Question 1(a) [3 marks]#
Draw and Explain the V-I Characteristics of TRIAC.
Answer: TRIAC (Triode for Alternating Current) is a bidirectional three-terminal semiconductor device that can conduct current in either direction when triggered.
Diagram:
I
↑
│ MT2
│ /│\
│ / │ \
│ / │ \
Quadrant III / G \ Quadrant I
│ / │ \
│ / │ \
──────┼──/──────┼──────\────→ V
│ / │ \
│/ │ \
│\ │ /
│ \ │ /
│ \ │ /
Quadrant IV \ │ / Quadrant II
│ \ │ /
│ \ │ /
↓ \ │ /
\ │ /
\│/
MT1
- Bidirectional operation: TRIAC conducts in both directions (positive and negative half cycles)
- Quadrant operation: Functions in all four quadrants based on polarity of MT2 and gate
- Triggering voltage: Breakdown occurs at ±VBO in either direction
- Holding current: Minimum current to maintain conduction
Mnemonic: “Two Rectifiers In A Case”
Question 1(b) [4 marks]#
Explain working of SCR using two transistor analogy.
Answer: SCR (Silicon Controlled Rectifier) can be represented as interconnected PNP and NPN transistors.
Diagram:
Anode
│
│
┌─┴─┐
│ │
┌───┤ P ├───┐
│ │ │ │
│ └───┘ │
│ │
│ ┌───┐ │
└───┤ N ├───┘
│ │
┌───┤ ├───┐
│ └───┘ │
│ │
│ ┌───┐ │
└───┤ P ├───┘
│ │
└─┬─┘
│
│
Cathode
- Two-transistor structure: PNP (Q1) and NPN (Q2) connected such that collector of each transistor drives the base of other
- Regenerative feedback: Once both transistors start conducting, they keep each other in saturation
- Triggering: Applying gate current to Q2 base starts the regenerative process
- Latching: Once triggered, SCR remains ON even if gate signal is removed
Mnemonic: “Pull Neat Path”
Question 1(c) [7 marks]#
Draw the circuit diagram of photo electric relay using LDR and explain it Working.
Answer: A photoelectric relay using LDR (Light Dependent Resistor) is a light-activated switching circuit.
Circuit Diagram:
+Vcc
│
├───────┐
│ │
R1 │
│ │
│ │
├───────┤
│ │
│ LDR
│ │
│ │
┌─┴─┐ │
│ B │ │
───┤ ├─────┘
│ C │
└─┬─┘
│
│
├─────────┐
│ │
R2 Relay
│ │
│ │
└─────────┴───── GND
- Light sensing: LDR resistance decreases in presence of light
- Transistor operation: When light falls on LDR, voltage at transistor base changes
- Relay switching: Transistor conducts/cuts off based on light, activating/deactivating relay
- Threshold adjustment: Potentiometer R1 sets light sensitivity level
- Applications: Automatic street lights, burglar alarms, automatic door openers
Mnemonic: “Light Detects Readily”
Question 1(c OR) [7 marks]#
Draw the gate pulse trigger circuit using UJT for SCR and explain its working.
Answer: UJT (Unijunction Transistor) provides reliable trigger pulses for SCR.
Circuit Diagram:
+Vcc
│
│
R1
│
│
┌────┴────┐
│ │
│ B2
│ UJT │
│ │
│ B1 │
└────┬────┘
│
R3
│
┌───┴────┐
│ │
C SCR Gate
│ │
│ │
└────────┴──── GND
- RC timing: R1 and C form charging circuit that determines pulse frequency
- UJT operation: UJT fires when capacitor voltage reaches peak point voltage
- Pulse generation: UJT discharges capacitor producing sharp trigger pulse
- SCR triggering: Pulse applied to SCR gate turns it ON at specific points in AC cycle
- Frequency control: Adjusting R1 changes pulse frequency for phase control
Mnemonic: “Uniform Junctions Trigger”
Question 2(a) [3 marks]#
State Triggering methods of SCR.
