What is a Triac?

Triac Definition

Do you know what is a triac?The triac is a bidirectional thyristor switch device widely used in AC circuits to regulate power, control motor speed, and dim lighting fixtures. It can conduct current during both positive and negative half-cycles, making it an efficient and versatile electronic component.

Material and structure of bidirectional thyristor

Material characteristics

The core material of triac is single crystal silicon, which is formed into P-type and N-type semiconductor regions through doping process. Its manufacturing process includes diffusion, photolithography and metallization processes, and finally forms a multi-layer PN junction structure. Common packaging materials are plastic (such as epoxy resin) or ceramic to ensure high temperature resistance and insulation performance.

Internal structure

Bidirectional thyristor adopts a five-layer three-terminal structure (NPNPN), with the two ends being the main electrodes A1 and A2 (without clear anode/cathode distinction), and the third end being the gate terminal (G). Its symmetrical design allows flow of current to be conducted in both negative directions and positive directions. Compared with the four-layer three-terminal structure of unidirectional thyristor (SCR), bidirectional thyristor achieves bidirectional control capability by adding an N region.

Working principle of bidirectional thyristor

Turn-on mechanism

The core characteristic of bidirectional thyristor is to achieve bidirectional conduction through gate triggering:

Forward triggering: When A2 is a positive voltage relative to A1, a positive pulse applied to the gate terminal can trigger conduction.

Reverse triggering: When A1 is a positive voltage relative to A2, a negative pulse applied to the gate terminal can also trigger conduction.

After triggering, the semiconductor device enters the on state until the current is lower than the holding current (usually a few mA to tens of mA) or the voltage is reversed.

Triggering quadrant

According to the relationship between the gate current direction and the main voltage polarity, the triggering mode of the bidirectional thyristor is divided into four quadrants (ⅠtoⅣ):

Ⅰ/Ⅲquadrant (commonly used): the gate electrode has the same polarity as A2, and the triggering sensitivity is high.

Ⅱ/Ⅳquadrant: the gate current required for triggering is larger, generally used for special circuit design.

Typical working circuits and applications

Basic voltage regulation circuit

In the AC voltage regulation circuit, the bidirectional thyristor is connected in series between the load and the power supply. The gate trigger phase angle is adjusted through the RC phase shift network. When the capacitor voltage reaches the breakdown threshold of the bidirectional trigger diode (DIAC), the TRIAC is turned on. The conduction angle can be adjusted by changing the resistance value to achieve a voltage output of 0%~100%.

Triac with optocoupler isolation drive

Use an optocoupler (such as MOC3041) to isolate the low-voltage control circuit from the high-voltage main circuit. The microcontroller output signal drives the LED inside the optocoupler, and the photosensitive TRIAC triggers the external high-power bidirectional thyristor, which is suitable for safety control in smart homes.

Motor speed controls and soft start

Motor control speed by adjusting the conduction period of the TRIAC. During the motor startup phase, gradually increasing the conduction angle can limit the starting current flow and avoid mechanical shock. Typical applications include industrial fans and pump equipment. IV.

Bidirectional thyristor application scenario expansion

Smart home system

Voice control lighting: Trigger TRIAC through voice commands, and combine with Wi-Fi module to achieve remote dimming.

Smart socket: Use TRIAC as a contactless switch to support variable power statistics and timed switch functions.

Industrial Automation

Electroplating power supply control: Accurately adjust the current density of the electrolytic cell to improve the uniformity of the coating. Injection molding machine temperature control: Cooperate with the PID algorithm to the power control of the heating coil to less than±1%.

New energy equipment

Photovoltaic inverter bypass: Quickly switch TRIAC when the power grid fails to achieve uninterrupted power supply.

Electric vehicle charging pile: Control the electronic lock and status indicator of the AC charging gun.

Medical equipment

Ultraviolet disinfection lamp: Adjust the intensity of the UV-C lamp through TRIAC application to meet the sterilization needs of different scenarios.

Ventilator flow control: Accurately adjust the speed of motor to ensure the stability of the airflow.

Agricultural Internet of Things

Greenhouse environment control: Link temperature and humidity sensors to automatically open and close ventilation equipment and heating systems.

Precision irrigation: Control the on and off time of the solenoid valve to achieve water-saving irrigation.

