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.
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.
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.
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?