The Diac

Objectives

After studying this unit, the student will be able to:

• Draw the schematic symbol for a diac

• Discuss the operation of a diac

• Connect a diac in a circuit

The diac is a special-purpose, bidirectional diode. The primary function of the diac is to phase shift a triac. The operation of the diac is very similar to that of a unijunction transistor, ex­ cept that the diac is a two-directional device. The diac has the ability to operate in an ac circuit while the UJT can operate only in a de circuit.

There are two schematic symbols for the diac,

figure 8-1. Both of these symbols are used in elec­ tronic schematics to illustrate the use of a diac.

Therefore, you should make your !.>elf familiar with both symbols.

FIGURE  8-1 Schematic symbols for the diac

The diac is a voltage sensitive switch that can operate on either polarity, figure 8-2. Voltage ap­ plied to the diac must reach a predetermined level before the diac will activate. For this example, as­ sume that the predetermined level is 15 volts. When the voltage reaches 15 volts, the diac will turn on, or fire. When the diac fires, it displays a negative resistance, which means that it will conduct at a lower voltage than the voltage needed to turn it on. In this example, assume that the volt­ age drops to 5 volts when the diac conducts. The diac will remain on until the applied voltage drops below its conduction level, which is 5 volts, fig­ ure 8-3.

FIGURE 8-2 The diac can operate on either polarity

FIGURE 8-3 The diac operates until the applied voltage falls below its conduction level

Since the diac is a bidirectional device, it will conduct on either half cycle of the alternating cur­ rent applied to it, figure 8-4. Note that the diac operates in the same manner on both halves of the accycle. The simplest way to summarize the op­ eration of the diac is to say that it is a voltage sen­ sitive ac switch.

FIGURE 8-4 The diac will conduct on either half of the altemating current

REVIEW QUESTIONS

1. Briefly explain how a diac operates.

2. Draw the two schematic symbols used to represent the diac.

3. What is the major use of the diac in industry?

4. When a diac first turns on, does the voltage drop, remain at the same level, or increase to a higher level?