DIAC, construction of Diac, working of Diac, applications of diac, Symbol of diac, VI characteristics of Diac

Hello everyone! Today, I'll give you a complete overview of the DIAC. A DIAC is a two-terminal device that can act as a switch based on the voltage applied across it. In this article, we will learn more about DIAC, its construction, working, and application. So let get started...



The (DIAC) stands for Diode for Alternating Current (DIAC). It is a bidirectional semiconductor switch that can be turned on in both forward and backward directions. This diode works on the complete cycle of AC, which means it can work during both the positive and negative half cycles of the AC waveform when polarity is active. The gate electrode is missing from the DIAC, which has only two electrodes. The device belongs to the Thyristor family and is used to trigger TRIACs and other Thyristor-based circuits. Only when the applied voltage exceeds the break-over voltage does the DIAC begin to conduct electric current

Symbol of DIAC

Because the DIAC is bidirectional, we can't call the terminals anode and cathode; instead, they're referred to as A1 and A2 or MT1 and MT2, where MT stands for Main Terminals. As a result, exactly like a resistor or ceramic capacitor, the pinouts of DIAC are reversible.
The positive or negative half cycle of the AC supply voltage can be used to turn on DIAC. A small current will flow through the device if the applied voltage is less than the forward break-over voltage. The leakage current is the name for this current. The GATE terminal is absent, the DIAC's break-over voltage remains unchanged.
Consider the PNP crystal structure, in which terminals 1 and 2 are linked to the P1 and P2 outer layers, separated by the N layer, respectively. Junction P1- N is forward biassed and P2-N is reverse biassed when terminal T1 is positive in relation to terminal 2. The entire structure enters the conduction mode when the P2-N break-over voltage is attained, and the diac conducts from terminal 1 to terminal 2. If terminal 2 is positive in relation to terminal 1, the opposite will occur.


They conduct in both positive and negative polarity, the DIAC's V-I characteristic curve will be in the shape of a Z, with the curve resting in the first and third quadrants. The first quadrant shows the positive half cycle, in which current flows from MT1 to MT2, while the second quadrant depicts the negative half cycle, in which current flows from MT2 to MT1. Because of the Reverse Bias junction between the layers, the resistance of the DIAC will be higher at first, resulting in a little leakage current flowing through the DIAC, which is referred to as the blocking condition in the curve.

Most of the DIACs will be having a breakdown voltage of around 30 Volts, the actual breakdown voltage will be dependent on the nature of the device. The DIAC will be in the conducting condition until the current reaches the specified value termed the holding current, where holding current is the minimum current that requires for a device to keep it in the ON state.


The DIAC is an important device of the thyristor family.
The main benefit of using this device is:

  • With an increase in current, the voltage drop reduces.

  • When used to activate other thyristors and TRIACs, it provides smooth power control.

  • It does not switch sharply to a low voltage condition at a low current level as done by SCR or TRIAC.

  • It has a low on-state voltage drop until its current falls below the holding current level.

A positive or negative pulse to the gate is required for a TRIAC to conduct, therefore the DIAC is commonly used in conjunction with the TRIAC circuit to provide symmetric firing. The DIACs are used to activate TRIACs and other types of thyristors, but they don't have many other applications. DIACs are utilized as a trigger device in a variety of applications, including motor speed control phase control circuits, light dimmers, heat controllers, and a variety of other control circuits. Let's look at some light dimmer and heat control circuit examples.
Heat Control:

Appilcations of DIAC Heat Control

The above circuit depicts the typical configuration of a DIAC-TRIAC combo used to manage AC power to a heater in a smooth manner. When the TRIAC is turned off, the capacitor C1 and the choke L form an LC circuit that reduces the voltage rise across it. The R2 potentiometer controls the applied voltage in both half-cycles, while the R4 resistor across the DIAC ensures smooth control. The heat dissipated from the heater increases as the TRIAC conducts longer, thus the DIAC is employed to provide smooth regulation of the heater's heat output.
Light Dimmer:

Applications of DIAC Light Dimmer

The RC phase shift network and DIAC controlled TRIAC for the light dimming application are shown in the circuit above. The RC arrangement using the Resistor R3 and the capacitor C3 changes the voltage provided to the TRIAC's gate terminal. The series R4-C1 network connected across the TRIAC limits the rate of voltage rise when the device is in its OFF state.
The capacitors C1 and C2 start charging at the rate indicated by the Variable resistor R2 once the input voltage from the 230V power source is supplied. When the voltage across the capacitor C3 exceeds the DIAC's breakdown voltage, the DIAC is activated, and the DIAC begins to conduct, causing the capacitor C3 to discharge and the TRIAC to receive the gate pulse. The TRIAC is turned on by the gate pulse, and the current begins to flow through the lamp. The rate of charging of the capacitor is also adjusted by adjusting the resistance in R2, which in turn changes the voltage rate at which the TRIAC is triggered in both the positive and negative half-cycles.

At last, hopefully, DIAC would be clear, now you can purchase any DIAC, Diode from our Website.

Active components

Leave a comment

All comments are moderated before being published