Diacs Information

Last revised: February 5, 2025

Diacs are bidirectional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. SCRs are four-layer (PNPN) thyristors with an input terminal (gate), an output terminal (anode), and a common terminal (cathode) for both the input and output. Triacs are three-terminal silicon devices that function as two SCRs configured in an inverse, parallel arrangement, so as to provide load current during both halves of the AC supply voltage. Diacs, which are similar to open base NPN transistors, exhibit a high-impedance blocking state up to a voltage breakover point above which negative resistance is achieved. Except for a small leakage amount, diacs do not conduct current until a breakover voltage is attained. Because they are bidirectional, diacs are used as firing devices in phase control such as light dimmers and motion speed controls.

Performance specifications for diacs include breakover voltage, breakover voltage symmetry, breakover current, output voltage, repetitive peak on-state current, and power dissipation. Breakover voltage (V_BO), the voltage at which diacs begin to conduct, is measured between the input and output terminals when diacs switch on. Breakover voltage symmetry ( V_BO) is the maximum breakover voltage range with a specified capacitance when diacs are connected in parallel. Measured during the “on” state, output voltage (V_O) is the voltage across a 20-ohm resistor in series with a diac during the discharge of a specified capacitor. Repetitive peak on-state current (I_TRM) is the maximum limiting peak on-state current, including all repetitive transient currents, for which diacs are rated. Power dissipation (P_d) is the power dissipated by diacs during the “on” state.

Diacs are available in a variety of IC package types with different numbers of pins and diodes. Basic IC packages types for diacs include discrete packaging (DPAK), power packaging (PPAK), and in-line packaging (IPAK). Other package types include diode outline (DO), transistor outline (TO), and small outline transistor (SOT). Diacs that use metal electrode leadless face (MELF) packaging have metallized terminals at each end of a cylindrical body. Other available package types for diacs include thin small outline package (TSOP), thin shrink small outline L-leaded package (TSSOP), and thin small outline J-lead (TSOJ) package.

Diacs FAQs

What are the key differences between diacs and other types of semiconductor devices like triacs?

Functionality

Diacs: Diacs are bidirectional diodes that do not conduct until a specific breakover voltage is reached. They are primarily used to trigger other devices like triacs or silicon-controlled rectifiers (SCRs) by providing a firing pulse when the breakover voltage is exceeded.

Triacs: Triacs are three-terminal silicon devices that function as two SCRs configured in an inverse, parallel arrangement. They can conduct current during both halves of the AC supply voltage, making them suitable for controlling AC power, such as in motor speed controls and light dimmers.

Structure

Diacs: Diacs are similar to open base NPN transistors and exhibit a high-impedance blocking state until the breakover voltage is reached, after which they enter a negative-resistance region.

Triacs: Triacs are essentially two SCRs cross-connected with their gate inputs tied together. They latch on and continue to conduct until the current through the device goes to zero.

Applications

Diacs: Used as triggering devices in phase control applications, such as light dimmers and motor speed controls, due to their ability to provide a precise firing pulse.

Triacs: Used for controlling AC power in applications like motor speed control and dimming of incandescent lamps, as they can handle current in both directions of the AC cycle.

Conductivity

Diacs: Do not conduct current (except for a small leakage current) until the breakover voltage is reached.

Triacs: Once triggered, they continue to conduct until the current falls below a certain level, even if the triggering signal is removed.

What are the performance specifications for diacs?

Breakover Voltage (VBO)

This is the voltage at which diacs begin to conduct. It is measured between the input and output terminals when diacs switch on.

Breakover Voltage Symmetry (ΔVBO)

This refers to the maximum breakover voltage range with a specified capacitance when diacs are connected in parallel.

Breakover Current:

The current flowing at the breakover voltage.

Output Voltage (VO)

Measured during the “on” state, this is the voltage across a 20-ohm resistor in series with a diac during the discharge of a specified capacitor.

Repetitive Peak On-State Current (ITRM)

This is the maximum limiting peak on-state current, including all repetitive transient currents, for which diacs are rated.

Power Dissipation (Pd)

The power dissipated by diacs during the “on” state.

What are the differences between diacs and sidacs?

Functionality

Sidacs: Sidacs are typically used in pulse-firing and high-voltage transformer applications. They are designed to handle higher voltages and currents compared to diacs and are often used in circuits where current limiting is provided by the transformer primary.

Applications

Diacs: Used as triggering devices in phase control applications, such as light dimmers and motor speed controls, due to their ability to provide a precise firing pulse.

Sidacs: Commonly used in ignition circuits or inexpensive high-voltage power supplies, where they can handle high current pulses.

Construction and Characteristics

Diacs: Similar to open base NPN transistors, diacs exhibit a high-impedance blocking state until the breakover voltage is reached, after which they enter a negative-resistance region.

Sidacs: Operate in a manner similar to avalanche diodes, switching to an on state when supplied with enough current. They remain on as long as the holding current is below a maximum value.

Packaging

Diacs: Available in various IC package types, including discrete packaging, power packaging, and in-line packaging.

Sidacs: Also available in a variety of IC package types, including discrete packaging, power packaging, and in-line packaging, with additional options like dual in-line packages.

What is the role of diacs in phase control circuits?

