Diac

Diac

• The diac is a two terminal, four layer device. It conducts in either directions hence it is also called bilateral trigger diode.

• It has a pair of four layer diodes a shown in Fig. 6.3.1 (a). The two different symbols used for the diac are shown in Fig. 6.3.1(b).

• The terminals of the diac are not named as it can be used in any direction. It is low power triggering device.

• There is no control terminal on the diac.

1. Basic Operation

• The diac can be treated as parallel inverse combination of the semiconductor layers that permits triggering in either direction.

• Fig. 6.3.2 (a) shows the diac equivalent circuit as two parallel Shockley diodes, connected in opposite directions. The Shockley diode is basically a four layer pnpn diode, with only two external terminals. The characteristics of Shockley diode are similar to SCR with I_G=0. It acts as a switch which is ON in one direction and OFF in the other.

• Both the Shockley diodes act as a switch as shown. in Fig. 6.3.2(b). The V is the applied voltage across A₁ and A₂.

• When the polarity of applied voltage is as shown in Fig. 6.3.2 (a) then Shockley diode D_S1 becomes ON when V is more than V_{BR (F)} of D_S1. Thus current flows from A₁ to A₂ as D_S1 acts as closed switch S₁.

• If the polarities of applied voltage V are reversed then for applied voltage more than V_{BR (F)} of D_S2, the D_S2 conducts and its acts like a closed switch S₂. The current flows from A₂ to A₁.

• Thus device can conduct in both the directions, depending on the polarities of the applied voltage across it.

2. Four Transistors Analogy

• The diac operation can be explained using four transistors analogy.

• Each pair of transistors form a Shockley diode. The four transistors analogy and the biasing conditions shown in Fig. 6.3.3 (a) and (b) also can be used to explain the basic operation of diac.

• The transistors Q₁, Q₂ form a Shockley diode D_S1 while the transistors Q₃, Q₄ form a Shockley diode D_S2. The basic operation remains same as explained earlier.

3. Characteristics of Diac

• The diac characteristics are shown in Fig. 6.3.4.

• As long as the forward voltage is less than the breakdown voltage, the diac is OFF and the current is small.

Fig. 6.3.3 Four transistors analogy

• When the applied forward voltage becomes more than V_{BR (F)} then diac becomes ON and conducts so that large current flows through it.

• The diac remains ON as long as the current through it is greater than the holding current.

• The diac characteristics are exactly similar in both the directions. But they are shown in the 1st quadrant and in the 3rd quadrant due to opposite polarities of voltage and current in the two directions.

4. Applications

The diac is not a control device, it is used as a triggering device.

• It is used in,

1. Triggering of triac

2. Motor speed control

3. Temperature control

4. Light dimming circuits.

Review Questions

1. Explain the basic construction of a diac.

2. Explain the working principle of diac.

3. Explain the four transistor analogy for a diac.

4. Draw and explain the characteristics of diac. Explain its operation.

5. Explain the working and characteristics of DIAC and its applications.