Answer:
| Triggering Method | Operating Principle | Advantages |
|---|---|---|
| Gate Triggering | Current applied to gate terminal | Most common, precise control |
| Thermal Triggering | Temperature rise causes leakage | Simple, no external circuit |
| Light Triggering | Photons create electron-hole pairs | Electrical isolation, used in LASCRs |
| dv/dt Triggering | Rapid voltage rise causes turn-on | Useful for protection circuits |
| Forward Voltage Triggering | Exceeding breakover voltage | No gate connection needed |
Mnemonic: “Good Triggers Let Devices Fire”
Question 2(b) [4 marks]#
What is Commutation of SCR? Explain class-E commutation.
Answer: Commutation is the process of turning OFF an SCR by reducing its anode current below holding current.
Class-E Commutation (Complementary Commutation):
L1
AC ┌────┐────┐
Source │ │ │
┌────┴─┐ │ ┌─┴────┐
│ │ │ │ │
│ SCR1 │ │ │ SCR2 │
│ │ │ │ │
└────┬─┘ │ └─┬────┘
│ │ │
└────┘────┘
Load
- Complementary switching: Uses another SCR in opposite half-cycle
- Natural commutation: AC source crosses zero, anode current falls below holding current
- Application: AC power control circuits, cycloconverters
- Advantage: No additional commutation components required
Mnemonic: “Complementary Elements”
Question 2(c) [7 marks]#
Draw and explain Snubber Circuit for SCR.
Answer: A snubber circuit protects SCR from voltage transients and dv/dt turn-on.
Circuit Diagram:
┌─────────┐
│ │
AC ┌──┴──┐ Rs
Source│ │ ├───┐
┌───┤ SCR ├─────┘ │
│ │ │ │
│ └──┬──┘ │
│ │ Cs│
│ │ │
│ │ │
│ └────────────┘
│
└─────────────────── Load
- RC network: Series resistor (Rs) and capacitor (Cs) connected across SCR
- Transient suppression: Capacitor absorbs voltage spikes that could damage SCR
- dv/dt protection: Prevents false triggering due to rapid voltage rise
- Turn-off assistance: Helps in commutation by providing alternate current path
- Component selection: Cs based on load current, Rs limits discharge current
Mnemonic: “Safely Neutralizes Unwanted Breakover”
Question 2(a OR) [3 marks]#
Explain over current protection method of SCR.
Answer:
| Protection Method | Working Principle | Applications |
|---|---|---|
| Fuses | Melts when current exceeds rating | Simple, economical protection |
| Circuit Breakers | Trips on overload, can be reset | Reusable protection |
| Current Limiting Reactors | Limits fault current magnitude | Industrial power control |
| Electronic Current Limiting | Senses current and controls gate | Precise protection |
| Crowbar Circuit | Shorts power supply on overload | Protects sensitive loads |
Mnemonic: “Fault Current Causes Equipment Damage”
Question 2(b OR) [4 marks]#
Explain the working of opto-SCR.
Answer: An opto-SCR (or Light Activated SCR) combines a light source and SCR in an isolated package.
Diagram:
┌───────────────┐
│ ┌───┐ │
│ │ │ │
LED │ │ ◄─┼───┐ │
Anode ├───┤LED│ │ │
│ │ │ │ │
│ └───┘ │ │
LED │ │ │
Cathode├───────────┘ │
│ │
│ ┌───┐ │
│ │ │ SCR│
SCR │ │ S ├───Anode
Gate ├──────┤ │ │
│ │ C │ │
│ │ R │ │
SCR │ └───┘ │
Cathode├───────────────┘
│ │
└───────────────┘
- Electrical isolation: LED optically triggers SCR without electrical connection
- Noise immunity: Immune to electrical noise and interference
- High-voltage isolation: Separates control and power circuits
- Applications: Industrial control, high-voltage switching
Mnemonic: “Light Activates Silicon Control”
Question 2(c OR) [7 marks]#
What is force commutation? Explain any two.
Answer: Force commutation is artificially turning OFF an SCR by reducing its anode current below holding level.
