteristics. In this mode triac is more sensitive than III+ mode.

 

2. Characteristics of Triac

• The triac has its characteristics in first quadrant and third quadrant.

• The characteristics are exactly similar to that of SCR, but in both the quadrants I and III and not like SCR only in first quadrant. 

• The mode I and II operations exist in first quadrant while mode III and IV operations exist in the third quadrant.

• The entire V-I characteristics of triac are shown in the Fig. 5.5.6.

 

3. Applications of Triac

• The various applications of triac are,

1. Static a.c. switching of applications.

2. In a three position static switch.

3. A.C. power flashers.

4. Heater controllers or temperature controllers.

5. As a triggering device for SCRs.

6. Light dimmer circuits.

7. The power control to the load.

8. Proximity detector circuit.

9. A.C. voltage stabilizers.

• As triac is a bi-directional device, it can easily be used to design variable full wave rectifier, as shown in the Fig. 5.5.7 (a).

• In this circuit, resistance R1 and capacitance C shift the phase angle of the gate signal. Because of this phase shift, the gate voltage lags the line voltage by an angle between 0 and 90°.

• This voltage lag delays the firing of triac controlling the current through the load.

• As triac conducts in either direction, current in both positive and negative half cycles can be controlled as shown in the Fig. 5.5.7 (b). Power in shaded portion only is available to the load.

 

 4. Comparison of SCR and Triao

 

5. Merits of Triac

1. It conducts in both the directions hence power control in both the half cycles is possible.

2. Triac turns off when voltage is reversed, no external circuit is required to turn off.

3. Single gate controls the conduction in both the directions.

4. Triacs with high voltage and current ratings are available.

5. More economical than SCR.

 

6. Demerits of Triac

1. Not suitable for d.c. power applications.

2. Gate has no control over the conduction when triac is on.

3. Triacs have very small switching frequencies.

4. Power rating is less than SCR.

5. The reliability is less than SCR.

6. Low dv/dt and di/dt rating than SCR hence accidental turning on due to high dv/dt of source voltage is possible.

Review Questions

1. What is triac ? Explain its construction, symbol and equivalent circuit.

2. Explain the working of triac.

3. Draw and explain the V-l characteristics of triac.

4. List the applications of triac.

5. Explain any one application of triac in detail.

6. Compare SCR and triac.

7. List the merits of triac.

8. List the demerits of triac.