Diac: Working and Characteristics

The Diac, also known as a bidirectional thyristor diode, is a two-terminal electronic device that can conduct current in both directions when it is triggered. The name "diac" is derived from the combination of the words "diode" and "AC," indicating its ability to work with alternating current.The structure of a diac consists of two layers of P-N junctions.The diac is referred to as a "gateless triac".

Introduction

DIACs are diodes that conduct current only after reaching their breakover voltage, VBO. They exhibit behavior similar to a triac's voltage breakdown without a gate terminal. When breakdown occurs, positive feedback mechanisms cause the diode to enter a region of negative dynamic resistance, resulting in a rapid increase in current and decrease in voltage drop across the diode. Conduction continues until the current drops below the holding current, IH, at which point the diode switches back to its high-resistance state. This behavior is bi-directional.

DIACs typically have a three-layer structure with a breakover voltage around 30 V and an on voltage below 3 V. Their characteristics resemble the striking and extinction voltages of a neon lamp but with better repeatability and lower voltages.

Unlike some other thyristors like TRIACs, DIACs lack a gate or trigger electrode. However, certain TRIACs, such as Quadrac, incorporate a built-in DIAC in series with the gate terminal for triggering purposes.

Due to the symmetrical characteristic curve, DIACs are often referred to as "symmetrical trigger diodes." Their terminals are labeled as A1 and A2 or MT1 and MT2, without anode and cathode designations.

Application of DIAC

The main application of a DIAC is in a TRIAC triggering circuit, where it is connected to the gate terminal of the TRIAC to control its conduction. Some other applications of a DIAC include:

1. Lamp dimmer circuit: The DIAC can be used in circuits that control the brightness of lamps or lights.
2. Heat control circuit: DIACs can be employed in circuits that regulate the amount of heat produced by a heating element.
3. Speed control of a universal motor: A DIAC can be utilized in circuits that control the speed of a universal motor, commonly found in appliances like mixers or power tools.
4. Triggering circuit for TRIAC: A DIAC can be combined with a TRIAC in a series configuration to trigger the conduction of the TRIAC. When the voltage across the DIAC exceeds its breakdown voltage, it conducts and triggers the TRIAC. Conversely, when the voltage across the DIAC falls below its breakdown voltage, it turns off, keeping the TRIAC in the off state.

Working of Diac

A Diac is a bidirectional device, which means it conducts current in both directions. It consists of two terminals, denoted as A1 and A2. The Diac is a type of diode that exhibits negative resistance characteristics. Negative resistance means that the current decreases with an increase in voltage across the device.

The Diac is primarily used for triggering triacs and thyristors in AC applications. When the voltage across the Diac reaches its breakdown voltage, it starts conducting. However, once the current drops below a certain threshold, the Diac switches off. This behavior allows the Diac to trigger the gate of a triac or thyristor, enabling the control of power delivered to a load.

Construction of DIAC

The DIAC is a four-layer semiconductor device with two p-type (P1 and P2) and two n-type (N1 and N2) layers. It does not have a gate terminal like some other semiconductor devices. Its construction is similar to a transistor but with a few differences:

1. No base terminal: Unlike a transistor, a DIAC does not have a base terminal.
2. Uniform doping: The three regions in a DIAC have nearly equal doping levels.
3. Symmetrical switching: The DIAC provides symmetrical switching characteristics, meaning it can switch for both positive and negative voltages.

DIAC Characteristics

A DIAC is a four-layer semiconductor device with two p-type layers and two n-type layers. It lacks a gate terminal.

The DIAC can conduct current in both polarities. When A2 is more positive than A1, current flows through P2-N2-P1-N1. When A1 is more positive than A2, current flows through P1-N2-P2-N3.

At low applied voltages, a small leakage current occurs due to electron and hole drift in the depletion region. This current is insufficient to cause avalanche breakdown, keeping the device non-conductive.

When the applied voltage exceeds the breakdown voltage, the DIAC conducts according to its V-I characteristics, which resemble the letter "Z". The DIAC behaves as an open circuit when the voltage is below its avalanche breakdown voltage. To turn off the device, the voltage must be reduced below the breakdown voltage.

Summary of Diac

The summary of Diac is that it is a bi-directional semiconductor device that conducts current when the voltage applied across it exceeds a certain threshold in either direction.Its main advantage is that it does not switch sharply to a low voltage condition at a low current level like SCR or TRIAC. Additionally, it has a low on-state voltage drop until its current falls below the holding current level, and the voltage drop decreases as the current increases. These characteristics make the DIAC suitable for applications where smooth voltage control and triggering are required, show on sith.