1. Class A Commutation (Self-Commutation):
┌───┐
│ │ L
│ ├────┐─────┐
AC │ │ │ │
Source┤ │ │ SCR
│ │ │ │
└───┘ C │
│ │
└─────┘
Load
- LC resonant circuit: Parallel L-C across SCR creates oscillations
- Reverse current: L-C circuit forces reverse current through SCR
- Applications: Inverters, choppers
2. Class B Commutation (Resonant Pulse Commutation):
Commutating
Switch
┌───┐ ┌───┐
│ │ │ │
AC │ │ L │ │
Source┤ ├────┐─┴─┐
│ │ │ │
└───┘ │ │
SCR C
│ │
└────┘
Load
- External switch: Additional SCR or switch triggers commutation
- Energy storage: L-C circuit stores energy then reverses SCR current
- Applications: DC choppers, controlled rectifiers
Mnemonic: “Force Circuit Reversal”
Question 3(a) [3 marks]#
Explain 1-φ full Wave bridge-controlled rectifier using four diodes & one SCR.
Answer: This circuit combines diodes and an SCR for controlled single-phase full-wave rectification.
Circuit Diagram:
D1 D2
┌───┬───┐───┬───┐
│ │ │ │ │
│ ▼ │ ▼ │
│ │ │
AC │ │ │ Load
Source┤ │ ├───R───┐
│ │ │ │
│ ▲ │ ▲ │ │
│ │ │ │ │ │
└───┴───┘───┴───┘ │
D3 SCR D4 │
GND
- Bridge configuration: Four diodes arranged in bridge with one replaced by SCR
- Variable output: SCR controls conduction angle and thus output voltage
- Economical design: Uses only one SCR instead of two or four
- Efficiency: Higher than half-wave controlled rectifier
Mnemonic: “Blend Diodes Smartly”
Question 3(b) [4 marks]#
What is Chopper? What are its application?
Answer:
| Aspect | Description |
|---|---|
| Definition | DC-DC converter that converts fixed DC input to variable DC output |
| Working Principle | Periodically switches DC input ON/OFF at high frequency |
| Types | Step-down (Buck), Step-up (Boost), Buck-Boost, Cuk |
| Control Methods | PWM, Frequency modulation, Current-limit control |
| Applications | DC motor speed control, Battery chargers, UPS, Solar systems, Electric vehicles |
Mnemonic: “Chops Current Perfectly”
Question 3(c) [7 marks]#
Draw and explain the circuit diagram of static switch using SCR for 1-φ A.C. Load.
Answer: A static switch using SCR provides non-mechanical switching for AC loads.
Circuit Diagram:
SCR1
┌──┐
┌───────┤ ├────┐
│ └──┘ │
│ │
AC │ │ AC
Source┤ ├── Load
│ │
│ ┌──┐ │
└───────┤ ├────┘
└──┘
SCR2
│
│
│
┌────┴────┐
│ Trigger │
│ Circuit │
└─────────┘
- Antiparallel SCRs: Two SCRs connected in inverse parallel for bidirectional conduction
- Gate control: Properly timed gate signals control power to load
- Zero-crossing switching: SCRs naturally turn OFF at zero crossing
- Applications: Heater control, motor soft-starting, lighting control
- Advantages: No moving parts, silent operation, long life
Mnemonic: “Solid Switching Technology”
Question 3(a OR) [3 marks]#
Explain basic principle of DC Chopper.
Answer:
| Component | Function |
|---|---|
| Switching Device | SCR, MOSFET, IGBT switches DC at high frequency |
| Control Circuit | Generates PWM gate signals to control ON/OFF time |
| Duty Cycle | Ratio of ON time to total time period determines output |
| Output Filter | Smooths chopped output to reduce ripple |
| Working Principle | Average voltage = Input voltage × Duty cycle |
Mnemonic: “Direct Current Control”
Question 3(b OR) [4 marks]#
Write short note on: Un-interrupted Power Supply (UPS).
Answer: UPS provides emergency power when main supply fails.
Block Diagram:
┌─────────┐ ┌─────────┐ ┌─────────┐
│ Mains │ │Rectifier │ │ Inverter│
│ Input ├────┤ & DC ├────┤ & AC ├─── Output
│ (AC) │ │ Section │ │ Section │ (AC)
└─────────┘ └─────────┘ └─────────┘
│
┌───┴───┐
│Battery │
│System │
└───────┘
- Backup power: Provides continuous power during outages
- Types: Online, Offline, Line-interactive UPS
- Protection: Against power surges, sags, and frequency variations
- Applications: Computers, medical equipment, telecommunications
Mnemonic: “Uninterrupted Power Securely”
Question 3(c OR) [7 marks]#
Draw the block diagram of SMPS and explain the function of each block.
