What is TRIAC? | How TRIAC Works
A TRIAC is a semiconductor device widely used for AC switching and control. In this detailed guide, we explain what a TRIAC is, how a TRIAC works, the construction, symbol, equivalent circuit, and various example circuits. Learn TRIAC switching, lamp dimming, solid-state relay applications, and AC motor control using TRIAC with a step-by-step explanation. This article explores TRIAC basics, TRIAC working principle, TRIAC applications, TRIAC examples, and TRIAC circuits with diagrams. Whether you’re a beginner or an experienced electronics hobbyist, this complete tutorial on TRIAC will help you understand the TRIAC device, TRIAC operation, and TRIAC usage in practical circuits. TRIAC is one of the most essential semiconductor components for power control in AC electronics.
Introduction
Hi friends, in this project we are going to see what a TRIAC is and how it works. We’ll also make some example circuits to better understand how a TRIAC functions in real-world applications. So without further ado, let’s start this project.
A TRIAC (Triode for Alternating Current) is a three-terminal semiconductor device that can conduct current in both directions when triggered. Unlike an SCR (Silicon Controlled Rectifier), which only allows current flow in one direction, the TRIAC can handle bidirectional AC power.
Because of this unique property, the TRIAC is one of the most widely used switching devices for controlling lamps, fans, heaters, and AC motors. With just a small gate trigger signal, the TRIAC can control large AC loads efficiently.
What is a TRIAC?
A TRIAC is a bidirectional device used for AC switching and power control.
It has three terminals:
MT1 (Main Terminal 1)
MT2 (Main Terminal 2)
Gate (G)
A TRIAC can be triggered by either a positive or negative gate pulse regardless of the polarity of the AC supply.
This makes it highly suitable for AC load control applications such as:
Lamp dimming
Motor speed control
Heater regulation
Solid-state relays
Construction of a TRIAC
The construction of a TRIAC is somewhat similar to two SCRs connected in anti-parallel with a common gate terminal.
It is made of five layers of semiconductor material.
This multilayer arrangement allows it to conduct in both positive and negative half-cycles of the AC waveform.
Symbol of a TRIAC
The symbol of a TRIAC looks like two SCRs joined together with a single gate terminal.
MT1 and MT2 represent the two main terminals.
The gate terminal is connected near MT1.
The arrow on the gate indicates the trigger input.
Equivalent Circuit of a TRIAC
The TRIAC can be represented as two SCRs connected in opposite directions but sharing a single gate terminal.
This equivalent circuit helps us understand that the TRIAC can conduct in both directions.
In each half-cycle of AC, one of the SCRs is triggered and allows conduction.
Working Principle of a TRIAC
When an AC signal is applied between MT1 and MT2:
No conduction occurs until a trigger signal is applied to the gate.
Once the gate receives a trigger pulse, the TRIAC switches ON and allows current to flow between MT1 and MT2.
The TRIAC remains conducting until the AC cycle reaches zero-crossing (natural commutation).
The firing angle (the point at which the TRIAC is triggered in each half-cycle) determines how much power is delivered to the load.
Key Points:
Positive or negative gate pulse can trigger the TRIAC.
Output depends on the triggering angle.
TRIAC naturally turns OFF at zero crossing of AC waveform.
Basic TRIAC Switching Circuit
One of the simplest applications of a TRIAC is as a switch for an AC lamp.
Circuit Description:
A push button is used to apply a trigger pulse to the gate.
When the button is pressed, the gate is triggered → TRIAC turns ON → Lamp lights up.
When the button is released, the TRIAC stops conducting at the next zero crossing → Lamp turns OFF.
Materials for the Project
Example Circuit 1: Solid-State Relay (SSR) with TRIAC
A solid-state relay (SSR) is an electronic switching device that uses a TRIAC and an optocoupler to control AC loads safely.
Working:
The low-voltage circuit (Arduino/microcontroller) sends a signal to the optocoupler.
The optocoupler triggers the TRIAC without any direct connection to AC mains.
This ensures isolation and safety.
Step-by-Step:
Connect the optocoupler input to Arduino.
Connect the optocoupler output to the TRIAC gate.
Connect the AC lamp to MT2.
Use Arduino code to send a HIGH/LOW signal to control the lamp.
Example Circuit 2: Digital Lamp Dimmer with TRIAC
Lamp dimming is one of the most popular uses of TRIAC.
Working:
Arduino monitors the zero-crossing point of AC waveform.
By delaying the gate trigger pulse, Arduino controls how much portion of AC is delivered to the lamp.
Smaller delay = Brighter lamp.
Larger delay = Dimmer lamp.
Step-by-Step:
Connect TRIAC in series with lamp.
Use MOC3021 optocoupler for triggering.
Use a zero-crossing detector circuit for synchronization.
Write Arduino code to vary the firing angle.
Example Circuit 3: Single-Phase AC Motor Speed Controller
TRIACs are also used for controlling single-phase induction motors.
Working:
By adjusting the firing angle, the effective voltage applied to the motor is controlled.
This changes the motor speed without needing bulky resistors or transformers.
Step-by-Step:
Connect motor in series with TRIAC.
Use optocoupler for gate triggering.
Use Arduino or manual phase control circuit.
Adjust delay to control motor speed.
Applications of TRIAC
Light dimmers
Fan speed regulators
Heater temperature control
Solid-state relays (SSR)
AC motor controllers
Industrial automation systems
Advantages of TRIAC
Controls AC power in both directions.
Requires only a small gate current.
Compact and lightweight compared to mechanical relays.
Cost-effective solution for AC load control.
Limitations of TRIAC
Cannot handle very high power (IGBTs and MOSFETs are preferred).
Generates harmonics in AC circuits.
Needs proper snubber circuits to avoid false triggering.
FAQs on TRIAC
Q1: What is a TRIAC used for?
A TRIAC is used for controlling AC loads such as lamps, heaters, and motors.
Q2: How does a TRIAC work?
A TRIAC works by receiving a gate pulse that allows it to conduct in both directions during AC cycles.
Q3: Is a TRIAC the same as an SCR?
No, an SCR conducts in one direction only, while a TRIAC can conduct in both directions.
Q4: Can I control a motor with a TRIAC?
Yes, a TRIAC can control single-phase AC motors by adjusting the firing angle.
Q5: Do I need an optocoupler with a TRIAC?
Yes, for safety, it’s best to use an optocoupler when triggering a TRIAC with a microcontroller.
Q6: What are common TRIAC models?
BT136, BT139, and MAC97A6 are common TRIACs for DIY projects.
Q7: Can TRIACs switch DC loads?
No, TRIACs are designed for AC; they don’t naturally commutate in DC circuits.
Conclusion
In this article, we learned what a TRIAC is, how a TRIAC works, and various example circuits. We covered TRIAC construction, symbol, equivalent circuit, and working principle. Then, we implemented practical circuits like a TRIAC switch, solid-state relay, lamp dimmer, and motor controller.
The TRIAC is a simple yet powerful device that makes AC load control easy, safe, and efficient.
So that’s it for today’s project. I hope you enjoyed it. For more projects like this, stay tuned, and I’ll see you in the next one.