Designing a Voltage-Controlled Rectifier Trigger Circuit

Introduction

• Objective: Design and test a voltage-controlled trigger circuit for a half-wave control rectifier.
• Goals:
  • Identify major features and issues.
  • Suggest modifications for different scenarios (e.g., converting to full-wave control).
  • Use a triggering circuit to obtain a pulse at a desired phase shift angle proportional to a control voltage.

Key Concepts

• Control Voltage: Alters the position of the triggering pulse.
• Triggering Circuit: Conceptualized as a black box that changes the position of the triggering pulse based on control voltage.
• Half-Wave Controlled Rectifier Setup: AC source, controlled rectifier, and R-L load.

Circuit Stages

1. Current Source (Transistor-Based):
   • Generates a ramp voltage by charging a capacitor.
2. Synchronizing Circuit:
   • Resets the ramp voltage to sync with zero crossings of the input AC voltage.
3. Comparator Circuit:
   • Produces the gating pulse by comparing ramp voltage with control voltage.

Working Principle

• Ramp Generation:
  • Ramp synchronized with zero crossings.
  • Compared with control voltage to produce a gating pulse.
• Zero Crossing Synchronization: Essential to ensure the gating pulse aligns with the input power supply zero crossings.
• Alpha Control: Controlled by adjusting the control voltage.

Design of Ramp Generator

• Components:
  • Transistor-based constant current source.
  • Capacitor charged to create the ramp voltage.
• Calculation:
  • Ramp height and duration based on AC cycle time (T/2).
  • Example: For a ramp of 5V in 10ms, use a 1µF capacitor charged by 0.5mA current.

Current Source Design

• Transistor Biasing:
  • Set by emitter resistance (RE) and base voltages.
  • Use standard resistor values close to calculated needs.
• Example Calculations:
  • If RE is 5.6kΩ, the current is fixed at 0.5mA.
  • Use potential divider rule to set base voltage and choose R1, R2.

Synchronizing Circuit

• Purpose: Reset ramp voltage at zero crossings.
• Design:
  • Transistor working as a switch to discharge capacitor.
  • Use standard resistor values to control discharge rate.

Comparator Circuit

• Control Voltage Adjustment:
  • Limit to a maximum (e.g., 5V).
  • Use potentiometer and fixed resistors for fine-tuning.
• Component Values:
  • Example: For a 12V VCC, use 1.3kΩ and 4.7kΩ potentiometer.

Summary of Design

• Ensure proper transistor biasing for constant current generation.
• Accurate resetting of ramp voltage for synchronization.
• Adjust control voltage to manage alpha effectively.
• Always use standard component values for reliability.