Answer: Switched-Mode Power Supply converts AC to regulated DC efficiently.
Block Diagram:
┌─────────┐ ┌─────────┐ ┌─────────┐ ┌─────────┐ ┌─────────┐
│ Mains │ │ Input │ │High-Freq│ │Output │ │Output │
│ Input ├────┤ Rectifier├────┤Switching├────┤Rectifier├────┤ Filter ├─── DC Output
│ (AC) │ │& Filter │ │ Circuit │ │& Filter │ │ │
└─────────┘ └─────────┘ └─────────┘ └─────────┘ └─────────┘
│
┌────┴────┐
│ Control │
│ Circuit │
└─────────┘
- Input rectifier: Converts AC to unregulated DC
- High-frequency switching: Converts DC to high-frequency AC using transistors
- Transformer: Provides isolation and voltage scaling
- Output rectifier: Converts high-frequency AC to DC
- Filter: Smooths DC output to reduce ripple
- Control circuit: Regulates output through feedback
Mnemonic: “Switch Mode Power System”
Question 4(a) [3 marks]#
Draw the circuit diagram using TRIAC for speed control of 1-φ DC Shunt motor and Explain its working.
Answer: TRIAC-based speed control for a DC shunt motor provides efficient variable speed.
Circuit Diagram:
┌────────┐ ┌────────┐ ┌───────┐
AC │ │ │ │ │ DC │
Source┤ TRIAC ├───┤ Bridge ├──────┤ Shunt │
│ │ │Rectifier│ │ Motor │
└────────┘ └────────┘ └───────┘
│
┌───┴───┐
│ DIAC │
│ │
└───┬───┘
│
┌───┴───┐
│ │
│ R │
│ │
└───┬───┘
│
┌───┴───┐
│ C │
│ │
└───────┘
- Phase control: TRIAC varies effective voltage through phase angle control
- Rectification: Bridge rectifier converts AC to DC for motor
- Speed variation: Motor speed proportional to applied voltage
- RC timing: RC network determines firing angle of TRIAC
Mnemonic: “TRIAC Regulates Speed”
Question 4(b) [4 marks]#
Draw and explain the circuit diagram four stage sequential timer using IC-556.
Answer: IC-556 dual timer can be configured as a multi-stage sequential timer.
Circuit Diagram:
Vcc
│
├─────┬─────┬─────┬─────┐
│ │ │ │ │
R1 R2 R3 R4 │
│ │ │ │ │
├─────┴─────┴─────┴─────┤
│ │
│ IC-556 │
│ │
├───┬───┬───┬───────────┤
│ │ │ │ │
C1 C2 C3 C4 │
│ │ │ │ │
└───┴───┴───┴───────────┘
│ │ │
O1 O2 O3 O4
- Dual timer IC: IC-556 contains two 555 timer circuits
- Cascaded configuration: Output of one stage triggers the next
- Timing control: RC time constants determine duration of each stage
- Applications: Industrial sequencing, process control, automation
Mnemonic: “Sequential Steps Timed Precisely”
Question 4(c) [7 marks]#
Explain induction heating.
Answer: Induction heating is a non-contact heating process using electromagnetic induction.
Diagram:
┌───────────────┐
│ High-Frequency│
│ Power Supply │
└───────┬───────┘
│
┌───────┴───────┐
│ Induction │
│ Coil │
└───────┬───────┘
│
┌───────┴───────┐
│ Workpiece │
│ (Conductive │
│ Material) │
└───────────────┘
| Principle | Description |
|---|---|
| Electromagnetic Induction | AC in coil creates alternating magnetic field |
| Eddy Currents | Magnetic field induces currents in workpiece |
| Resistive Heating | Eddy currents generate heat due to material resistance |
| Skin Effect | Current concentrates near surface at high frequencies |
| Applications | Heat treatment, melting, forging, brazing, cooking |
Mnemonic: “Induced Heating Efficiently”
Question 4(a OR) [3 marks]#
Draw and explain three stage IC555 timer circuit.
