A strong understanding of DIAC, TRIAC, and UJT is very important for ECET 2026 aspirants. These devices are widely used in power control, triggering circuits, and AC voltage regulation. Many ECET questions come directly from their working principles, VI characteristics, and applications. Knowing these devices will also help in later topics like SCR firing circuits, lamp dimmers, and motor speed control.
📘 Concept Notes
1. DIAC (Diode for Alternating Current)
- A two-terminal device that conducts current only after its breakover voltage (V_BO) is reached.
- It is a bidirectional device, meaning it can conduct in both directions once breakdown occurs.
- Mostly used to trigger TRIACs.
Key features:
- No gate terminal → conduction begins only after
. - VI characteristic → symmetrical in both quadrants.
- Application: Light dimmers, motor speed control, triggering circuits.
2. TRIAC (Triode for Alternating Current)
- A three-terminal, bidirectional thyristor.
- Can conduct current in both half cycles of AC, controlled by gate triggering.
- Equivalent to two SCRs connected in anti-parallel with a common gate.
Terminals:
- MT1, MT2 (main terminals), Gate (G).
Applications:
- Fan regulators
- AC motor speed control
- Lamp dimming circuits
Operation:
- Triggered into conduction by applying a gate pulse.
- Latches until current falls below holding current
.
3. UJT (Unijunction Transistor)
- A three-terminal device: Emitter (E), Base1 (B1), Base2 (B2).
- Has a negative resistance region in its VI characteristic.
- Mainly used for triggering SCRs and TRIACs in phase control circuits.
Working principle:
- When emitter voltage
reaches the peak-point voltage 
, the device switches ON. 
→ intrinsic stand-off ratio
→ interbase voltage
→ diode drop
Applications:
- Relaxation oscillators
- Triggering SCRs and TRIACs
- Timing circuits
⚙️ Formulas
- DIAC breakover voltage:
- TRIAC holding current:
- UJT peak-point voltage:
- Intrinsic stand-off ratio:
🔟 10 MCQs
Q1. DIAC is mainly used for:
a) Rectification
b) Amplification
c) Triggering TRIAC
d) Switching DC
Q2. TRIAC can conduct in:
a) One direction only
b) Both directions
c) Only when forward biased
d) None
Q3. UJT has how many terminals?
a) 2
b) 3
c) 4
d) 5
Q4. The VI characteristic of DIAC is:
a) Unidirectional
b) Bidirectional
c) Linear
d) None
Q5. The peak-point voltage of UJT is given by:
a)
b) 
c) 
d) 
Q6. The intrinsic stand-off ratio of a UJT is:
a) 
b) 
c) 
d) 
Q7. Which device shows a negative resistance characteristic?
a) DIAC
b) TRIAC
c) UJT
d) SCR
Q8. The holding current of a TRIAC is:
a) Minimum current to keep it ON
b) Maximum current it can conduct
c) Gate triggering current
d) None
Q9. Which device is equivalent to two SCRs in anti-parallel?
a) UJT
b) DIAC
c) TRIAC
d) MOSFET
Q10. If 
, find of UJT.
a) 10.7V
b) 12.7V
c) 15.7V
d) 20.7V
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | c |
| 2 | b |
| 3 | b |
| 4 | b |
| 5 | a |
| 6 | b |
| 7 | c |
| 8 | a |
| 9 | c |
| 10 | b |
🧠 Explanations
- Q1: DIAC is mainly used to trigger TRIACs → (c).
- Q2: TRIAC can conduct in both directions → (b).
- Q3: UJT has 3 terminals (E, B1, B2) → (b).
- Q4: DIAC has symmetrical bidirectional VI curve → (b).
- Q5: Formula → → (a).
- Q6: By definition → (b).
- Q7: UJT shows negative resistance → (c).
- Q8: TRIAC requires minimum current to keep conducting → Holding current → (a).
- Q9: TRIAC = two SCRs in anti-parallel → (c).
- Q10:
→ (b).
🎯 Motivation / Why Practice Matters
In ECET 2026, questions from DIAC, TRIAC, and UJT are sure-shot scoring topics. They often appear as direct formula-based numericals or conceptual device behavior questions. Practicing them daily helps you gain speed in solving triggering circuit problems and builds confidence for advanced Power Electronics topics like SCR and inverter control.
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