Diac, Symbol, Construction, Working and Characteristics

 What is Diac?


Diac is a two terminal bidirectional semiconductor switch. ‘Di’ stands for two and ‘ac’ stands for alternating current. Thus Diac is a diode for alternating current.


It belongs to Thyristor family. It can be turned ON in both forward and negative direction. It starts conducting current when the applied voltage is above breakover voltage.


Symbol of Diac:


Symbol of Diac
Symbol of Diac



The symbol of Diac consists of two diodes connected in parallel but opposite to one another. It has two terminals, anode1 and anode2.


MT stands for main terminal. Since it is bidirectional the terminals are reversible like resistor and capacitor. It does not have a gate terminal.


Construction of Diac:


Construction of Diac
Construction of Diac



Diac is made up of five layers and two terminals. It has two P type layer and three N type layer. It has no base or gate terminal. The layers which are close to the terminals are made up of both N type and P type.


The doping concentration in all the layers is identical, whereas in transistor the doping concentration is different in each layer. Since all the layers are identical they are bidirectional and terminals can be used the either way.


Working of Diac:

Working of Diac
Working of Diac



When the terminal MT1 is positive the direction of the flow of current will be in the order P1-N2-P2-N3. The junction between P1 and N2 is forward biased, the junction between N2 and P2 is reverse biased and the junction between P2 and N3 is forward biased.

working of Diac
working of Diac



When the terminal MT2 is positive the direction of the flow of current will be in the order N1-P1-N2-P2. The junction between N1 and P1 is forward biased, the junction between P1 and N2 is reverse biased and the junction between N2 and P2 is forward biased.


V-I characteristics of Diac:


V-I Characteristics of Diac
V-I Characteristics of Diac



When the external voltage is applied at the terminals, Diac does not conduct immediately. Only a small leakage current flows. When the applied voltage is further increased and when it crosses the breakover voltage the junction which is reverse biased breaks and they start conducting. 

Till the breakover voltage is reached they remain in forward and reverse blocking state. After the applied voltage is increased above the breakover voltage avalanche breakdown takes place and the current increases. It happens for both the polarity of voltages. To turn OFF the device the applied voltage is decreased below the breakover voltage.

    Advantages :

  1.  It is of low cost
  2. It is used for low power applications
  3. Smooth control of power 
  4. Diac can be switched ON or OFF just by lowering the voltages.

    Disadvantages :

  1. Trigger voltage is 30v. So the incoming voltage to trigger the diac should be minimum of 30v.

Application of Diac:

  • Used in TRAIC triggering circuit
  • Used in Lamp dimmer circuit
  • Used in heat control circuit
  • Used in speed control of a universal motor.

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