Answer: A three-stage timer using IC555 provides sequential timing operations.
Circuit Diagram:
Vcc
│
┌───────┴───────┐
│ Reset │
┌────┤4 8├──┐
│ │ │ │
│ ┌──┤2 IC555 3├──┴────┐
│ │ │ │ │
R1│ │ │7 │ │
│ │ │ │ R4
│ │ │6 │ │
├─┘ │ │ │
│ C1 C2 │
│ │ │ │
└────┴──────┬──────┴──────┘
│
O1
- Monostable mode: Each stage operates in monostable mode with fixed time delay
- Cascaded connection: Output of first timer triggers second, and so on
- Timing components: R-C network determines time delay of each stage
- Applications: Automatic sequencing, process timing, industrial control
Mnemonic: “Time Intervals Created”
Question 4(b OR) [4 marks]#
Explain the principle of dielectric heating.
Answer:
| Principle | Description |
|---|---|
| High-Frequency Electric Field | Material placed between electrodes with RF voltage (1-100 MHz) |
| Molecular Friction | Dipole molecules vibrate/rotate trying to align with alternating field |
| Heat Generation | Internal friction between molecules generates heat uniformly |
| Non-Conductive Materials | Effective for heating non-conductive materials (plastics, wood, food) |
| Applications | Plastic welding, wood drying, food processing (microwave ovens) |
Mnemonic: “Dielectric Energy Heats”
Question 4(c OR) [7 marks]#
Make comparison between Induction heating and Dielectric heating.
Answer:
| Parameter | Induction Heating | Dielectric Heating |
|---|---|---|
| Basic Principle | Electromagnetic induction | High-frequency electric field |
| Suitable Materials | Conductive materials (metals) | Non-conductive materials (plastics, wood) |
| Frequency Range | 1 kHz to 1 MHz | 1 MHz to 1 GHz |
| Heating Mechanism | Eddy currents and hysteresis | Molecular friction (dipole rotation) |
| Heat Distribution | Surface heating (skin effect) | Volumetric (uniform throughout) |
| Efficiency | 80-90% for magnetic materials | 50-70% depending on material |
| Applications | Metal melting, forging, heat treatment | Plastic welding, food processing, drying |
| Equipment | Induction coil, work piece | Electrodes, dielectric material |
Mnemonic: “ICED” - Induction Conductive, Eddy currents; Dielectric, Dipoles
Question 5(a) [3 marks]#
Explain Construction and working of Universal Motor.
Answer: Universal motor operates on both AC and DC power sources.
Diagram:
┌───┐
│ │
│ │
│ │
┌─────┴───┴─────┐
│ Field Winding │
│ ┌─────┐ │
│ │ │ │
│ │Rotor│ │
│ │ │ │
│ └─────┘ │
│ │
└───────────────┘
Brushes
- Series connection: Field winding in series with armature winding
- Construction: Stator with field winding, rotor with commutator and brushes
- Operating principle: Same direction torque on both AC and DC
- Characteristics: High starting torque, high speed at low load
- Applications: Portable tools, household appliances, blenders
Mnemonic: “Universally Motorized”
Question 5(b) [4 marks]#
Draw and explain the construction of DC servo motor.
Answer: DC servo motor provides precise position or speed control.
Diagram:
┌─────────────┐
│ Permanent │
│ Magnet │
│ Stator │
│ ┌─────┐ │
│ │ │ │
│ │Rotor│ │
│ │ │ │
│ └─────┘ │
│ │
└─────┬───────┘
│
┌─────┴─────┐
│ Encoder │
│ Feedback │
└───────────┘
- Construction: Permanent magnet stator, lightweight rotor, feedback device
- Control system: Closed-loop control with position/velocity feedback
- Low inertia: Allows quick response and precise positioning
- Applications: Robotics, CNC machines, positioning systems
- Features: High torque-to-inertia ratio, fast response, accuracy
Mnemonic: “Servo System Control”
Question 5(c) [7 marks]#
Draw the block diagram of Programmable logic Control (PLC) and explain the Function of each block.
Answer: PLC is an industrial digital computer for automation control.
