Lecture Notes: Overvoltage Protection Using NPN Transistors

Introduction

• Overvoltage Protection: Shuts down/clamps output voltage when it exceeds a maximum value.
  • Causes: Power supply faults, external surges.
• Objective: Implement overvoltage protection using NPN transistors in a BJT circuit.
  • Use Case: Protecting load (e.g., LEDs, embedded systems).

Concept and Application

• Threshold Voltage: Set a limit above which control circuit reacts.
• BJT Usage: Protect circuits from high voltage.
• Example: Load (e.g., CCR with LEDs, embedded system) with input voltage range.

NPN Transistor in Overvoltage Protection

Circuit Design

• Series Connection: BJT in series with the load and supply.
  • Acts as a switch; cuts off supply when voltage exceeds.
  • Low Side Switch: Transistor placed after load (Q2).
• Resistor: Connect from supply to BJT base to keep it on.
• Second BJT: Added to pull base of Q2 to ground, turning it off.

Mechanism

• Voltage Divider: Provides ~0.7V to turn on the transistor.
  • Activates second BJT, which turns off Q2, cutting off the circuit.
• Example Setup:
  • Input Voltage: 13V.
  • Threshold: 18V; circuit cutoff designed for voltages above this value.
  • Current: Handling capacity of Q2 > current through circuit.

Simulation

• Normal Operation: 13V with 0.5A current; circuit operates normally.
• Overvoltage Condition: Increase voltage to 19V; Q2 cuts off current flow.
  • Ensures protection by shutting down the circuit.

Enhancements with Zener Diode

• Stability: Zener diode can replace resistor (R1) for a more stable circuit.
  • Zener Voltage: 18V.
  • Additional Resistor: Current limiting for base of the transistor.
• Operation:
  • 14V Input: Circuit functions normally.
  • 19V Input: Zener diode conducts beyond 18V, turning on Q1; Q2 turns off, shutting down circuit.

Next Steps

• Next lecture: Overvoltage protection using PNP transistors.
• Instructions: Watch video again if needed, ask doubts in the comments, like, and subscribe.

Conclusion

• NPN-based overvoltage protection is effective for handling voltage surges and protecting circuits.
• Simulation and enhancements with zener diodes offer stability and reliability.