Differences between bidirectional thyristors and unidirectional thyristors

As two core components in the field of power electronic components, bidirectional thyristors (TRIACs) and unidirectional thyristors (SCRs) have significant differences in structure, function and application. Understanding these differences is crucial for practical circuit design.

Structural differences

Bidirectional thyristors use a five-layer three-terminal (NPNPN) semiconductor structure, with main electrodes A1 and A2 symmetrically distributed and the gate signal located in the middle layer. This design enables it to have bidirectional conduction capability, and current can flow in both positive half-cycle and reverse directions. The unidirectional thyristor is a four-layer three-terminal (PNPN) structure, which only allows current to flow from the anode (A) to the cathode (K) in one direction. It is essentially a unidirectional semiconductor device.

Triggering and conduction characteristics

The triggering method of bidirectional thyristors is more flexible. Regardless of the polarity of the main electrode voltage, it can be triggered to conduct by positive or negative gate current. Quadrant I (A2 positive voltage + positive trigger voltage) and quadrant III (A2 negative voltage + negative trigger voltage) modes are commonly used.

The unidirectional thyristor strictly relies on forward triggering: it can only be turned on when the anode voltage is higher than the cathode voltage, and the positive gate current is applied, and the reverse voltage will directly block the current. This feature requires an additional rectifier bridge or reverse parallel device when SCR controls AC power, while TRIAC can directly control AC waveform, greatly simplifying the circuit design.

Performance and application scenarios

Bidirectional thyristors have voltage crossover losses due to their bidirectional conduction characteristics, and are prone to electromagnetic interference (EMI) during the switching process. They need to be used with RC buffer circuits to suppress spikes. They are suitable for cost-sensitive AC control scenarios, such as light dimmer, fan speed regulation, and home appliance switches. Its typical advantage is that a single device can achieve full-wave control, but the operating frequency is usually limited to less than 1kHz.

Unidirectional thyristors are known for their low conduction losses and high-frequency characteristics. The electronic switch speed can reach MHz level, which is suitable for high-frequency inverters, DC power supply protection, and high-voltage DC transmission. For example, in photovoltaic inverters, SCRs can quickly cut off fault currents; in induction cookers, their high-frequency switching capabilities can accurately control heating power level. However, SCRs need to be used in pairs in AC applications, which increases circuit complexity and cost.

Selection considerations

The core advantage of typical TRIAC is that it simplifies the AC control architecture and is suitable for medium and low frequency scenarios such as smart homes and small motors; SCR is irreplaceable in the field of high frequency, high voltage, and high reliability. The actual selection requires a comprehensive evaluation of voltage level, current capacity, heat dissipation conditions, and EMI requirements. For example, TRIAC is preferred for controlling 220V AC lamps, while SCR arrays must be used in megawatt-level DC transmission systems. The two are not substitutes, but complementary technical solutions that jointly support the diversified needs of modern power electronic systems.

Selection and failure case analysis

Selection points

Voltage matching: Select VDRM≥1.5 times the peak voltage of the power supply (for example, select a model above 600V for 220V AC waveform). Heat dissipation design: At least 20mm²heat dissipation area is required for every 1A current, and ceramic package models are recommended for high temperature environments.

Typical faults and countermeasures

False triggering: Connect a 100Ωresistor + 0.01μF capacitor in parallel between the gate and A1 to absorb interference.

Overheating damage: Check whether the load current exceeds the standard and ensure that the heat sink is in good contact.

Voltage breakdown: Add a varistor (such as 14D471K) at both ends of the TRIAC for overvoltage protection.

Future development trends

Third-generation semiconductor applications

SiC-TRIAC: The withstand voltage is increased to 1700V, which is suitable for new energy grid-connected systems. GaN integrated module: The drive circuit is integrated with the TRIAC monolithic chip, and the volume is reduced by 50%.

Intelligent upgrade

Self-diagnosis function: Built-in temperature/current sensor, real-time feedback on the health status of electronic device.

Wireless control: Support Zigbee/Bluetooth communication, directly receive wireless signal trigger current.

Bidirectional thyristors continue to penetrate into emerging fields such as smart home, Industry 4.0, and new energy with their advantages of bidirectional conduction and simple control. With the advancement of materials and packaging technology, its application boundaries will be further expanded. Engineers need to have a deep understanding of its characteristics and optimize the design based on specific scenarios in order to fully realize the potential of the electronic components.