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DIAC: Introduction, Definition, Symbol, Configuration, Working Principle, V-I characteristics, Application 



DIAC  is a bi-directional semiconductor switch which can use in both forward and backward direction. Diode Ac Switches(DIAC) are mainly used to triggering the Thyristor or Bilateral triode Thyristor(TRIAC). Before discus about DIAC, it's better a short brief about DIODE, A diode is outlined as a two-terminal electronic part that carries current in one direction (so long because it is operated at intervals a specific voltage level). A perfect diode can have zero resistance in one direction, and infinite resistance within the reverse direction. The diode is a Single Directional electrical component. These types of electronic switches are used in a starter circuit of any type of starter kit. DIAC is also known as Symmetrical Trigger Diodes. DIAC has various types mainly they are selected according to the starter circuit.


 DIAC is a two-directional electrical Diodes that can use after it's breakdown voltage. DIAC is "Diode for Alternating Current". So we can use it in both the direction in forward and also backward direction. In a starter circuit, it acts as two polarities switch which can optimize the device in forward and also backward. 

DIAC Symbol:-

DIAC is an electronic component that is made up of two parallel diodes and the diodes are connected in the opposite direction.

 Configuration Of DIAC:-

The DIAC is made up of two parallel diodes and they are connected opposite direction. The configuration of DIAC and Transistor are primarily the same but some working principles are different:-
  1.  DIAC has 2 terminals (MT1, MT2)  but the Transistor has 3 terminals (Emitter, Collector, Base).
  2. The doping concentrations in the P-N junctions are identical but in Transistor highly doped in the emitter and lightly doped in the collecter and moderately doped in the base.
So don't mess up with the DIAC and Transistor. They just look alike but the working principle and the configuration are different.

The DIAC can be configured in 3,4 or 5 layer structures depend upon where it's used in the Starter circuit. A 3 layer structure is more commonly used than other structures. The 3 layer DIAC can be configured in either PNP or NPN structure. In PNP form, 2 terminals are connected to the outer silicon P-regions separated by the N-region. The DIAC configuration is the same as the PNP transistor with no base connection.

Consider the PNP crystal structure, within which terminals one and a couple are connected to the P1 and P2 outer layers severally which are separated by the N layer. once the terminal T1 is positive with relation to a terminal a pair of, junction P1N is forward biased and P2N is reverse biased. once the breakdown voltage of P2N is reached, the complete structure comes into the conductivity mode and thence DIAC conducts from terminal one to terminal a pair of. The reverse can happen if the terminal a pair is positive to the terminal one.

It is a tool that consists of 4 layers and 2 terminals. the development is nearly similar to that of the electronic transistor. however, there are sure points that deviate from the development from the electronic transistor. The differentiating points are-

There is no base terminal within the DIAC

The 3 regions have nearly a similar level of doping

It offers symmetrical shift characteristics for either polarity of voltages.

Characteristics DIAC:-

From the figure on top of, we will see that a DIAC has 2 p-type material and 3 n-type materials. Also, it doesn't have any gate terminal in it.

The DIAC is turned on for each of the polarities of voltages. once A2 is a lot of positive with relation to A1 then the present doesn't flow through the corresponding N-layer, however, flows from P2-N2-P1-N1. once A1 is a lot of positive A2 then the present flows through P1-N2-P2-N3.

The construction resembles the diode-connected serial.
When the applied voltage is forward polarity either backward polarity, an awfully tiny current flows that are understood as outflow current thanks to the drift of electrons and holes within the depletion region. though a tiny low current flows, it's not adequate to supply avalanche breakdown, thence the device remains within the non-conducting state.

Working Principle Of The DIAC:-

As before long because the offer voltage whether or not positive or negative is applied across the terminals of a DIAC, solely low leak current flows through the device. That the device operates in either forward or reverses backward direction. once the applied voltage is raised to a price specified it's up to the breakdown voltage, an avalanche breakdown happens at the reverse-biased NPN junction.

Then, it starts conducting and exhibits negative resistance characteristics, i.e., the present will increase with decreasing values of an applied voltage. The fall throughout the conductivity is incredibly less and is up to the ON state drop of the DIAC. The present flow will increase quickly once it comes into the forward direction. Therefore, for a secure operation level of this conductivity current in either direction, a resistance is connected in series with the DIAC.

V-I Characteristics Operation of DIAC:-

The figure below shows the V-I characteristics of DIAC that indicates this flow through the DIAC with relevancy the voltage across it. As long because the voltage across it's among its breakdown limits that are from –VBO(Backward Voltage) to +VBO(Forward Voltage), the resistance offered by the DIAC is extremely high.

So, a tiny low leakage current flows through the device for applying a positive voltage that is a smaller amount than +VBO and negative voltage but –VBO as shown in the figure. The region OA within the portion of the characteristics is that the block region. below these conditions, DIAC operates as an associate open switch. The voltages +VBO and –VBO square measure the breakdown voltages that square measure typically within the vary of thirty to fifty volts.
Once the positive or negative applied voltage is quite the several breakdown voltages which means at purpose A within the on top of figure the DIAC begins to conduct and also the fall across the device becomes few volts. The portion AB represents the conductivity of DIAC.

This conductivity continuous until the device current falls below its holding current level. From the figure, it's noted that the holding current and breakdown voltage values square measure identical for reverse and forward region of operation. the primary and third quadrant characteristics represent the forward and reverse bias conditions of the DIAC.

Application of DIAC:-


The main application of a DIAC is its use in a very TRIAC triggering circuit. The DIAC is connected to the gate terminal of the TRIAC. once the voltage across the gate decreases below a planned worth, the gate voltage is zero and therefore the TRIAC is turned off.

One of the main uses of DIACs inside TRIAC circuits. TRIACs don't hearth symmetrically as a result of slight variations between the 2 halves of the device.

The non-symmetrical firing and ensuing undulations bring about to the generation of unwanted harmonics – the less symmetrical the waveform the larger the extent of harmonic generation.

DIACs are a wide used electronic part. The chief application of DIACs is to be used in conjunction with TRIACs to equalize their shift characteristics. By equalizing the shift characteristics of those TRIACs, the extent of harmonics generated once shift AC signals will be reduced. Despite this, for large applications, 2 thyristors are typically used. withal the DIAC / TRIAC combination is incredibly helpful for lower power applications as well as light dimmers, etc.

Some other applications of a DIAC include:- 

  1. It will be utilized in the lamp rheostat circuit
  2. It is utilized in a heat feedback loop
  3. It is utilized in the speed control of a universal motor
  4. A DIAC will be used with a TRIAC in a very series combination for triggering. 
  5. The gate of TRIAC is connected with a terminal of the DIAC. once the applied voltage across the DIAC will increase higher than the avalanche breakdown voltage, only then it will conduct.

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