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 the Fig. 5.4.1 (a). The two different symbols used for the diac are shown in the Fig. 5.4.1(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.

• The Fig. 5.4.2 (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 IQ =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 the Fig. 5.4.2 0?). The V is the applied voltage across A-j and A2.

• When the polarity of applied voltage is as shown in the Fig. 5.4.2 (a) then Shockley diode DS1 becomes ON when V is more than VBRof DS1. Thus current flows from A1 to A2 as DS1 acts as closed switch S₁.

•  If the polarities of applied voltage V are reversed then for applied voltage more than VBRof DS2, the DS2 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 the Fig. 5.4.3 (a) and 0?) 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 DS2. The basic operation remains same as explained earlier.

3. Characteristics of Diac

• The diac characteristics are shown in the Fig. 5.4.4.

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

• 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 3 rd 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.

5. List the applications of diac.