Diacs play a crucial role in phase control circuits, particularly in applications like light dimmers and motor speed controls. Here is how they function in these circuits:

Triggering Device

Diacs are used as triggering devices in phase control circuits. They are bidirectional diodes that do not conduct until a specific breakover voltage is reached. Once this voltage is exceeded, diacs provide a firing pulse to trigger other devices like triacs or silicon-controlled rectifiers (SCRs).

Bidirectional Operation

Diacs are capable of conducting current in both directions, which makes them suitable for AC applications. This bidirectional capability allows them to effectively control the phase of the AC voltage applied to the load, such as a lamp or motor.

Phase Control

In phase control circuits, diacs help in controlling the point in the AC cycle at which the triac is triggered. By delaying the firing angle of the triac, diacs enable the control of the power delivered to the load. This is achieved by adjusting the phase angle at which the diac conducts, thereby controlling the amount of time the triac is on during each AC cycle.

Negative Resistance Region

Diacs exhibit a high-impedance blocking state up to a voltage breakover point, above which they enter a negative-resistance region. This characteristic is utilized in phase control circuits to create a bidirectional pulsing oscillator in a resistor-capacitor AC circuit, which is essential for triggering the triac at the desired phase angle.

What is the concept of breakover voltage in diacs?

The concept of breakover voltage in diacs is an important characteristic that defines their operation. Here is an explanation based on the information available:

Definition

Breakover voltage (VBO) is the voltage at which diacs begin to conduct. It is a critical parameter that determines when the diac will switch from a high-impedance blocking state to a conducting state.

Functionality

Diacs are bidirectional diodes that do not conduct current (except for a small leakage current) until the breakover voltage is reached. Once this voltage is exceeded, the diac provides a firing pulse to trigger other devices like triacs or silicon-controlled rectifiers (SCRs).

Operation

Up to the breakover voltage, diacs exhibit a high-impedance blocking state. When the voltage across the diac exceeds the breakover voltage, the device enters a negative-resistance region, allowing it to conduct current. This characteristic is utilized in phase control circuits to create a bidirectional pulsing oscillator in a resistor-capacitor AC circuit.

Applications

The breakover voltage is crucial in applications such as light dimmers and motor speed controls, where diacs are used as triggering devices. By controlling the breakover voltage, the phase angle at which the diac conducts can be adjusted, thereby controlling the power delivered to the load.

How are diacs are used in conjunction with triacs for phase control?

Diacs are used in conjunction with triacs for phase control in AC circuits, such as light dimmers and motor speed controls. Here is how they work together:

Triggering Mechanism

Diacs act as triggering devices for triacs. They are bidirectional diodes that do not conduct until a specific breakover voltage is reached. Once this voltage is exceeded, the diac provides a firing pulse to the gate of the triac, initiating its conduction.

Bidirectional Operation

Both diacs and triacs are capable of conducting current in both directions, which is essential for AC applications. This bidirectional capability allows them to control the phase of the AC voltage applied to the load effectively.

Phase Control

In phase control circuits, the diac helps control the point in the AC cycle at which the triac is triggered. By delaying the firing angle of the triac, the diac enables control over the power delivered to the load. This is achieved by adjusting the phase angle at which the diac conducts, thereby controlling the amount of time the triac is on during each AC cycle.

Negative Resistance Region

What are the applications of diacs in electronic circuits?

Diacs are versatile components used in various electronic circuits, particularly for phase control applications. Here are some key applications of diacs in electronic circuits:

Phase Control Circuits

Diacs are commonly used in phase control circuits, such as light dimmers and motor speed controls. They serve as triggering devices for triacs, enabling precise control over the power delivered to AC loads by adjusting the phase angle at which the triac is triggered.

Bidirectional Triggering

Due to their bidirectional nature, diacs can conduct current in both directions, making them suitable for AC applications. This characteristic is essential for controlling the phase of the AC voltage applied to the load, allowing for effective power regulation.

Negative Resistance Region

Diacs exhibit a high-impedance blocking state up to a voltage breakover point, after which they enter a negative-resistance region. This property is utilized in phase control circuits to create a bidirectional pulsing oscillator in a resistor-capacitor AC circuit, which is crucial for triggering the triac at the desired phase angle.

Triggering Triacs and SCRs

Diacs are used to trigger triacs and silicon-controlled rectifiers (SCRs) by providing a firing pulse once the breakover voltage is exceeded. This makes them integral components in circuits requiring precise control of power delivery.

What is the role of diacs in light dimmer circuits?

Diacs play a significant role in light dimmer circuits by serving as triggering devices for triacs. Here is how they function in these circuits:

Triggering Mechanism

Diacs are bidirectional diodes that do not conduct until a specific breakover voltage is reached. Once this voltage is exceeded, the diac provides a firing pulse to the gate of the triac, initiating its conduction.

Bidirectional Operation

Diacs can conduct current in both directions, which is essential for AC applications like light dimmers. This bidirectional capability allows them to control the phase of the AC voltage applied to the load effectively.

Phase Control

In light dimmer circuits, the diac helps control the point in the AC cycle at which the triac is triggered. By delaying the firing angle of the triac, the diac enables control over the power delivered to the light bulb. This is achieved by adjusting the phase angle at which the diac conducts, thereby controlling the amount of time the triac is on during each AC cycle.

Negative Resistance Region

Diacs exhibit a high-impedance blocking state up to a voltage breakover point, above which they enter a negative-resistance region. This characteristic is utilized in phase control circuits to create a bidirectional pulsing oscillator in a resistor-capacitor AC circuit, which is crucial for triggering the triac at the desired phase angle.