--- title: "DIAC drawback; TRIAC four-quadrant operation" subject: "Advanced Electronics" chapter: 7 key: "semester_4/advanced_electronics:diac-drawback-triac-quadrants" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/diac-drawback-triac-quadrants site: BCT Engineering --- # DIAC drawback; TRIAC four-quadrant operation Advanced Electronics · Chapter 7: Introduction to Power Electronics - Asked in: 2071 Chaitra, 2+5 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/diac-drawback-triac-quadrants - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7 ## Question Mention the drawback of DIAC. Describe the operating principle of TRIAC with necessary diagrams and explain why it is called four quadrant operation. ## Short answer DIAC has no gate, so its breakover voltage is fixed (~30V) and it is only a low-power trigger device. A TRIAC is a bidirectional thyristor; it conducts in both directions after gate triggering. Four-quadrant operation means triggering is possible with any combination of MT2 and gate polarity (I+, I−, III+, III−). ## Answer #### Drawbacks of DIAC - It has **no gate terminal**, so its switching point cannot be controlled; it turns on only when the applied voltage reaches its fixed breakover voltage (about 30 V). - It is a **low-power device**; it cannot handle large load currents, so it is used only as a trigger device for TRIACs/SCRs. - It has a **negative resistance region** and a fairly large voltage drop after breakover, and its breakover voltage varies from device to device. #### Operating principle of TRIAC A **TRIAC** is a bidirectional, three-terminal thyristor (MT1, MT2, gate G), equivalent to two SCRs in inverse parallel with a common gate. It is a five-layer device (N-P-N-P-N between MT1 and MT2), with an N region also near the gate. ```text MT2 | +----+----+ | | SCR1 -> <- SCR2 | | +----+----+ | G MT1 ----+ ``` - **MT2 positive w.r.t. MT1:** the P1-N1-P2-N2 path is forward biased. A gate pulse turns it ON and current flows MT2 → MT1. - **MT2 negative w.r.t. MT1:** the P2-N1-P1-N4 path is forward biased. A gate pulse turns it ON and current flows MT1 → MT2. - Once ON it latches until the current falls below the holding current (at every AC zero crossing). **V-I characteristic:** ```text I ^ on (quadrant I) | / | | -VBO | | ----|--------+---|-----> V(MT2-MT1) | | +VBO | | / | on (quadrant III) ``` Increasing gate current reduces the breakover voltage in both directions, just like an SCR. #### Why four-quadrant operation Both the MT2 voltage and the gate current can be of either polarity, giving four triggering modes: | Mode | MT2 (w.r.t. MT1) | Gate (w.r.t. MT1) | Sensitivity | |---|---|---|---| | I+ | Positive | Positive | Highest | | I− | Positive | Negative | Lower | | III+ | Negative | Positive | Lowest | | III− | Negative | Negative | High | Since the TRIAC can be triggered with any of the four combinations of anode (MT2) and gate polarity, i.e. in all four quadrants of the V–IG plane, it is called a **four-quadrant device**. In practice, modes I+ and III− are preferred because they need the least gate current. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a main drawback of a DIAC? A. No gate terminal, so its breakover cannot be controlled B. It needs a large gate current C. It cannot block voltage D. It conducts only in one direction Answer: A. It turns on only at its fixed breakover voltage (about 30 V). (Key point: DIAC drawback) 2. Because DIAC is a low-power device, it is used for? A. Driving heavy loads B. Triggering TRIACs/SCRs C. Rectification D. Voltage regulation Answer: B. It cannot handle large load currents. (Key point: DIAC use) 3. When MT2 is negative w.r.t. MT1, a gate pulse turns the TRIAC ON with current flowing? A. MT2 to MT1 B. It does not turn on C. MT1 to MT2 D. From the gate to MT2 Answer: C. The P2-N1-P1-N4 path is forward biased. (Key point: TRIAC operation) 4. Why is a TRIAC called a four-quadrant device? A. It has four terminals B. It has four PN junctions only C. It conducts at four different frequencies D. It can be triggered with MT2 and gate each positive or negative Answer: D. The four combinations I+, I−, III+, III− give four triggering modes. (Key point: Four quadrants) ## Related in Advanced Electronics - [Four- vs single-quadrant multiplier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Afour-vs-single-quadrant-multiplier) — Advanced Electronics, chapter 6 - [Short note: Working principle of TRIAC](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ashort-note-triac) — Advanced Electronics, chapter 7 - [Four-quadrant multiplier circuit and derivation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Afour-quadrant-multiplier-derive) — Advanced Electronics, chapter 6 - [Four-quadrant multiplier: concept and output](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Afour-quadrant-multiplier-concept) — Advanced Electronics, chapter 6 - [SCR two-transistor analogy; TRIAC firing angles](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ascr-analogy-triac-firing-25v) — Advanced Electronics, chapter 7 - [TRIAC firing angles (lamp circuit)](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atriac-firing-20v-lamp) — Advanced Electronics, chapter 7 ## Related in other subjects - [Four quadrant operation of a hoist](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-hoisting-mechanism) — Utilization of Electrical Energy, chapter 2 - [Four quadrant operation in DC motor control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-dc-motor-control) — Utilization of Electrical Energy, chapter 3 - [Triac vs diac; BJT vs MOSFET selection](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Atriac-diac-bjt-mosfet-selection) — Power Electronics, chapter 1 - [Static Ward-Leonard four quadrant control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Astatic-ward-leonard-four-quadrant) — Utilization of Electrical Energy, chapter 3