---
title: "Short note: Working principle of TRIAC"
subject: "Advanced Electronics"
chapter: 7
key: "semester_4/advanced_electronics:short-note-triac"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-triac
site: BCT Engineering
---

# Short note: Working principle of TRIAC

Advanced Electronics · Chapter 7: Introduction to Power Electronics

- Asked in: 2072 Kartik, 3 marks
- Video: [TRIAC (Basics, Working, Structure, Symbol, Characteristics & Application) Explained](https://www.youtube.com/watch?v=rIMexAWE6Cc) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-triac
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7

## Question

Write a short note on working principle of TRIAC.

## Short answer

A TRIAC is a bidirectional thyristor equivalent to two inverse-parallel SCRs. It conducts in both half-cycles when gated, turning off when current falls below holding current at each AC zero crossing; used in dimmers and motor speed control.

## Answer

A **TRIAC** (Triode AC switch) is a three-terminal, bidirectional thyristor that can conduct in both directions. Its terminals are MT1, MT2 and gate G. It is equivalent to **two SCRs connected in inverse parallel** with a common gate.

```text
        MT2
         |
    +----+----+
    |         |
   SCR1 ->   <- SCR2    (inverse parallel)
    |         |
    +----+----+
         |       G
        MT1 -----+
```

**Working principle:**

- When **MT2 is positive** with respect to MT1, a gate pulse (positive or negative) turns ON the left PNPN path and current flows MT2 → MT1.
- When **MT1 is positive** with respect to MT2, a gate pulse turns ON the other path and current flows MT1 → MT2.
- Once ON it latches until the current falls below the holding current, which happens naturally at each zero crossing of AC.
- It can be triggered in **four modes** (MT2 +/−, gate +/−); sensitivity is highest in quadrants I+ and III−.

**V-I characteristic** is identical in the first and third quadrants: blocking until breakover (reduced by gate current), then a low on-state voltage.

**Applications:** light dimmers, fan speed regulators, heater control, AC motor speed control, AC static switches. It is usually triggered by a DIAC.

**Limitations:** lower voltage/current and dv/dt ratings than SCRs, and suitable mainly for 50/60 Hz.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A TRIAC is equivalent to which arrangement?
   A. Two SCRs in series
   B. Two SCRs in inverse parallel with a common gate
   C. An SCR and a DIAC in parallel
   D. A single SCR with a diode
   Answer: B. This lets it conduct in both directions. (Key point: TRIAC equivalent)

2. How does a TRIAC turn OFF in an AC circuit?
   A. The gate pulse is removed
   B. A reverse gate pulse is applied
   C. Current falls below holding current at each zero crossing
   D. It never turns off
   Answer: C. Like an SCR, the gate loses control once ON. (Key point: Turn-off)

3. Which statement about TRIAC triggering is correct?
   A. Only a positive gate pulse works
   B. It conducts only when MT2 is positive
   C. It needs a DIAC reverse voltage to turn on
   D. It can be triggered by a positive or negative gate pulse in four modes, I+ and III− being most sensitive
   Answer: D. Gate polarity and MT2 polarity both can be either sign. (Key point: Triggering)

## Related in Advanced Electronics

- [Boost regulator working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aboost-regulator-principle) — Advanced Electronics, chapter 8, asked 2×
- [DIAC drawback; TRIAC four-quadrant operation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adiac-drawback-triac-quadrants) — Advanced Electronics, chapter 7
- [Dual slope ADC working; why accurate](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adual-slope-adc-accurate) — Advanced Electronics, chapter 3, asked 3×

## Related in other subjects

- [GPS, components and working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-components-working-principle) — Surveying II, chapter 8, asked 19×
- [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4×
- [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4×
- [MOSFET construction and working principle](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-construction-working) — Basic Electronics Engineering, chapter 3, asked 2×