Concept Notes
1. Silicon Controlled Rectifier (SCR)
- SCR is a 4-layer, 3-junction, 3-terminal device.
- Terminals: Anode (A), Cathode (K), Gate (G).
- Symbol: diode + gate terminal.
- Works as a controlled switch.
Modes of Operation:
- Forward Blocking: Anode +ve, gate open, no conduction.
- Forward Conduction: Anode +ve, gate pulse applied → conducts.
- Reverse Blocking: Cathode +ve, behaves like reverse-biased diode.
Key Parameters:
- Latching current (
) → minimum current to latch ON after triggering. - Holding current (
) → minimum current required to keep SCR ON.
Formula for average current in resistive load (HWR with firing angle 
):
2. TRIAC (Triode for Alternating Current)
- A bidirectional device (like two SCRs connected in anti-parallel with one gate).
- Terminals: MT1, MT2, Gate (G).
- Can conduct current in both directions → used in AC power control.
- Triggered by positive or negative gate signal.
Key Advantages:
- Single device controls both half-cycles.
- Compact and cheaper than two SCRs.
Applications:
- Fan regulators
- Lamp dimmers
- Speed control of induction motors
⚙️ Formulas
SCR average output voltage (HWR with resistive load):
SCR RMS output voltage:
Average current in HWR:
Full-wave controlled rectifier average voltage:
🧮 Examples
Example 1:
An SCR half-wave rectifier has 
, firing angle 
.
Find average DC output voltage.
Example 2:
A TRIAC is used to control lamp brightness. If gate is fired at 
, only half power is delivered.
🔟 10 MCQs
Q1. An SCR is a device with how many layers and junctions?
a) 2 layers, 1 junction
b) 3 layers, 2 junctions
c) 4 layers, 3 junctions
d) 5 layers, 4 junctions
Q2. Which parameter defines minimum current to keep SCR ON?
a) Breakover current
b) Latching current
c) Holding current
d) Leakage current
Q3. The average output voltage of an SCR half-wave rectifier is:
a) 
b) 
c) 
d) 
Q4. In a full-wave controlled rectifier, average DC output voltage is:
a)
b) 
c) 
d)
Q5. TRIAC can conduct current in:
a) One direction
b) Two directions
c) No conduction
d) Only positive half cycle
Q6. Which is true for TRIAC?
a) Has 3 terminals: A, K, G
b) Has 3 terminals: MT1, MT2, G
c) Has 2 terminals only
d) Cannot work in AC
Q7. The firing angle in an SCR controls:
a) Output frequency
b) Output voltage
c) Input voltage
d) Current only
Q8. Application of TRIAC is:
a) Rectification only
b) Lamp dimmer
c) Amplifier
d) Oscillator
Q9. For 
, SCR rectifier acts as:
a) Normal diode rectifier
b) Switch OFF
c) Short circuit
d) Amplifier
Q10. A device that can replace two SCRs in anti-parallel is:
a) DIAC
b) TRIAC
c) UJT
d) BJT
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | c |
| Q2 | c |
| Q3 | b |
| Q4 | b |
| Q5 | b |
| Q6 | b |
| Q7 | b |
| Q8 | b |
| Q9 | a |
| Q10 | b |
🧠 Explanations
- Q1: SCR → 4 layers, 3 junctions.
- Q2: Holding current keeps device ON.
- Q3: Formula →
. - Q4: Full-wave SCR →
. - Q5: TRIAC → bidirectional.
- Q6: TRIAC terminals = MT1, MT2, Gate.
- Q7: Output voltage depends on .
- Q8: TRIAC → lamp dimmer, fan regulator.
- Q9: At
, acts like diode rectifier. - Q10: TRIAC replaces 2 SCRs in anti-parallel.
🎯 Motivation / Why Practice Matters
Industrial electronics = power control in real world 🌍.
- SCR → controls high voltage DC power.
- TRIAC → controls AC appliances like fans, lamps.
- ECET frequently asks formulas for
with firing angle .
👉 “Mastering SCR & TRIAC = mastering power control circuits.”
📲 CTA
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