Block Diagram:
┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│ │ │ │ │ │
│ Input │ │ Central │ │ Output │
│ Modules ├────┤ Processing ├────┤ Modules │
│ │ │ Unit │ │ │
└─────────────┘ └──────┬──────┘ └─────────────┘
│
┌───────────────────┼───────────────────┐
│ │ │
┌────┴─────┐ ┌─────┴─────┐ ┌────┴─────┐
│ Memory │ │Programming│ │ Power │
│ Unit │ │ Device │ │ Supply │
└──────────┘ └───────────┘ └──────────┘
- CPU (Central Processing Unit): Executes program, processes I/O data, makes decisions
- Input modules: Convert field signals (sensors, switches) to digital signals for CPU
- Output modules: Convert CPU commands to actuator signals (motors, valves)
- Memory unit: Stores program and data (ROM for OS, RAM for user program)
- Programming device: PC or console for program development and monitoring
- Power supply: Provides regulated power to PLC components
Mnemonic: “Programs Logic Completely”
Question 5(a OR) [3 marks]#
Draw and explain the construction of Stepper motor.
Answer: Stepper motor rotates in discrete steps for precise positioning.
Diagram:
┌───────────┐
│ │
│ Stator │
│ ┌─────┐ │
│ │ │ │
│ │Rotor│ │
│ │ │ │
│ └─────┘ │
│ │
└───────────┘
Phases
- Stator: Contains multiple coil windings (phases)
- Rotor: Permanent magnet or variable reluctance type
- Types: Permanent magnet, variable reluctance, hybrid
- Step angle: Typically 1.8° (200 steps/rev) or 0.9° (400 steps/rev)
- Applications: Printers, disk drives, robotics, CNC machines
Mnemonic: “Steps Precisely Moved”
Question 5(b OR) [4 marks]#
Draw explain solid state circuit to control DC shunt Motor Speed.
Answer: Solid-state circuit provides efficient and smooth DC motor speed control.
Circuit Diagram:
+Vdc
│
│ ┌──────┐
├───────────┤ Field │
│ │ Winding│
│ └──────┘
│
┌─┴─┐
│ │ ┌──────┐
│PWM├─────┤ MOSFET│ ┌──────┐
│ │ │Driver │────┤MOSFET│
└───┘ └──────┘ │ │
┌┴──────┴┐
│Armature│
│Winding │
└────────┘
- PWM controller: Generates variable duty cycle pulses to control speed
- MOSFET driver: Provides gate drive to power MOSFET
- Power MOSFET: Controls current through armature winding
- Feedback: Tachogenerator or encoder provides speed feedback
- Advantages: Efficient, smooth control, wide speed range
Mnemonic: “Power With MOSFET”
Question 5(c OR) [7 marks]#
Explain the Working of VFD (Variable Frequency Drive).
Answer: VFD controls AC motor speed by varying frequency and voltage.
Block Diagram:
┌─────────┐ ┌─────────┐ ┌─────────┐ ┌─────────┐
│ AC │ │Rectifier │ │DC Link │ │Inverter │ ┌─────────┐
│ Input ├────┤ Circuit ├────┤Capacitor├────┤ Circuit ├────┤ AC │
│ │ │ │ │ │ │ │ │ Motor │
└─────────┘ └─────────┘ └─────────┘ └─────────┘ └─────────┘
│
┌────┴────┐
│ Control │
│ Circuit │
└─────────┘
| Component | Function |
|---|---|
| Rectifier | Converts AC input to DC (diode bridge or active front end) |
| DC Link | Filters DC and stores energy (capacitors, sometimes inductors) |
| Inverter | Converts DC to variable frequency AC (IGBTs with PWM) |
| Control Circuit | Regulates frequency/voltage based on speed requirement |
| Braking Circuit | Dissipates regenerative energy during deceleration |
- Speed control: Motor speed proportional to frequency (RPM = 120f/P)
- Torque control: Maintains V/f ratio for constant torque
- Energy savings: Reduces energy consumption at lower speeds
- Applications: Pumps, fans, conveyors, process control
- Features: Soft start, overcurrent protection, regenerative braking
Mnemonic: “Vary Frequency, Drive motor”