Introduction to DIAC – Operation and Construction

By Basar

Posted on July 26, 2021

A diac is the important member of the thyresitor family and it is use to triggering the triac circuit. It is bidirectional semiconductor switching circuit or full wave. It can work both forward and reverse polarities. It has three layers and two PN junctions. The diac word is the combination of two words DI and AC. The DI stands for the diode and AC stands for Alternating Current. The synonym is diode for alternating current. It is almost similar to the triac except the gate terminal. The equivalent circuit is a pair of the inverted connection of diode. It is use in light dimmers and fluorescent lamp etc.

What is a DIAC?

The diac word is the combination of two words DI and AC, DIode for Alternating Current (DIAC). It is a bi-directional semiconductor switching device, which work in forward and reverse bias regions. It belongs to the thyrsistor family and use to triggering triac and other thyrsistor based circuits. It starts conducting when the applied voltage goes beyond its break-over voltage. It is also similar to the transistor without base. It can be turn “ON” and “OFF” for both polarities of voltage. It is available in different type packages. These packages are discrete components in small leaded packages, surface-mount packages and large packages. The DIAC and TRIAC are used together in most of electronics circuit. So, they are available in integrated packages also.

Symbol of DIAC

A diac is a two terminal device and it is constructed by using two diodes, which are connected in inverse. It has no gate terminal unlike other thyrsistor that they are use for triggering the circuit such as triac. Since the DIAC is bidirectional semiconductor device. So, we can’t name those terminals as an anode and cathode. The DIAC terminals are known MT1 (anode 1) and MT2 (anode 2) where MT stands for Main terminals. The symbol of diac is shown below:

symbol-of-diac

DIAC Construction

The diac construction is similar to the transistor structure, but is has no base terminal. The doping of all three layers are same and it act symmetrical properties in both polarities of applied voltage. The basic difference of diac and transistor is given below:-

The doping concentration of diac is identical in all layers but in transistor, emitter is highly doped collector lightly doped and moderately doped base.
Diac has two terminals but transistor has three terminals.
In diac all regions are in equal.

daic-structure

It is made of three, four and five layer of semiconductor material. The above figure shows the three, four and five layers construction. Mostly use three layers of construction PNP or NPN. In PNP, the two main terminals (MT1 and MT2) come from outer P-region and separated by N-region. As we know, It has two main terminals that are MT1 and MT2 and the current flow in both directions. When the voltage is apply across terminals. These layers with respective polarity to the voltage get activated. This combination of both polarities helps into conduction of DIAC in both forward and reverse direction.

In three layers of PNP structure, the layers P1 and P2 are connected to the MT1 and MT2 respectively and these layers are separated by N-layer. When the terminal 1 is positive with respect to the terminal 2, the junction P1 and N is forward bias and the P2 and N junction is reverse bias. When P2-N junction reaches the breakover voltage, the device comes into the conduction mode. Hence, it is “ON” from terminal 1 to terminal 2. The reverse bias will become when terminal 2 is positive with respect to terminal 1.

V-I Characteristics of DIAC

V-I-Characteristics-of-DIAC

When it starts conduction. It operates in both directions of voltage polarities. The above figure shows the V-I characteristics graph of diac. When breakover voltage limit that is from –VBO to +VBO cross diac, the resistance will be high.

So, small leakage current flow due to apply positive voltage. which is lower than +VBO and negative voltage lower than –VBO as shown in figure. From the figure the region AO is blocking region in this region the device act as an open switch. The breakover voltage range is 30 to 50 volts. The portion “AB” represents the conduction of device. The device will continuously conduct until the device current fall below its holding current level. It is noted that the value of holding and blocking currant are identical in forward and reverse region. The forward and reveres characteristics are shown in first and third quadrant of graph.

Advantages of DIAC

The device will be turn “ON” and “OFF” by reducing the voltage level below its breakdown voltage.
It is cheap in price using DIAC as a triggering circuit.

Applications of DIAC

It can be used in the TRIAC triggering circuit by connecting the gate terminal to the DIAC.
It is used in dimmer lamp circuit.
In heater control circuit.
It controls the speed of the universal motor.

Difference between SCR, DIAC and TRIAC

SCR (Silicon controlled rectifier)

It is a three terminals device.
It is a unidirectional device means it can only conducted in forward bias (one direction).
It has a gate terminal for control the device operation.
Its power handling capacity is high.
The SCR firing angle lies between 0 to 180°.
It is use in various type of electronics application like inverters, choppers, rectifiers, 3 phase systems.

DIAC (Diode for alternating current)

The word DIAC stands for Diode for alternating current.
It is have only two terminals.
It is a bi-directional device.
It has no gate terminal. So, it is an uncontrolled device.
It has low power handling capacity.
No firing angle for DIAC.
It operate in both positive and negative half cycles of AC signal input.

TRIAC ( Triode for alternating current)

Stands for Triode for alternating current.
It has also three terminals
Its working is bi-directional.
It has gate terminal. So, it is a controlled device
It has high power handling capacity.
The firing angle of TRIAC is 0 to 180° and 180° to 360°.
It is Used in light dimmer, fan control and many more applications.

Frequently ask questions;

What is a DIAC used for?

The diac is an electronics device that is use in various types of applications. The uses of applications are given below;

It can be used in the TRIAC triggering circuit by connecting the gate terminal to the DIAC.
It is used in dimmer lamp circuit.
In heater control circuit and control universal speed of motor.

What is meant by DIAC?

A diac is the important member of the thyresitor family and it is use to triggering the triac circuit. It is bidirectional semiconductor switching circuit or full wave.

The diac word is the combination of two words DI and AC, DIode for Alternating Current. It is a bi-directional semiconductor switching device, which work in forward and reverse bias regions.

What is the operation of DIAC?

In three layers of PNP structure, the layers P1 and P2 are connected to the MT1 and MT2 respectively and these layers are separated by N-layer. When the terminal 1 is positive with respect to the terminal 2, the junction P1 and N is forward bias and the P2 and N junction is reverse bias. When P2-N junction reaches the breakover voltage, the device comes into the conduction mode. Hence it is “ON” from terminal 1 to terminal 2. The reverse bias will become when terminal 2 is positive with respect to terminal 1.